TY - JOUR
AU - Doo-Hyung, Lee,
AB - Abstract Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) is a major agricultural pest on leguminous plants and tree fruit in South Korea and Japan. Only anecdotal information is currently available about its overwintering behavior and ecology. Therefore, we conducted laboratory experiments and field sampling to characterize overwintering structures and landscapes that R. pedestris use and prefer in South Korea. Under laboratory conditions, we identified the overwintering structure preference of R. pedestris adults and analyzed their spatial distributions. Among tested structures including pile of rocks, rotten wood, and leaf litter, R. pedestris was almost exclusively found in leaf litter. Spatial analysis using Spatial Analysis by Distance IndicEs (SADIE) indicated that most overwintering R. pedestris showed no spatial-aggregation behavior in the test arena. Field surveys were also conducted to characterize overwintering landscapes during the two winter seasons between 2014 and 2016. We selected two distinct landscapes: mountain areas and agricultural areas. Mountain areas were high-elevation mountains remote from agricultural practice, whereas agricultural areas were low-elevation forested landscapes adjacent to agricultural fields, including soybeans. From the 2-yr field survey, 92% (11 out of 12 individuals) of overwintering R. pedestris were found in the agricultural area without a significant aggregation pattern. crop pest, SADIE, winter ecology, winter behavior, overwintering As poikilothermic animals, the biology and ecology of insects are largely affected by temperature change, but they have limited ability to regulate body temperature, because they lack internal thermal homeostasis (Salt 1961; Danks 1991, 2006; Bale and Hayward 2010). Winter is a particularly harsh season for insects, during which many may die because of thermally stressful environments (Leather et al. 1993). Despite such an innate disadvantage, insects have successfully thrived from the equator to the pole, displaying a suite of physiological and behavioral strategies to mitigate the adverse effects of thermal stress (Sinclair et al. 2003, Bowler and Terblanche 2008, Clark and Worland 2008). Although it is obvious that overwintering is a critical life stage that affects the population dynamics of insects, surprisingly few studies have investigated this ‘quiescent’ life stage compared to ‘active’ stages (Leather et al. 1993). Especially, very few studies have focused on the ecological information about overwintering insects, such as their behavior and overwintering sites. However, understanding the overwintering ecology of pest populations is particularly important, because overwintered populations can provide baseline information as a barometer of pest pressure in the following growing season. A bean bug, Riptortus pedestris (Fabricius) (Hemiptera: Alydidae), is a polyphagous insect in East Asia, including South Korea, Japan, and Taiwan (Kono 1989, Kikuhara 2005, Lim 2013). R. pedestris was regarded as an occasional pest until the mid-1990s, because the volume of economic loss by this species was less severe than that for other crop pests in South Korea (Lee et al. 2004, Bae et al. 2008). However, the population size of R. pedestris has sharply increased, causing serious economic losses since the 2000s and is ranked as the most serious crop pest among the 20 hemipteran species recorded in South Korea (Annual Research Report [ARR] 2002, Kang et al. 2003, Bae et al. 2004). This insect attacks various leguminous plants, such as soybeans, tree fruit and medicinal plants, by piercing and feeding on fruit and other plant parts (Kono 1989, Son et al. 2000, Kang et al. 2003, Lee et al. 2004). R. pedestris enters a facultative diapause in adults that responds to a short-day photoperiod under 13.5 h (Numata and Hidaka 1982, 1983, 1984). Furthermore, physiological studies related to the reproductive diapause of R. pedestris were also conducted and addressed pheromone response, body coloration, and winter acclimation (Kobayashi and Numata 1995; Mizutani et al. 2008a,b; Rozsypal et al. 2017). Although physiological aspects of its diapause have been relatively well understood, the overwintering behavior and ecology of R. pedestris have been barely described in scientific literature, but it has been suggested that leaf litter might serve as overwintering structures (Ishikura et al. 1955, Tabuchi et al. 2004, Moriya 2005). Moriya (2005) reexamined leaf litter samples collected from a coppice forest floor over 14 yr to locate an overwintering stink bug species and found only one male R. pedestris from these samples. Historically, several attempts have been made to describe overwintering sites of R. pedestris, especially in Japan (Ishikura et al. 1955, Tabuchi et al. 2004, Moriya 2005). However, current knowledge on the overwintering structure and sites of R. pedestris remains unclear, due to limited