Resinosis Inhibits Monochamus spp. (Coleoptera: Cerambycidae) Colonization of Healthy Shortleaf Pines in Southeastern United States

Resinosis Inhibits Monochamus spp. (Coleoptera: Cerambycidae) Colonization of Healthy Shortleaf... Abstract The genus Monochamus Dejean (Coleoptera: Cerambycidae) includes large, woodboring, longhorned beetles, which colonize pine trees in North America. Many authors have classified the genus as saprophagous, but one recent study reported successful colonization of standing jack pine trees (Pinus banksiana Lamb.) (Pinales: Pinaceae) following severe wind disturbance in Minnesota. We tested whether two Monochamus species native to the southeastern United States (M. titillator (Fabricius) and M. carolinensis (Olivier)) could successfully colonize healthy shortleaf pines (Pinus echinata Mill.) (Pinales: Pinaceae) in recently harvested stands without coincident abiotic or biotic stressors, such as lightning strikes or bark beetle attacks. We attached commercially available semiochemical lures, including monochamol, ethanol, and ipsenol, to healthy shortleaf pine trees and observed Monochamus spp. oviposition response. Egg development was monitored following oviposition by harvesting attacked trees and dissecting oviposition pits. High numbers of oviposition pits were observed on trees treated with lures containing the bark beetle pheromone ipsenol and pits were highly concentrated on the tree bole near lures. Although egg deposition occurred, pit dissection revealed large amounts of resin present in almost all dissected pits and that egg hatch and subsequent larval development were rare. Our results demonstrate that southeastern Monochamus spp. are unlikely to be primary pests of healthy shortleaf pines due to resinosis. To better understand the host finding behavior of these two Monochamus species, we also conducted trapping trials with several semiochemical combinations. Both species and sexes demonstrated similar attraction to compounds, and the most attractive lure combined host volatiles, pheromone, and sympatric insect kairomone. Monochamus, shortleaf pine, oviposition, monochamol, ipsenol Longhorned beetles (Coleoptera: Cerambycidae) in the genus Monochamus Dejean are among the largest insects that colonize conifers. Fourteen Monochamus species are native to North America where pines (Pinus L.) are their primary hosts, although other hosts may include fir (Abies Mill.), spruce (Picea Mill.), and other conifer species (e.g., Pseudotsuga menziesii (Mirb.) Franco) (Linsley and Chemsak 1984, Roguet 2015). Monochamus spp. initiate the onset of wood decomposition and can be economically important, yet many of their intra- and interspecific interactions are poorly understood. The life cycles of Monochamus spp. are strongly influenced by host physiological status and response to volatile compounds. Following sexual maturation Monochamus spp. aggregate on susceptible conifer hosts in response to volatile compounds associated with stressed hosts and sympatric woodborers (Hanks 1999, Allison et al. 2001, Miller et al. 2013), and male-produced pheromones promote subsequent mating (Allison et al. 2012, Fierke et al. 2012, Macias-Samano et al. 2012). Following copulation females create oviposition pits and place one to nine eggs beneath the bark surrounding the pit (Webb 1909, Alya and Hain 1985). Developing larvae consume cambium and phloem tissue (Rose 1957, Alya and Hain 1985) and, as they mature, create galleries into the sapwood while periodically returning to feed in subcortical tissue. Larval tunnels may score the heartwood before turning upwards and into the xylem, forming U-shaped galleries where pupation occurs (Webb 1909). After eclosion adults emerge from natal hosts and fly to canopies of healthy host trees, where they feed for up to 14 d on needles, twigs, and branches (Linsley 1959, Walsh and Linit 1985). In forest ecosystems Monochamus spp. play a role in wood nutrient recycling and can be economically detrimental to timber products. Larval beetles initiate wood decomposition by digesting subcortical tree tissue (Linsley 1959). The extensive larval galleries also provide access to the interior of the tree for various saprophagic insects, fungi, and bacteria (Linsley 1959, Dajoz 1998). These galleries, so-called ‘worm holes’ in cut wood, can facilitate the introduction of certain wood-staining fungi that can decrease the value of lumber by more than 30% (Wilson 1962, Safranyik and Raske 1970, Raske 1972). Adult Monochamus spp. are also the primary vectors of the North American nematode Bursaphelenchus xylophilus (Steiner and Buhrer) (Linit 1988). This nematode causes pine wilt disease, responsible for widespread mortality of native pines in Europe and Asia as well as exotic pines planted in North America (Yamane and Oda 1975, Linit and Tamura 1987, Sousa et al. 2001). Although Monochamus spp. can negatively affect pine growth and production, they are typically classified as secondary colonizers that develop in injured or moribund trees (Webb 1909, Hanks 1999). The ability of host trees to resist colonization by woodborers relies on both constituitive and induced defenses. In pines, constituitive defenses include a system of preformed resin and oleoresin ducts, which borers encounter as they tunnel into hosts (Berryman 1972). Induced metabolism in tissues near wounds activates the release of toxic terpenes and phenolics, and increases oleoresin flow and resin duct production (Berryman 1972, Raffa and Berryman 1983, Keeling and Bohlmann 2006). Resinosis is the primary pine defense against many woodboring beetles, as it acts to physically trap or expel colonizing beetles or progeny thus limiting successful colonization (Paine et al. 1997, Trapp and Croteau 2001). Diminished host vigor decreases both constituitive and induced resistance in wounded or stressed trees, thereby increasing susceptibility to colonization by woodboring insects (Raffa and Berryman 1983, Paine and Stephen 1987). Although previous research has suggested that Monochamus spp. are saprophagous (Webb 1909, Hanks 1999), recent observations in an extensively wind-damaged forest in Minnesota reported that Monochamus spp. at high densities successfully colonized residual jack pine trees (Pinus banksiana Lamb.) without prior infestation by bark beetles (Gandhi 2005, Gandhi et al. 2007). Gandhi (2005) concluded that Monochamus scutellatus (Say), M. mutator LeConte, and M. notatus (Drury) were capable of being primary colonizers and could contribute to increased tree mortality within disturbed pine stands. The primary objective of our study was to determine if Monochamus species native to the southeastern United States (M. titillator (Fabricius) and M. carolinensis (Olivier)) could successfully colonize apparently healthy shortleaf pines in recently harvested stands. We used semiochemicals lures, including pheromones and kairomones, to attract southeastern Monochamus spp. to healthy trees and monitored oviposition and subsequent egg development. Host trees baited with attractive semiochemicals have been used to induce colonization in many species of bark beetles (e.g., Pureswaran and Borden 2003, Ranger et al. 2015). Colonization was considered successful if eggs developed to adults in the host tree. In addition, to further understand the host finding behavior of southeastern Monochamus spp., we examined species- and sex-specific differences in attraction to known semiochemicals between M. titillator and M. carolinensis in a field trapping study. Materials and Methods Site and Tree Selection Our study was conducted 21 May–26 September 2014 in four recently thinned forest plots within the Ozark National Forest in Arkansas. Within each plot, shortleaf pine was the dominant tree species (15.6–30.2 m2/ha pine basal area), while the surrounding forest contained mixed oak-hickory-pine species (1.6–8.7 m2/ha pine basal area). Standing pines in each plot appeared healthy, with little or no crown flagging or injury, and the amount of downed woody material appeared sufficient to support substantial Monochamus spp. populations. Two separate experiments, in which treatments were applied early and late season, were conducted simultaneously in each plot. A combined total of 11 healthy, medium-sized (20–35 cm dbh [diameter at breast height ca. 1.5 m]) shortleaf pine trees were selected for the experiments in each plot. Trees were classified as healthy if they showed no serious defects in growth or crown condition. Serious defects included split trunks or abnormal limb growth, flagging needles and limbs, split branches, major scarring or weeping wounds, symptoms of previous insect attack, and poor crown condition. All selected trees were located at least 20 m from other selected trees. All trees were sampled for presence of pinewood nematode (Kinn and Linit 1992) prior to treatment. Pinewood nematode has been observed in