supporting empirical data. Therefore, in this study, we investigated the overwintering behavior of R. pedestris under laboratory conditions and characterized overwintering sites under field conditions. In the laboratory, we first identified the potential structure that R. pedestris might use as overwintering microhabitats. Then, we assessed the spatial distribution patterns of R. pedestris in the selected potential overwintering structures under laboratory conditions. Last, we conducted a 2-yr field survey to validate the results obtained from laboratory experiments and characterized the overwintering sites of R. pedestris in South Korea. Materials and Methods Insects R. pedestris adults in reproductive diapause were collected every week by using synthetic aggregation pheromone traps (Green-Agrotech Co., Ltd., Gyeongsangbuk-do, South Korea) in wooded areas of Gachon University, Seongnam-si, Gyeonggi-do, South Korea (37° 27′ 2.44″ N, 127° 7′ 50.74″ E) from October 2015 to November 2015. The reproductive diapause of R. pedestris is known to initiate under a 13.5-h photoperiod, which occurs in mid-September in South Korea (Numata and Hidaka 1982). Immediately after collection, adults were brought to the laboratory and kept in fine mesh cages (BugDorm-4, MegaView Science Education Services Co., Ltd., Taiwan). Mesh cages were placed in a growth chamber (SJP-250MRI, Sejong Plus Co., Ltd., Gyeonggi-do, South Korea) with a 12:12 (L:D) h light at 15 ± 0.5°C and 30 ± 0.5% RH to maintain the reproductive diapause of R. pedestris (Numata and Hidaka 1983, 1984). As food and water, dried soybean grains (Glycine max) and distilled water with 0.05% ascorbic acid were provided. R. pedestris adults were randomly selected from the colony and sub-sampled to confirm their reproductive diapause status according to the descriptions of ovaries in Numata and Hidaka (1982) before tests. Preference of R. pedestris for Natural Materials as Overwintering Structure To identify the materials that R. pedestris use as overwintering structures, adults were exposed to three natural materials commonly found in natural wooded areas: pile of rocks (ca. 5–10 cm [diameter]), Fagaceae sp. deciduous leaf litter (average leaf size was 5 ± 2.4 × 14 ± 3.1 cm [width × length]), and rotten wood (ca. 8 × 10 cm [diameter × length]) with porous crevices where the adults can crawl in. For the test, acrylic boxes (20 × 20 × 15 cm [length × width × height]) were filled with soil up to 5 cm as a foundation for each structure. These three designated materials were then placed on the soil (Fig. 1). An adult R. pedestris was introduced into each acrylic box, which was then covered by fine mesh. Experimental arenas were randomly arranged and placed inside a low-temperature incubator (SJP-250MRI, Sejong Plus Co., Ltd., Gyeonggi-do, South Korea) at 11°C for 3 h with light. The temperature was selected based on average temperature in September of a 30-yr period in South Korea (Korea Meteorological Administration [KMA] 2011). After 3 h, the location of R. pedestris was carefully checked to evaluate whether the individual hid in the structure. Hiding behavior was judged to be a sign of seeking a place for overwintering by R. pedestris. This experiment was replicated 30 times (sex ratio = 1:1). Fig. 1. View largeDownload slide Experimental arenas to test natural materials preferred by Riptortus pedestris as overwintering structure. Acrylic boxes were filled with soil up to 5 cm as a foundation for the test material. Fig. 1. View largeDownload slide Experimental arenas to test natural materials preferred by Riptortus pedestris as overwintering structure. Acrylic boxes were filled with soil up to 5 cm as a foundation for the test material. Spatial Distribution of Overwintering R. pedestris Under Laboratory Condition Spatial distribution of overwintering R. pedestris adults was investigated in a 120 × 100 × 25 cm (L × W × H) arena filled with 15-cm-thick Fagaceae sp. deciduous leaf litter. The litter was collected from wooded areas of Gachon University, Seongnam-si, Gyeonggi-do, South Korea (37° 27′ 2.44″ N, 127° 7′ 50.74″ E). The experimental arena was placed in a room at 11 to 15°C and 30 to 40% RH. The arena was divided into 6 × 5 rectangular grids. At the beginning of the experiment, an individual R. pedestris adult was placed in each grid. The R. pedestris population used in this experiment was confirmed in reproductive diapause as described above. A total of 30 individuals (sex ratio = 1:1) were introduced into the arena, and the locations of males and females were randomized. Then, the experimental arena was covered by mesh to prevent the insects from escaping. After 24 h, individuals above the leaf litter and on the mesh cover or walls were counted and removed, because they were not trying to hide in the potential overwintering structure. Then, the leaf litter was carefully removed from the top to the bottom to locate R. pedestris hidden in the structure. The