asymptomatic shortleaf pines, so we sampled trees to determine whether the presence of nematodes might interact with woodborer colonization to influence tree health and egg development. Phloem shavings were obtained at breast height from all trees by drilling into the phloem, using a 2.54 cm auger bit, and collecting the shavings into plastic bags. Each bag of pine shavings was soaked in water for 24 h and then examined for nematode presence under a 400× stereomicroscope. Nematode sampling was repeated for all trees at the end of the experiment. Before experimental treatments, resin flow on all trees was measured to determine if oleoresin pressure might be correlated with Monochamus spp. pit creation and beetle development. Resin was sampled by hammering through the bark and 0.5 cm into the xylem, with a 2.54 cm diameter arc punch. A plastic collector (USDA FS MTDC, Missoula, MT), with an attached plastic 15-ml collection tube, was inserted below the outer bark to allow resin to drain (Karsky et al. 2004). The sample from each tree consisted of two collectors, attached on opposite sides of the bole at approximately 1.5 m from ground level. Tubes were collected 24 h after placement and held in an upright position for an additional 24 h to allow resin to settle. The height of resin in each tube was measured and averaged to give the sample measurement for each tree. Resin was sampled prior to and following tree treatments. Pretreatment Trapping To confirm that Monochamus spp. adults were present in each plot, a short trapping study was conducted prior to the experiment. Within each plot one black cross-vane panel trap (APTIV Intercept, APTIV Inc., Portland, OR) was suspended between two nonhost trees, at a height of approximately 2 m, and baited with a combination of ethanol and alpha-pinene. In previous studies this lure combination attracted Monochamus spp. beetles (Fatzinger 1985, Phillips et al. 1988). Collection containers on traps contained propylene glycol (Splash RV antifreeze, SPLASH, St. Paul, MN) as a preservative. Insects were collected weekly and Monochamus spp. beetles were sexed and identified to species (Lingafelter 2007). Traps were erected on 28 March and checked until 21 May, when the first experiment commenced. Tree Treatments Each tree was treated by driving an aluminum nail into the trunk, at approximately 6 m above ground level, and securing a semipermeable lure bag on the nail. Bubble caps or pouches containing individual compounds were placed inside each bag, which was loosely sealed and had one small corner removed to prevent rain from collecting inside. The following compounds were used to treat trees: low-release ethanol (100 mg/d at 25°C), 2-undecyloxyethan-1-ol (hereafter monochamol, 0.7 mg/d at 25°C), 2-methyl-6-methylene-7-octen-4-ol (hereafter ipsenol, 0.2 mg/d at 25°C), and cis-3-pinene-2-ol (no elution data available) (Synergy Semiochemicals, Burnaby, BC, Canada). Each tree was baited with the compounds in one of four lure combinations: 1) monochamol + ethanol (ME), 2) monochamol + ethanol + ipsenol (MEI), 3) ipsenol, and 4) cis-3-pinene-2-ol (CIS-OL). Trees were treated during two separate, sequential periods (Experiment 1 and Experiment 2) corresponding to two periods of 8 wk during high Monochamus spp. flight activity (Alya and Hain 1985). During experiment 1, 21 May–16 July 2014, three trees were selected from each of the four plots, for a total of 12 trees. In each plot, two trees were treated with a lure and one tree was an untreated control. Trees were treated with a monochamol + ethanol (ME) lure, a combination of host volatiles and male-produced sex pheromone shown to attract Monochamus species (Pajares et al. 2010, Teale et al. 2011) (Table 1). Table 1. Number of trees treated with lures in each experiment Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 A total of 40 trees were treated with lures, while four trees remained untreated as controls throughout both experiments. View Large Table 1. Number of trees treated with lures in each experiment Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 A total of 40 trees were treated with lures, while four trees remained untreated as controls throughout both experiments. View Large Due to low numbers of oviposition pits in experiment 1 (<0.5 pits per tree per week), we considered alternative Monochamus spp. attractants. Several Ips De Geer (Coleoptera: Curculionidae: Scolytinae) bark beetle pheromones, including ipsenol, are known to attract Monochamus spp. in natural settings (Allison et al. 2001, Miller and Asaro 2005). We conducted a short test treatment by placing ipsenol lures on four healthy shortleaf pines for 4 wk (3 July–31 July), and observed a large number of oviposition pits (data not included). In light of these observations ipsenol was included in experiment 2. In experiment 2, 1 August–26 September 2014, eight new trees were selected in each plot, for a total of 32 treated trees. In an attempt to increase oviposition on trees, treatments included ipsenol alone as well as in combination with the ME lure. Cis-3-pinene-2-ol was also included as it attracts male M. alternatus Hope in Japan (Sakai and Yamasaki 1991) and may act as an attractant to southeastern Monochamus spp. Therefore, in each plot two trees were treated with each of the following lure combinations: 1) monochamol + ethanol (ME), 2) monochamol + ethanol + ipsenol (MEI), 3) ipsenol, and 4) cis-3-pinene-2-ol (CIS-OL). This resulted in a total of 32 treated trees in experiment 2, with untreated trees from experiment 1 reused as controls (Table 1). Periodic Tree Inspection All trees were examined every 2 wk for the presence of new oviposition pits. Sectional 3 m tall Swedish tree-climbing ladders were attached to the trees to allow for observation of the tree bole from ground level to approximately 7 m. Small punctures in the bark were created by metal prongs on the ladders, but no serious injury to the tree was created by ladder placement. A mirror with an extended handle was used to inspect the bole surface on the side opposite from the observer. During tree examination, Monochamus spp. oviposition pits were counted and marked with small, colored, map pins, which were inserted into the middle of pits to avoid damaging any eggs. Oviposition pits of Monochamus spp. are usually oblong, bowl-shaped pits (ca. 8 mm × 3 mm), although in thinner bark they may be slits in the bark (ca. 2 mm long), with a small hole in the middle (Alya and Hain 1985). The spatial distribution of pits was noted by demarcating 0.5 m sections of the bole, above and below the lure, and counting all pits in each section. Trees were also inspected for any observable change in condition. Changes in bole, branch, or crown condition, such as flagging needles or weeping resin, or wounds from abiotic factors, were noted during inspections. If the tree was free from signs of negative change in condition, it was noted as healthy. Bolt Dissection To determine if successful Monochamus spp. colonization occurred, 18 trees with varying numbers of oviposition pits were destructively sampled. Trees were felled in December 2014 and portions of each bole containing pits were cut into sections and transported to the University of Arkansas Forest Entomology Lab. Pits in each bole section were dissected by centering a 2.54 cm diameter arc punch over the pit and hammering through outer and inner bark. Within the circular cuts, the outer bark was removed and eggs, larvae, and larval galleries were counted. The presence, or absence, of resin in dissected pits was also noted. Trapping Study To identify possible species- and sex-specific differences in attraction to our lure combinations, a trapping study was undertaken 26 May‒21 July 2015. Two plots were selected within the St. Francis-Ozark National Forest near Lake Wedington, in northwest Arkansas. Stands were dominated by shortleaf pine (21.5–30.2 m2/ha pine basal area), with a mixed hardwood understory (2.8–3.7 m2/ha basal area). Both plots had been recently thinned, but standing pines appeared healthy. In each plot a series of four traps was placed in a roughly square pattern with traps approximately 25 m from one another. Black cross-vane panel traps with modified collection containers (Hartshorn et al. 2016) were used. Containers consisted of a 121-liter garbage can with a hole in the lid and screened ventilation holes in each side. A panel trap was permanently affixed to the hole in the lid, where collected insects entered the container. Fresh pine branches were placed inside the cans as feeding material and refuge for Monochamus spp. adults. Each panel trap was baited with one of four lure combinations: 1) ipsenol, 2) monochamol + ethanol (ME), 3) monochamol + ethanol + ipsenol (MEI), or 4) monochamol + ipsenol + UHR alpha-pinene (MIA) (Synergy Semiochemicals, Burnaby, BC, Canada). These attractants were similar to treatments applied to