locations and sex of R. pedestris were individually marked. This experiment was replicated 10 times. Spatial distributions of R. pedestris found in the leaf litter were analyzed using spatial analysis by distance indices (SADIE). SADIE measures the degree of spatial aggregation by calculating the minimum distance to regularity (Perry et al. 1999). To find out whether R. pedestris aggregated among the 30 grids, two SAIDE indices (index of aggregation [Ia] and P value of Ia [Pa]) were calculated to address spatial distribution of overwintering R. pedestris. Furthermore, correlation coefficients between the clustering indices of male and female R. pedestris were measured to determine spatial associations between the two sexes using SADIE (Perry and Dixon 2002). Overall association (X) was achieved by calculating the mean of the local correlation coefficient between the clustering indices of the males and females; it was used to evaluate the spatial association of males and females, with its associated possibility (p) being calculated based on randomized tests. We calculated the level of aggregation by individual overwintering R. pedestris as well as the association between males and females using SADIE Shell, provided by Dr. Joe Perry of Rothamsted Research, with 153 permutations. Field Survey to Locate Overwintering Sites of R. pedestris Based on the results of the laboratory experiments, a field survey was conducted during two winter seasons to locate and characterize overwintering sites of R. pedestris adults in South Korea. For the survey, sampling sites were established in all four different climate zones in South Korea (Fig. 2). In each climate zone, a high-elevation mountain landscape, ranging from 247 to 1,260 m in elevation, was selected for sampling (referred to as ‘mountain area’ hereafter). In climate zone II, an additional mountain was selected to include the middle area of South Korea (Fig. 2). Within 4 to 45 km from the mountains, low-elevation wooded landscapes, ranging from 39 to 498 m in elevation, that were adjacent to soybean fields were also selected for the survey (referred to ‘agricultural area’ hereafter). Soybean fields are commonly selected as agricultural areas, because soybean is a major cultivated host plant of R. pedestris in South Korea (Kang et al. 2003, Lim 2013). However, not all agricultural areas were assigned to all mountain areas, because of regional differences in South Korea. Especially, soybeans are not major crops in the areas near Mt. Chiak and Mt. Songni; therefore, we surveyed additional agricultural areas near the bean fields in the Gyeonggi area instead (Fig. 2). The survey was conducted during two winter seasons from December 2014 to February 2016. In the five mountain areas chosen, sampling sites were established every 100 m in elevation. In the six agricultural areas chosen, sampling sites were established at 20 ± 5 m apart from each other along with similar elevations. At each sampling site, a 1 × 1 m quadrant was drawn and investigated. In the quadrant, the depth of leaf litter was measured at the four corners and the center. Then all the materials above the soil base layer were carefully collected. Two samples were collected at every sampling site. A total of 130 leaf litter samples were collected in each winter season; 66 and 64 samples from the mountain area and the agricultural area, respectively. The collected samples were brought to the laboratory and carefully examined to find overwintering R. pedestris. Finally, sampled leaf litter was measured for fresh and dry weights and categorized for dominant leaf types as coniferous leaves or broad leaves. Fig. 2. View largeDownload slide Locations of sampling sites and results of field surveys to locate overwintering Riptortus pedestris during two winter seasons. The pie chart shows proportions of samples from which insects were found. The climate zones were established by KMA (Korea Meteorological Administration 2011) based on average temperatures in January in South Korea. Fig. 2. View largeDownload slide Locations of sampling sites and results of field surveys to locate overwintering Riptortus pedestris during two winter seasons. The pie chart shows proportions of samples from which insects were found. The climate zones were established by KMA (Korea Meteorological Administration 2011) based on average temperatures in January in South Korea. Results Preference of R. pedestris for Natural Materials as Overwintering Structure Behavioral response of adult R. pedestris to three natural materials was evaluated under a no-choice condition to find out if the insect showed hiding behavior when seeking a potential overwintering structure (Fig. 1). Among the three materials tested, leaf litter was the most preferred material by R. pedestris as a potential overwintering structure (Fisher’s exact test: P < 0.001). In the leaf litter arena, 19 out of 30 individuals were found hiding in the leaf litter, whereas the