trees, although alpha-pinene was added to the MIA lure, as alpha-pinene was assumed to be present on live trees and may have increased due to wounds from the climbing ladders. Each trash can lid with the attached intercept trap and lures was rotated weekly to the next trap position, in a clockwise direction, to account for possible positional effects. Traps were checked weekly and collected beetles were identified to species and sex (Lingafelter 2007). Statistical Analysis All data were analyzed using R (R Core Team 2018). All tests were conducted at significance levels of P = 0.05 and normality was tested using the Shapiro–Wilk test. One-way analysis of variance (ANOVA) was used to determine significant differences in oviposition pit numbers between lure treatments. Individual trees were used as the unit of replication and the number of oviposition pits was utilized as the response variable. Treatments with zero pits (control and CIS-OL) were removed and the response variable was log-transformed to comply with the assumptions of normality. Homogeneity of variance was assessed using the Bartlett’s test (k2 = 0.618, df = 2, P = 0.7341). Post hoc Tukey’s HSD test was used to isolate differences. Correlation between the number of oviposition pits and tree dbh, initial resin flow, and resin flow change, was tested using a generalized linear model with a Poisson distribution. Due to a lack of normality, differences in the spatial distribution of oviposition pits, as well as in number of each species and sex of beetles captured by each lure, were analyzed using the Kruskal–Wallis ANOVA by ranks (Kruskal and Wallis 1952). Individual trees and oviposition pits, and individual traps and beetles captured, were used as units of replication and the response variable, respectively. Post hoc multiple comparisons of differences were analyzed using the ‘PMCMR’ package in R (Pohlert 2014). Results Pretreatment Trapping Both M. titillator and M. carolinensis were initially captured on 12 May. Each sex of both species was captured in all plots from the initial capture date until experiments were initiated. The number of Monochamus spp. captured in each plot ranged from 4 to 11 beetles per week, with a mean of 7.25 ± 1.6 (SE) beetles. The most common Monochamus species and sex were M. titillator females (2.38 ± 0.63 beetles per plot per week) and the least common were M. carolinensis females (1.13 ± 0.48 beetles per plot per week). Associated insects commonly captured included: Cerambycidae: Rhagium inquisitor (Linnaeus); Trogossitidae: Temnoscheila virescens (Fabricius); Cleridae: Thanasimus dubius (Fabricius); Histeridae: Platysoma cylindricum (Paykull), as well as a variety of bark and ambrosia beetles. Of note was the low number of Ips species bark beetles captured in the plots. Tree Measurements Experimental tree dbhs ranged from 19.0 to 47.5 cm, with a mean of 29.0 ± 1.20 cm. Nematodes were not observed in samples from any trees, either before or after the experiment. Initial tree resin volumes ranged from 0 to 4.75 cm, with a mean of 1.68 ± 0.27 cm for trees used in experiment 1, and 1.39 ± 0.19 cm for trees used in experiment 2. At the end of the field season resin volume ranged from 0 to 3.65 cm with a mean of 1.68 ± 0.24 cm for experiment 1 and 1.09 ± 0.16 cm for experiment 2 trees. Average change in resin volume was 0.11 ± 0.36 cm. Numbers of oviposition pits were not significantly correlated with tree dbh (Z = −0.998, df = 32, P = 0.318), initial resin (Z = −0.117, df = 32, P = 0.907), or final resin measurements (Z = −0.316, df = 32, P = 0.752). Tree Inspection Throughout the experiment treated trees showed no visible symptoms of negative health that could be correlated with Monochamus spp. attack. Resin was seen pooling in wounds caused by ladder prongs and oviposition pits, but rarely running down the trunk. A number of trees experienced low levels of sapsucker (Picidae: Sphyrapicus spp.) feeding damage along the length of the main stem. In total, 556 pits were observed throughout the two experiments. The number of oviposition pits in treated trees ranged from 0 to 14 for experiment 1, and from 0 to 75 pits for experiment 2. There were significant differences in the number of oviposition pits among treatments (F = 32.94, df = 2, P < 0.001). Lure combinations that included ipsenol, the ipsenol and MEI treatments, had significantly more oviposition pits than those lacking ipsenol, the ME and control treatments (Fig. 1). Fig. 1. View largeDownload slide Mean number of Monochamus spp. oviposition pits on individual trees (n = 40) treated with attractive lures for 8 wk in 2014. Treatments included were ipsenol, monochamol + ethanol + ipsenol (MEI), monochamol + ethanol (ME), and cis-3-pinene-2-ol (CIS-OL). Results for ME treatments from experiment 1 and 2 are pooled (not significantly different), while all other results are only from experiment 2. Significant differences (P = 0.05; one-way ANOVA, Tukey’s HSD) are designated by different letters. Fig. 1. View largeDownload slide Mean number of Monochamus spp. oviposition pits on individual trees (n = 40) treated with attractive lures for 8 wk in 2014. Treatments included were ipsenol, monochamol + ethanol + ipsenol (MEI), monochamol + ethanol (ME), and cis-3-pinene-2-ol (CIS-OL). Results for ME treatments from experiment 1 and 2 are pooled (not significantly different), while all other results are only from experiment 2. Significant differences (P = 0.05; one-way ANOVA, Tukey’s HSD) are designated by different letters. Oviposition pits were distributed in a nonrandom pattern, with pits highly concentrated near the lure. More oviposition pits were located within 0.5 m of the lure bag (Kruskal–Wallis χ2 = 91.3548, df = 5, P < 0.001) than in other bole sections where pits were present. Furthermore, 90% of total pits were found on the same 180° arc of the trunk where lures were suspended (‘front’) and only 10% of total pits were found on the opposing trunk 180° arc (‘back’) (Fig. 2). Fig. 2. View largeDownload slide Distribution of Monochamus spp. oviposition pits along tree boles in relation to location of the attractive lure. ‘Front’ refers to the same 180° arc of the bole where the lure bag was located, whereas ‘back’ refers to the opposite 180° arc. Sections marked with an asterisk contained significantly more oviposition pits (P = 0.05; Kruskal–Wallis rank sum test) than all other sections, but were not significantly different from each other. Fig. 2. View largeDownload slide Distribution of Monochamus spp. oviposition pits along tree boles in relation to location of the attractive lure. ‘Front’ refers to the same 180° arc of the bole where the lure bag was located, whereas ‘back’ refers to the opposite 180° arc. Sections marked with an asterisk contained significantly more oviposition pits (P = 0.05; Kruskal–Wallis rank sum test) than all other sections, but were not significantly different from each other. Bolt Dissection From the 18 destructively sampled trees, 562 pits were identified and dissected. A total of 993 eggs were found beneath the bark of dissected pits, but only 62% of the oviposition pits contained any eggs. The range of eggs within pits was 1–11, with an average of 2.85 ± 0.16 eggs for those pits with eggs present. The majority of eggs were placed in a circular pattern surrounding the oviposition pit, while few pits had eggs laid side by side. Only 20 larvae were found in dissected pits, and of these, only 5 had created any galleries. Substantial amounts of resin, indicative of resinosis, were observed in 95.6% of the dissected pits. Trapping Study A total of 933 adult Monochamus beetles were caught in 2015 over 7 wk of trapping. Of 770 M. titillator captured, 423 were male and 347 were female, a male to female ratio of 1.22:1. Of 163 M. carolinensis captured 63 were male and 100 were female, for a male to female ratio of 0.63:1. Traps baited with MEI and MIA captured significantly more beetles than all other lures, ipsenol captured an intermediate number, and ME captured the least (Kruskal–Wallis χ2 = 18.7479, df = 3, P < 0.001; Fig. 3). Each species differed in their response to different lure combinations. For M. carolinensis, the MEI and MIA lure combinations were most attractive, although MIA was not significantly different from ipsenol, which itself was not significantly greater than ME (Kruskal–Wallis χ2 = 7.6531, df = 3, P = 0.053; Fig. 4). For M. titillator, MEI and MIA were the most attractive lure combinations, with both lures capturing significantly more beetles than both the ipsenol and ME lure; ipsenol was intermediate between the two most attractive lures and the ME lure (Kruskal–Wallis χ2 = 3.7883, df = 3, P = 0.2853; Fig. 5). No sex-specific differences in either species occurred in the response to the various lure combinations (Kruskal–Wallis χ2 = 0.175, df = 3, P = 0.9130). Fig. 3. View largeDownload slide Mean number of Monochamus spp. captured in cross-vane panel traps baited with four attractive lure combinations during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) are designated by different letters. Fig. 3. View largeDownload slide Mean number of Monochamus spp. captured in cross-vane panel traps baited with four attractive lure combinations during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) are designated by different letters. Fig. 4. View largeDownload slide Mean number of each sex of M. carolinensis captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. carolinensis captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 4. View largeDownload slide Mean number of each sex of M. carolinensis captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. carolinensis captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 5. View largeDownload slide Mean number of each sex of M. titillator captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. titillator captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 5. View largeDownload slide Mean number of each sex of M. titillator captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. titillator captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Discussion Our objective was to determine if southeastern Monochamus spp. could successfully colonize healthy shortleaf pines in the absence of external stressors such as lightning strikes, drought, or coincident bark beetle attack. Although we could successfully induce oviposition in healthy pines, only 2.5% of eggs hatched, and larval feeding and development was almost absent. The primary cause of Monochamus spp. egg death in our experiment was resinosis, a general conifer defense responsible for mortality of many woodboring insects (Langor and Raske 1988, Trapp and Croteau 2001, Akbulut et al. 2004). In healthy conifers, constitutive resin ducts, prevalent in Pinus species, and induced resin formation act as the primary defense against beetle colonization (Berryman 1972, Paine et al. 1997). Dodds and Stephen (2000) found that the egg stage of M. titillator was particularly vulnerable to mortality from resinosis. As wounds or stress limit nutrient flow, oleoresin and resin duct formation quickly decreases and trees become susceptible to woodborer colonization (Raffa and Berryman 1983, Paine et al. 1997). Monochamus spp. most likely colonize recently dead or moribund trees to avoid host defenses, such as resinosis, which are often overcome by mass attack of primary or secondary bark beetles (Hanks 1999, Stephen 2011). Our results do not support Gandhi’s (2005) findings that Monochamus spp. can successfully colonize residual trees in disturbed pine stands. Possible explanations for these differences may relate to both abiotic and biotic factors. Gandhi (2005) reported approximately 2.5 times more beetles present in wind-damaged plots than in fire-treated or control plots, while Monochamus spp. populations in Arkansas appeared somewhat lower. Oviposition in residual pines in Minnesota occurred in wind-damaged stands that contained more coarse woody debris than harvested stands in Arkansas, thereby increasing the unprotected material available for Monochamus spp. development. Additional stand stressors, such as increased exposure to wind or nutrient loss, are also known to diminish host resistance to woodboring species (Paine and Stephen 1987) and may have been more pervasive in Minnesota stands due to the extensive wind throw. Southeastern Monochamus species use a variety of volatile compounds to find susceptible hosts and mates. Sexually mature beetles are attracted to volatiles associated with declining pines, including α-pinene, ethanol, and turpentine (Fatzinger 1985, Fan et al. 2007, Allison et al. 2012). These compounds are abundant near wounded or stressed conifers, which may have diminished resinous defenses and are therefore more susceptible to Monochamus spp. colonization (Berryman 1972, Paine et al. 1997, Hanks 1999). In addition to host volatiles, Monochamus spp. also respond to the pheromones of sympatric organisms, which function as kairomones. Our tree and trap treatments that included the Ips bark beetle pheromone ipsenol, incurred higher oviposition and captured more beetles, respectively, than treatments without the kairomone. Our results complement other findings that many Monochamus species are attracted to Ips bark beetle pheromones, especially ipsenol (Billings and Cameron 1984, Allison et al. 2001, Groot and Nott 2004, Pajares et al. 2004, Miller and Asaro 2005, Ibeas et al. 2007). Contrary to Groot and Nott (2004) we found that both M. carolinensis and M. titillator are attracted to ipsenol in the absence of other attractants. Ipsenol is produced by Ips bark beetles which infest stressed or moribund hosts, thus sharing larval host resources with Monochamus species (Vite and Renwick 1971, Hanks 1999). Attraction to pheromones of feeding-guild members that share hosts effectively increases the number of beetles searching for susceptible hosts, which can be geographically dispersed and temporally ephemeral (Allison et al. 2001, Ibeas et al. 2007). Attraction to Ips pheromones may also serve to increase nutrient intake in a nitrogen-poor environment, as larval Monochamus spp. can act as facultative predators of bark beetles (Allison et al. 2001, Dodds et al. 2001). In addition to behavioral cues, conspecific pheromones are important in host finding for many woodboring insects. Monochamol, 2-undecyloxy-1-ethanol, is a commercially available pheromone produced by male Monochamus spp. that attracts mature beetles of both sexes (Pajares et al. 2010, Allison et al. 2012, Fierke et al. 2012). We used monochamol combined with the host volatile ethanol in our study, but observed the lowest response to this lure combination in both oviposition and trapping experiments. Low response may indicate that monochamol is only a weak attractant for these two Monochamus species, or that the pheromone is only detectable at short distances. Although cerambycid sex pheromones can operate at long distances, some compounds are only attractive at 5 cm or less (Wang et al. 1990, Wang et al. 1991, Lacey et al. 2004). Teale (2011) proposed a sequence for mating behavior in Monochamus spp., whereby mature adults are attracted to host volatiles at long distances, followed by attraction to the pheromone at short distances once on hosts. In our experiments, the increased number of pits and captured beetles in response to ipsenol may indicate that bark beetle pheromones also attract Monochamus spp. at long distances. Although solitary compounds (e.g., α-pinene) can attract Monochamus spp., our results support previous findings (e.g., Miller et al. 2011) that combinations of cues and pheromones are more attractive than isolated chemicals. In southeastern Monochamus spp. the combination of monochamol and α-pinene is more attractive than either compound by itself, although M. carolinensis is attracted to monochamol alone (Allison et al. 2012, Hanks et al. 2012). The addition of sympatric insect pheromones (e.g., ipsenol) to host volatiles drastically increases the attraction above that of host volatiles alone (Ibeas et al. 2007, Miller et al. 2011). We found that more beetles of each species and sex were collected in traps baited with combinations of monochamol, Ips pheromones, and host volatiles, suggesting synergy between the compounds (Miller et al. 2011, 2013). Although our lures used different host volatiles (ethanol, α-pinene), there was no significant difference in beetle capture between the two, perhaps alluding to a lack of differences in compound-specific response to general host stress volatiles. Acknowledgments We thank Jake Bodart, Dave Dalzatto, and Ryan Rastok for their help in conducting fieldwork. We also appreciate technical input and semiochemical lures provided by David Wakarchuk and Synergy Semiochemicals, Burnaby, BC, Canada. Keith Whalen and Mike Hennigan, USDA Forest Service, assisted in site selection on the Boston Mountain Ranger District of the Ozark National Forest. We also thank the USDA Forest Service for permission to conduct experiments in the Ozark National Forest. This research was supported in part by the University of Arkansas, Division of Agriculture. References Cited Akbulut , S. , W. T. Stamps , and M. J. Linit . 2004 . 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The pine bark and wood-boring beetles are secondary, then what is primary? A brief review of the studies on the cause of heavy mortality of pines in Japan . FAO/IUFRO World Technical Consultation on Forest Diseases and Insects, New Delhi, India . © The Author(s) 2018. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environmental Entomology Oxford University Press

Resinosis Inhibits Monochamus spp. (Coleoptera: Cerambycidae) Colonization of Healthy Shortleaf Pines in Southeastern United States

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Abstract

Abstract The genus Monochamus Dejean (Coleoptera: Cerambycidae) includes large, woodboring, longhorned beetles, which colonize pine trees in North America. Many authors have classified the genus as saprophagous, but one recent study reported successful colonization of standing jack pine trees (Pinus banksiana Lamb.) (Pinales: Pinaceae) following severe wind disturbance in Minnesota. We tested whether two Monochamus species native to the southeastern United States (M. titillator (Fabricius) and M. carolinensis (Olivier)) could successfully colonize healthy shortleaf pines (Pinus echinata Mill.) (Pinales: Pinaceae) in recently harvested stands without coincident abiotic or biotic stressors, such as lightning strikes or bark beetle attacks. We attached commercially available semiochemical lures, including monochamol, ethanol, and ipsenol, to healthy shortleaf pine trees and observed Monochamus spp. oviposition response. Egg development was monitored following oviposition by harvesting attacked trees and dissecting oviposition pits. High numbers of oviposition pits were observed on trees treated with lures containing the bark beetle pheromone ipsenol and pits were highly concentrated on the tree bole near lures. Although egg deposition occurred, pit dissection revealed large amounts of resin present in almost all dissected pits and that egg hatch and subsequent larval development were rare. Our results demonstrate that southeastern Monochamus spp. are unlikely to be primary pests of healthy shortleaf pines due to resinosis. To better understand the host finding behavior of these two Monochamus species, we also conducted trapping trials with several semiochemical combinations. Both species and sexes demonstrated similar attraction to compounds, and the most attractive lure combined host volatiles, pheromone, and sympatric insect kairomone. Monochamus, shortleaf pine, oviposition, monochamol, ipsenol Longhorned beetles (Coleoptera: Cerambycidae) in the genus Monochamus Dejean are among the largest insects that colonize conifers. Fourteen Monochamus species are native to North America where pines (Pinus L.) are their primary hosts, although other hosts may include fir (Abies Mill.), spruce (Picea Mill.), and other conifer species (e.g., Pseudotsuga menziesii (Mirb.) Franco) (Linsley and Chemsak 1984, Roguet 2015). Monochamus spp. initiate the onset of wood decomposition and can be economically important, yet many of their intra- and interspecific interactions are poorly understood. The life cycles of Monochamus spp. are strongly influenced by host physiological status and response to volatile compounds. Following sexual maturation Monochamus spp. aggregate on susceptible conifer hosts in response to volatile compounds associated with stressed hosts and sympatric woodborers (Hanks 1999, Allison et al. 2001, Miller et al. 2013), and male-produced pheromones promote subsequent mating (Allison et al. 2012, Fierke et al. 2012, Macias-Samano et al. 2012). Following copulation females create oviposition pits and place one to nine eggs beneath the bark surrounding the pit (Webb 1909, Alya and Hain 1985). Developing larvae consume cambium and phloem tissue (Rose 1957, Alya and Hain 1985) and, as they mature, create galleries into the sapwood while periodically returning to feed in subcortical tissue. Larval tunnels may score the heartwood before turning upwards and into the xylem, forming U-shaped galleries where pupation occurs (Webb 1909). After eclosion adults emerge from natal hosts and fly to canopies of healthy host trees, where they feed for up to 14 d on needles, twigs, and branches (Linsley 1959, Walsh and Linit 1985). In forest ecosystems Monochamus spp. play a role in wood nutrient recycling and can be economically detrimental to timber products. Larval beetles initiate wood decomposition by digesting subcortical tree tissue (Linsley 1959). The extensive larval galleries also provide access to the interior of the tree for various saprophagic insects, fungi, and bacteria (Linsley 1959, Dajoz 1998). These galleries, so-called ‘worm holes’ in cut wood, can facilitate the introduction of certain wood-staining fungi that can decrease the value of lumber by more than 30% (Wilson 1962, Safranyik and Raske 1970, Raske 1972). Adult Monochamus spp. are also the primary vectors of the North American nematode Bursaphelenchus xylophilus (Steiner and Buhrer) (Linit 1988). This nematode causes pine wilt disease, responsible for widespread mortality of native pines in Europe and Asia as well as exotic pines planted in North America (Yamane and Oda 1975, Linit and Tamura 1987, Sousa et al. 2001). Although Monochamus spp. can negatively affect pine growth and production, they are typically classified as secondary colonizers that develop in injured or moribund trees (Webb 1909, Hanks 1999). The ability of host trees to resist colonization by woodborers relies on both constituitive and induced defenses. In pines, constituitive defenses include a system of preformed resin and oleoresin ducts, which borers encounter as they tunnel into hosts (Berryman 1972). Induced metabolism in tissues near wounds activates the release of toxic terpenes and phenolics, and increases oleoresin flow and resin duct production (Berryman 1972, Raffa and Berryman 1983, Keeling and Bohlmann 2006). Resinosis is the primary pine defense against many woodboring beetles, as it acts to physically trap or expel colonizing beetles or progeny thus limiting successful colonization (Paine et al. 1997, Trapp and Croteau 2001). Diminished host vigor decreases both constituitive and induced resistance in wounded or stressed trees, thereby increasing susceptibility to colonization by woodboring insects (Raffa and Berryman 1983, Paine and Stephen 1987). Although previous research has suggested that Monochamus spp. are saprophagous (Webb 1909, Hanks 1999), recent observations in an extensively wind-damaged forest in Minnesota reported that Monochamus spp. at high densities successfully colonized residual jack pine trees (Pinus banksiana Lamb.) without prior infestation by bark beetles (Gandhi 2005, Gandhi et al. 2007). Gandhi (2005) concluded that Monochamus scutellatus (Say), M. mutator LeConte, and M. notatus (Drury) were capable of being primary colonizers and could contribute to increased tree mortality within disturbed pine stands. The primary objective of our study was to determine if Monochamus species native to the southeastern United States (M. titillator (Fabricius) and M. carolinensis (Olivier)) could successfully colonize apparently healthy shortleaf pines in recently harvested stands. We used semiochemicals lures, including pheromones and kairomones, to attract southeastern Monochamus spp. to healthy trees and monitored oviposition and subsequent egg development. Host trees baited with attractive semiochemicals have been used to induce colonization in many species of bark beetles (e.g., Pureswaran and Borden 2003, Ranger et al. 2015). Colonization was considered successful if eggs developed to adults in the host tree. In addition, to further understand the host finding behavior of southeastern Monochamus spp., we examined species- and sex-specific differences in attraction to known semiochemicals between M. titillator and M. carolinensis in a field trapping study. Materials and Methods Site and Tree Selection Our study was conducted 21 May–26 September 2014 in four recently thinned forest plots within the Ozark National Forest in Arkansas. Within each plot, shortleaf pine was the dominant tree species (15.6–30.2 m2/ha pine basal area), while the surrounding forest contained mixed oak-hickory-pine species (1.6–8.7 m2/ha pine basal area). Standing pines in each plot appeared healthy, with little or no crown flagging or injury, and the amount of downed woody material appeared sufficient to support substantial Monochamus spp. populations. Two separate experiments, in which treatments were applied early and late season, were conducted simultaneously in each plot. A combined total of 11 healthy, medium-sized (20–35 cm dbh [diameter at breast height ca. 1.5 m]) shortleaf pine trees were selected for the experiments in each plot. Trees were classified as healthy if they showed no serious defects in growth or crown condition. Serious defects included split trunks or abnormal limb growth, flagging needles and limbs, split branches, major scarring or weeping wounds, symptoms of previous insect attack, and poor crown condition. All selected trees were located at least 20 m from other selected trees. All trees were sampled for presence of pinewood nematode (Kinn and Linit 1992) prior to treatment. Pinewood nematode has been observed in asymptomatic shortleaf pines, so we sampled trees to determine whether the presence of nematodes might interact with woodborer colonization to influence tree health and egg development. Phloem shavings were obtained at breast height from all trees by drilling into the phloem, using a 2.54 cm auger bit, and collecting the shavings into plastic bags. Each bag of pine shavings was soaked in water for 24 h and then examined for nematode presence under a 400× stereomicroscope. Nematode sampling was repeated for all trees at the end of the experiment. Before experimental treatments, resin flow on all trees was measured to determine if oleoresin pressure might be correlated with Monochamus spp. pit creation and beetle development. Resin was sampled by hammering through the bark and 0.5 cm into the xylem, with a 2.54 cm diameter arc punch. A plastic collector (USDA FS MTDC, Missoula, MT), with an attached plastic 15-ml collection tube, was inserted below the outer bark to allow resin to drain (Karsky et al. 2004). The sample from each tree consisted of two collectors, attached on opposite sides of the bole at approximately 1.5 m from ground level. Tubes were collected 24 h after placement and held in an upright position for an additional 24 h to allow resin to settle. The height of resin in each tube was measured and averaged to give the sample measurement for each tree. Resin was sampled prior to and following tree treatments. Pretreatment Trapping To confirm that Monochamus spp. adults were present in each plot, a short trapping study was conducted prior to the experiment. Within each plot one black cross-vane panel trap (APTIV Intercept, APTIV Inc., Portland, OR) was suspended between two nonhost trees, at a height of approximately 2 m, and baited with a combination of ethanol and alpha-pinene. In previous studies this lure combination attracted Monochamus spp. beetles (Fatzinger 1985, Phillips et al. 1988). Collection containers on traps contained propylene glycol (Splash RV antifreeze, SPLASH, St. Paul, MN) as a preservative. Insects were collected weekly and Monochamus spp. beetles were sexed and identified to species (Lingafelter 2007). Traps were erected on 28 March and checked until 21 May, when the first experiment commenced. Tree Treatments Each tree was treated by driving an aluminum nail into the trunk, at approximately 6 m above ground level, and securing a semipermeable lure bag on the nail. Bubble caps or pouches containing individual compounds were placed inside each bag, which was loosely sealed and had one small corner removed to prevent rain from collecting inside. The following compounds were used to treat trees: low-release ethanol (100 mg/d at 25°C), 2-undecyloxyethan-1-ol (hereafter monochamol, 0.7 mg/d at 25°C), 2-methyl-6-methylene-7-octen-4-ol (hereafter ipsenol, 0.2 mg/d at 25°C), and cis-3-pinene-2-ol (no elution data available) (Synergy Semiochemicals, Burnaby, BC, Canada). Each tree was baited with the compounds in one of four lure combinations: 1) monochamol + ethanol (ME), 2) monochamol + ethanol + ipsenol (MEI), 3) ipsenol, and 4) cis-3-pinene-2-ol (CIS-OL). Trees were treated during two separate, sequential periods (Experiment 1 and Experiment 2) corresponding to two periods of 8 wk during high Monochamus spp. flight activity (Alya and Hain 1985). During experiment 1, 21 May–16 July 2014, three trees were selected from each of the four plots, for a total of 12 trees. In each plot, two trees were treated with a lure and one tree was an untreated control. Trees were treated with a monochamol + ethanol (ME) lure, a combination of host volatiles and male-produced sex pheromone shown to attract Monochamus species (Pajares et al. 2010, Teale et al. 2011) (Table 1). Table 1. Number of trees treated with lures in each experiment Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 A total of 40 trees were treated with lures, while four trees remained untreated as controls throughout both experiments. View Large Table 1. Number of trees treated with lures in each experiment Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 Treatment period (ME) Monochamol + ethanol (MEI) Monochamol + ethanol + ipsenol Ipsenol (CIS-OL) Cis-3-pinene-2-ol Control Total Experiment 1 (21 May–16 July) 8 – – – 4 12 Experiment 2 (1 Aug.–26 Sept.) 8 8 8 8 4 32 Total 16 8 8 8 4 44 A total of 40 trees were treated with lures, while four trees remained untreated as controls throughout both experiments. View Large Due to low numbers of oviposition pits in experiment 1 (<0.5 pits per tree per week), we considered alternative Monochamus spp. attractants. Several Ips De Geer (Coleoptera: Curculionidae: Scolytinae) bark beetle pheromones, including ipsenol, are known to attract Monochamus spp. in natural settings (Allison et al. 2001, Miller and Asaro 2005). We conducted a short test treatment by placing ipsenol lures on four healthy shortleaf pines for 4 wk (3 July–31 July), and observed a large number of oviposition pits (data not included). In light of these observations ipsenol was included in experiment 2. In experiment 2, 1 August–26 September 2014, eight new trees were selected in each plot, for a total of 32 treated trees. In an attempt to increase oviposition on trees, treatments included ipsenol alone as well as in combination with the ME lure. Cis-3-pinene-2-ol was also included as it attracts male M. alternatus Hope in Japan (Sakai and Yamasaki 1991) and may act as an attractant to southeastern Monochamus spp. Therefore, in each plot two trees were treated with each of the following lure combinations: 1) monochamol + ethanol (ME), 2) monochamol + ethanol + ipsenol (MEI), 3) ipsenol, and 4) cis-3-pinene-2-ol (CIS-OL). This resulted in a total of 32 treated trees in experiment 2, with untreated trees from experiment 1 reused as controls (Table 1). Periodic Tree Inspection All trees were examined every 2 wk for the presence of new oviposition pits. Sectional 3 m tall Swedish tree-climbing ladders were attached to the trees to allow for observation of the tree bole from ground level to approximately 7 m. Small punctures in the bark were created by metal prongs on the ladders, but no serious injury to the tree was created by ladder placement. A mirror with an extended handle was used to inspect the bole surface on the side opposite from the observer. During tree examination, Monochamus spp. oviposition pits were counted and marked with small, colored, map pins, which were inserted into the middle of pits to avoid damaging any eggs. Oviposition pits of Monochamus spp. are usually oblong, bowl-shaped pits (ca. 8 mm × 3 mm), although in thinner bark they may be slits in the bark (ca. 2 mm long), with a small hole in the middle (Alya and Hain 1985). The spatial distribution of pits was noted by demarcating 0.5 m sections of the bole, above and below the lure, and counting all pits in each section. Trees were also inspected for any observable change in condition. Changes in bole, branch, or crown condition, such as flagging needles or weeping resin, or wounds from abiotic factors, were noted during inspections. If the tree was free from signs of negative change in condition, it was noted as healthy. Bolt Dissection To determine if successful Monochamus spp. colonization occurred, 18 trees with varying numbers of oviposition pits were destructively sampled. Trees were felled in December 2014 and portions of each bole containing pits were cut into sections and transported to the University of Arkansas Forest Entomology Lab. Pits in each bole section were dissected by centering a 2.54 cm diameter arc punch over the pit and hammering through outer and inner bark. Within the circular cuts, the outer bark was removed and eggs, larvae, and larval galleries were counted. The presence, or absence, of resin in dissected pits was also noted. Trapping Study To identify possible species- and sex-specific differences in attraction to our lure combinations, a trapping study was undertaken 26 May‒21 July 2015. Two plots were selected within the St. Francis-Ozark National Forest near Lake Wedington, in northwest Arkansas. Stands were dominated by shortleaf pine (21.5–30.2 m2/ha pine basal area), with a mixed hardwood understory (2.8–3.7 m2/ha basal area). Both plots had been recently thinned, but standing pines appeared healthy. In each plot a series of four traps was placed in a roughly square pattern with traps approximately 25 m from one another. Black cross-vane panel traps with modified collection containers (Hartshorn et al. 2016) were used. Containers consisted of a 121-liter garbage can with a hole in the lid and screened ventilation holes in each side. A panel trap was permanently affixed to the hole in the lid, where collected insects entered the container. Fresh pine branches were placed inside the cans as feeding material and refuge for Monochamus spp. adults. Each panel trap was baited with one of four lure combinations: 1) ipsenol, 2) monochamol + ethanol (ME), 3) monochamol + ethanol + ipsenol (MEI), or 4) monochamol + ipsenol + UHR alpha-pinene (MIA) (Synergy Semiochemicals, Burnaby, BC, Canada). These attractants were similar to treatments applied to trees, although alpha-pinene was added to the MIA lure, as alpha-pinene was assumed to be present on live trees and may have increased due to wounds from the climbing ladders. Each trash can lid with the attached intercept trap and lures was rotated weekly to the next trap position, in a clockwise direction, to account for possible positional effects. Traps were checked weekly and collected beetles were identified to species and sex (Lingafelter 2007). Statistical Analysis All data were analyzed using R (R Core Team 2018). All tests were conducted at significance levels of P = 0.05 and normality was tested using the Shapiro–Wilk test. One-way analysis of variance (ANOVA) was used to determine significant differences in oviposition pit numbers between lure treatments. Individual trees were used as the unit of replication and the number of oviposition pits was utilized as the response variable. Treatments with zero pits (control and CIS-OL) were removed and the response variable was log-transformed to comply with the assumptions of normality. Homogeneity of variance was assessed using the Bartlett’s test (k2 = 0.618, df = 2, P = 0.7341). Post hoc Tukey’s HSD test was used to isolate differences. Correlation between the number of oviposition pits and tree dbh, initial resin flow, and resin flow change, was tested using a generalized linear model with a Poisson distribution. Due to a lack of normality, differences in the spatial distribution of oviposition pits, as well as in number of each species and sex of beetles captured by each lure, were analyzed using the Kruskal–Wallis ANOVA by ranks (Kruskal and Wallis 1952). Individual trees and oviposition pits, and individual traps and beetles captured, were used as units of replication and the response variable, respectively. Post hoc multiple comparisons of differences were analyzed using the ‘PMCMR’ package in R (Pohlert 2014). Results Pretreatment Trapping Both M. titillator and M. carolinensis were initially captured on 12 May. Each sex of both species was captured in all plots from the initial capture date until experiments were initiated. The number of Monochamus spp. captured in each plot ranged from 4 to 11 beetles per week, with a mean of 7.25 ± 1.6 (SE) beetles. The most common Monochamus species and sex were M. titillator females (2.38 ± 0.63 beetles per plot per week) and the least common were M. carolinensis females (1.13 ± 0.48 beetles per plot per week). Associated insects commonly captured included: Cerambycidae: Rhagium inquisitor (Linnaeus); Trogossitidae: Temnoscheila virescens (Fabricius); Cleridae: Thanasimus dubius (Fabricius); Histeridae: Platysoma cylindricum (Paykull), as well as a variety of bark and ambrosia beetles. Of note was the low number of Ips species bark beetles captured in the plots. Tree Measurements Experimental tree dbhs ranged from 19.0 to 47.5 cm, with a mean of 29.0 ± 1.20 cm. Nematodes were not observed in samples from any trees, either before or after the experiment. Initial tree resin volumes ranged from 0 to 4.75 cm, with a mean of 1.68 ± 0.27 cm for trees used in experiment 1, and 1.39 ± 0.19 cm for trees used in experiment 2. At the end of the field season resin volume ranged from 0 to 3.65 cm with a mean of 1.68 ± 0.24 cm for experiment 1 and 1.09 ± 0.16 cm for experiment 2 trees. Average change in resin volume was 0.11 ± 0.36 cm. Numbers of oviposition pits were not significantly correlated with tree dbh (Z = −0.998, df = 32, P = 0.318), initial resin (Z = −0.117, df = 32, P = 0.907), or final resin measurements (Z = −0.316, df = 32, P = 0.752). Tree Inspection Throughout the experiment treated trees showed no visible symptoms of negative health that could be correlated with Monochamus spp. attack. Resin was seen pooling in wounds caused by ladder prongs and oviposition pits, but rarely running down the trunk. A number of trees experienced low levels of sapsucker (Picidae: Sphyrapicus spp.) feeding damage along the length of the main stem. In total, 556 pits were observed throughout the two experiments. The number of oviposition pits in treated trees ranged from 0 to 14 for experiment 1, and from 0 to 75 pits for experiment 2. There were significant differences in the number of oviposition pits among treatments (F = 32.94, df = 2, P < 0.001). Lure combinations that included ipsenol, the ipsenol and MEI treatments, had significantly more oviposition pits than those lacking ipsenol, the ME and control treatments (Fig. 1). Fig. 1. View largeDownload slide Mean number of Monochamus spp. oviposition pits on individual trees (n = 40) treated with attractive lures for 8 wk in 2014. Treatments included were ipsenol, monochamol + ethanol + ipsenol (MEI), monochamol + ethanol (ME), and cis-3-pinene-2-ol (CIS-OL). Results for ME treatments from experiment 1 and 2 are pooled (not significantly different), while all other results are only from experiment 2. Significant differences (P = 0.05; one-way ANOVA, Tukey’s HSD) are designated by different letters. Fig. 1. View largeDownload slide Mean number of Monochamus spp. oviposition pits on individual trees (n = 40) treated with attractive lures for 8 wk in 2014. Treatments included were ipsenol, monochamol + ethanol + ipsenol (MEI), monochamol + ethanol (ME), and cis-3-pinene-2-ol (CIS-OL). Results for ME treatments from experiment 1 and 2 are pooled (not significantly different), while all other results are only from experiment 2. Significant differences (P = 0.05; one-way ANOVA, Tukey’s HSD) are designated by different letters. Oviposition pits were distributed in a nonrandom pattern, with pits highly concentrated near the lure. More oviposition pits were located within 0.5 m of the lure bag (Kruskal–Wallis χ2 = 91.3548, df = 5, P < 0.001) than in other bole sections where pits were present. Furthermore, 90% of total pits were found on the same 180° arc of the trunk where lures were suspended (‘front’) and only 10% of total pits were found on the opposing trunk 180° arc (‘back’) (Fig. 2). Fig. 2. View largeDownload slide Distribution of Monochamus spp. oviposition pits along tree boles in relation to location of the attractive lure. ‘Front’ refers to the same 180° arc of the bole where the lure bag was located, whereas ‘back’ refers to the opposite 180° arc. Sections marked with an asterisk contained significantly more oviposition pits (P = 0.05; Kruskal–Wallis rank sum test) than all other sections, but were not significantly different from each other. Fig. 2. View largeDownload slide Distribution of Monochamus spp. oviposition pits along tree boles in relation to location of the attractive lure. ‘Front’ refers to the same 180° arc of the bole where the lure bag was located, whereas ‘back’ refers to the opposite 180° arc. Sections marked with an asterisk contained significantly more oviposition pits (P = 0.05; Kruskal–Wallis rank sum test) than all other sections, but were not significantly different from each other. Bolt Dissection From the 18 destructively sampled trees, 562 pits were identified and dissected. A total of 993 eggs were found beneath the bark of dissected pits, but only 62% of the oviposition pits contained any eggs. The range of eggs within pits was 1–11, with an average of 2.85 ± 0.16 eggs for those pits with eggs present. The majority of eggs were placed in a circular pattern surrounding the oviposition pit, while few pits had eggs laid side by side. Only 20 larvae were found in dissected pits, and of these, only 5 had created any galleries. Substantial amounts of resin, indicative of resinosis, were observed in 95.6% of the dissected pits. Trapping Study A total of 933 adult Monochamus beetles were caught in 2015 over 7 wk of trapping. Of 770 M. titillator captured, 423 were male and 347 were female, a male to female ratio of 1.22:1. Of 163 M. carolinensis captured 63 were male and 100 were female, for a male to female ratio of 0.63:1. Traps baited with MEI and MIA captured significantly more beetles than all other lures, ipsenol captured an intermediate number, and ME captured the least (Kruskal–Wallis χ2 = 18.7479, df = 3, P < 0.001; Fig. 3). Each species differed in their response to different lure combinations. For M. carolinensis, the MEI and MIA lure combinations were most attractive, although MIA was not significantly different from ipsenol, which itself was not significantly greater than ME (Kruskal–Wallis χ2 = 7.6531, df = 3, P = 0.053; Fig. 4). For M. titillator, MEI and MIA were the most attractive lure combinations, with both lures capturing significantly more beetles than both the ipsenol and ME lure; ipsenol was intermediate between the two most attractive lures and the ME lure (Kruskal–Wallis χ2 = 3.7883, df = 3, P = 0.2853; Fig. 5). No sex-specific differences in either species occurred in the response to the various lure combinations (Kruskal–Wallis χ2 = 0.175, df = 3, P = 0.9130). Fig. 3. View largeDownload slide Mean number of Monochamus spp. captured in cross-vane panel traps baited with four attractive lure combinations during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) are designated by different letters. Fig. 3. View largeDownload slide Mean number of Monochamus spp. captured in cross-vane panel traps baited with four attractive lure combinations during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) are designated by different letters. Fig. 4. View largeDownload slide Mean number of each sex of M. carolinensis captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. carolinensis captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 4. View largeDownload slide Mean number of each sex of M. carolinensis captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. carolinensis captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 5. View largeDownload slide Mean number of each sex of M. titillator captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. titillator captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Fig. 5. View largeDownload slide Mean number of each sex of M. titillator captured in cross-vane panel traps baited with attractive lures during 7 wk in 2015. Treatments included were monochamol + ethanol + ipsenol (MEI), monochamol + ipsenol + α-pinene (MIA), ipsenol, and monochamol + ethanol (ME). Significant differences (P = 0.05; Kruskal–Wallis rank sum test) in M. titillator captured by each lure combination are designated by different letters. Male and female means are shown, but were not significantly different. Discussion Our objective was to determine if southeastern Monochamus spp. could successfully colonize healthy shortleaf pines in the absence of external stressors such as lightning strikes, drought, or coincident bark beetle attack. Although we could successfully induce oviposition in healthy pines, only 2.5% of eggs hatched, and larval feeding and development was almost absent. The primary cause of Monochamus spp. egg death in our experiment was resinosis, a general conifer defense responsible for mortality of many woodboring insects (Langor and Raske 1988, Trapp and Croteau 2001, Akbulut et al. 2004). In healthy conifers, constitutive resin ducts, prevalent in Pinus species, and induced resin formation act as the primary defense against beetle colonization (Berryman 1972, Paine et al. 1997). Dodds and Stephen (2000) found that the egg stage of M. titillator was particularly vulnerable to mortality from resinosis. As wounds or stress limit nutrient flow, oleoresin and resin duct formation quickly decreases and trees become susceptible to woodborer colonization (Raffa and Berryman 1983, Paine et al. 1997). Monochamus spp. most likely colonize recently dead or moribund trees to avoid host defenses, such as resinosis, which are often overcome by mass attack of primary or secondary bark beetles (Hanks 1999, Stephen 2011). Our results do not support Gandhi’s (2005) findings that Monochamus spp. can successfully colonize residual trees in disturbed pine stands. Possible explanations for these differences may relate to both abiotic and biotic factors. Gandhi (2005) reported approximately 2.5 times more beetles present in wind-damaged plots than in fire-treated or control plots, while Monochamus spp. populations in Arkansas appeared somewhat lower. Oviposition in residual pines in Minnesota occurred in wind-damaged stands that contained more coarse woody debris than harvested stands in Arkansas, thereby increasing the unprotected material available for Monochamus spp. development. Additional stand stressors, such as increased exposure to wind or nutrient loss, are also known to diminish host resistance to woodboring species (Paine and Stephen 1987) and may have been more pervasive in Minnesota stands due to the extensive wind throw. Southeastern Monochamus species use a variety of volatile compounds to find susceptible hosts and mates. Sexually mature beetles are attracted to volatiles associated with declining pines, including α-pinene, ethanol, and turpentine (Fatzinger 1985, Fan et al. 2007, Allison et al. 2012). These compounds are abundant near wounded or stressed conifers, which may have diminished resinous defenses and are therefore more susceptible to Monochamus spp. colonization (Berryman 1972, Paine et al. 1997, Hanks 1999). In addition to host volatiles, Monochamus spp. also respond to the pheromones of sympatric organisms, which function as kairomones. Our tree and trap treatments that included the Ips bark beetle pheromone ipsenol, incurred higher oviposition and captured more beetles, respectively, than treatments without the kairomone. Our results complement other findings that many Monochamus species are attracted to Ips bark beetle pheromones, especially ipsenol (Billings and Cameron 1984, Allison et al. 2001, Groot and Nott 2004, Pajares et al. 2004, Miller and Asaro 2005, Ibeas et al. 2007). Contrary to Groot and Nott (2004) we found that both M. carolinensis and M. titillator are attracted to ipsenol in the absence of other attractants. Ipsenol is produced by Ips bark beetles which infest stressed or moribund hosts, thus sharing larval host resources with Monochamus species (Vite and Renwick 1971, Hanks 1999). Attraction to pheromones of feeding-guild members that share hosts effectively increases the number of beetles searching for susceptible hosts, which can be geographically dispersed and temporally ephemeral (Allison et al. 2001, Ibeas et al. 2007). Attraction to Ips pheromones may also serve to increase nutrient intake in a nitrogen-poor environment, as larval Monochamus spp. can act as facultative predators of bark beetles (Allison et al. 2001, Dodds et al. 2001). In addition to behavioral cues, conspecific pheromones are important in host finding for many woodboring insects. Monochamol, 2-undecyloxy-1-ethanol, is a commercially available pheromone produced by male Monochamus spp. that attracts mature beetles of both sexes (Pajares et al. 2010, Allison et al. 2012, Fierke et al. 2012). We used monochamol combined with the host volatile ethanol in our study, but observed the lowest response to this lure combination in both oviposition and trapping experiments. Low response may indicate that monochamol is only a weak attractant for these two Monochamus species, or that the pheromone is only detectable at short distances. Although cerambycid sex pheromones can operate at long distances, some compounds are only attractive at 5 cm or less (Wang et al. 1990, Wang et al. 1991, Lacey et al. 2004). Teale (2011) proposed a sequence for mating behavior in Monochamus spp., whereby mature adults are attracted to host volatiles at long distances, followed by attraction to the pheromone at short distances once on hosts. In our experiments, the increased number of pits and captured beetles in response to ipsenol may indicate that bark beetle pheromones also attract Monochamus spp. at long distances. Although solitary compounds (e.g., α-pinene) can attract Monochamus spp., our results support previous findings (e.g., Miller et al. 2011) that combinations of cues and pheromones are more attractive than isolated chemicals. In southeastern Monochamus spp. the combination of monochamol and α-pinene is more attractive than either compound by itself, although M. carolinensis is attracted to monochamol alone (Allison et al. 2012, Hanks et al. 2012). The addition of sympatric insect pheromones (e.g., ipsenol) to host volatiles drastically increases the attraction above that of host volatiles alone (Ibeas et al. 2007, Miller et al. 2011). We found that more beetles of each species and sex were collected in traps baited with combinations of monochamol, Ips pheromones, and host volatiles, suggesting synergy between the compounds (Miller et al. 2011, 2013). Although our lures used different host volatiles (ethanol, α-pinene), there was no significant difference in beetle capture between the two, perhaps alluding to a lack of differences in compound-specific response to general host stress volatiles. Acknowledgments We thank Jake Bodart, Dave Dalzatto, and Ryan Rastok for their help in conducting fieldwork. We also appreciate technical input and semiochemical lures provided by David Wakarchuk and Synergy Semiochemicals, Burnaby, BC, Canada. Keith Whalen and Mike Hennigan, USDA Forest Service, assisted in site selection on the Boston Mountain Ranger District of the Ozark National Forest. We also thank the USDA Forest Service for permission to conduct experiments in the Ozark National Forest. This research was supported in part by the University of Arkansas, Division of Agriculture. References Cited Akbulut , S. , W. T. Stamps , and M. J. Linit . 2004 . 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Environmental EntomologyOxford University Press

Published: Aug 1, 2018

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