other 11 individuals of R. pedestris were located over the litter or on the walls. Only one individual was found hiding in the pile of rocks; no individuals were found under the rotten wood or in crevices of the wood. In the leaf litter arena, there was no significant difference between males and females in the number of hiding individuals (Fisher’s exact test: P = 0.77). Spatial Distribution of Overwintering R. pedestris Under Laboratory Condition Since R. pedestris showed overwintering behavior in an indoor arena filled with leaf litter, the spatial distribution of R. pedestris was then investigated using SADIE analysis. In the experiment, 62.9 ± 4.6% (mean ± SE) of adult R. pedestris showed overwintering behaviors by crawling and hiding inside the leaf litter. First, the overall spatial distribution of overwintering R. pedestris (both sexes) was random in the experimental arena. The SADIE index of aggregation was not significantly different from 1 in nine out of 10 replications (P > 0.10), indicating the individuals were randomly distributed in most cases (Table 1). Only one replication yielded an aggregated distribution of overwintering individuals (P = 0.04). Furthermore, no significant spatial association was detected between male and female overwintering R. pedestris across all replications (P > 0.15) (Table 1). Table 1. Results of SADIE analysis for spatial distributions of Riptortus pedestris (both sexes) found hiding in leaf litter in the experimental arena (3 × 3 m) and spatial associations between males and females in the arena Replication Number of detected R. pedestris Index of aggregation (Ia)a P-value of Ia (Pa) Index of spatial association between two sexes (X)b P-value of X (PX) 1 12 0.97 0.49 0.13 0.30 2 21 0.79 0.98 0.21 0.18 3 20 0.82 0.92 −0.11 0.60 4 14 1.17 0.12 −0.22 0.72 5 26 1.17 0.13 −0.25 0.88 6 14 1.30 0.04 0.07 0.44 7 18 0.93 0.60 −0.20 0.76 8 20 1.00 0.41 −0.05 0.44 9 23 0.85 0.84 −0.12 0.63 10 20 0.90 0.68 0.09 0.40 Replication Number of detected R. pedestris Index of aggregation (Ia)a P-value of Ia (Pa) Index of spatial association between two sexes (X)b P-value of X (PX) 1 12 0.97 0.49 0.13 0.30 2 21 0.79 0.98 0.21 0.18 3 20 0.82 0.92 −0.11 0.60 4 14 1.17 0.12 −0.22 0.72 5 26 1.17 0.13 −0.25 0.88 6 14 1.30 0.04 0.07 0.44 7 18 0.93 0.60 −0.20 0.76 8 20 1.00 0.41 −0.05 0.44 9 23 0.85 0.84 −0.12 0.63 10 20 0.90 0.68 0.09 0.40 aIa = 1, random distribution; Ia > 1, aggregated distribution; Ia < 1, regular distribution. bPX = 1, no spatial association; PX > 1, positive spatial association; and PX < 1, negative spatial association. View Large Table 1. Results of SADIE analysis for spatial distributions of Riptortus pedestris (both sexes) found hiding in leaf litter in the experimental arena (3 × 3 m) and spatial associations between males and females in the arena Replication Number of detected R. pedestris Index of aggregation (Ia)a P-value of Ia (Pa) Index of spatial association between two sexes (X)b P-value of X (PX) 1 12 0.97 0.49 0.13 0.30 2 21 0.79 0.98 0.21 0.18 3 20 0.82 0.92 −0.11 0.60 4 14 1.17 0.12 −0.22 0.72 5 26 1.17 0.13 −0.25 0.88 6 14 1.30 0.04 0.07 0.44 7 18 0.93 0.60 −0.20 0.76 8 20 1.00 0.41 −0.05 0.44 9 23 0.85 0.84 −0.12 0.63 10 20 0.90 0.68 0.09 0.40 Replication Number of detected R. pedestris Index of aggregation (Ia)a P-value of Ia (Pa) Index of spatial association between two sexes (X)b P-value of X (PX) 1 12 0.97 0.49 0.13 0.30 2 21 0.79 0.98 0.21 0.18 3 20 0.82 0.92 −0.11 0.60 4 14 1.17 0.12 −0.22 0.72 5 26 1.17 0.13 −0.25 0.88 6 14 1.30 0.04 0.07 0.44 7 18 0.93 0.60 −0.20 0.76 8 20 1.00 0.41 −0.05 0.44 9 23 0.85 0.84 −0.12 0.63 10 20 0.90 0.68 0.09 0.40 aIa = 1, random distribution; Ia > 1, aggregated distribution; Ia < 1, regular distribution. bPX = 1, no spatial association; PX > 1, positive spatial association; and PX < 1, negative spatial association. View Large Field Survey to Locate Overwintering Sites of R. pedestris In each winter season, a total of 130 leaf litter samples were collected; 64 and 66 samples from the agricultural area and the mountain area, respectively. Locations of sampling sites and results of field survey are summarized in Fig. 2. From the 2-yr field survey, a total of 12 overwintering R. pedestris were found from 10 samples (Supp. Table S1 and S2). There was a significant difference in the proportion of samples containing overwintering R. pedestris between the agricultural area and the mountain area (Fisher’s exact test: P = 0.0094); 11 individuals were found from the agricultural areas, but only one individual was located from a mountain area. From the samples, a total of 23 other overwintering hemipteran insects, including family Acanthosomatidae (Hemiptera: Pentatomidae) and Dolycoris baccarum (L.) (Hemiptera: Pentatomidae), were found during the two winter seasons (Supp. Table S1 and S2). In contrast to R. pedestris, no significant difference was found in the proportion of samples harboring other hemipterans between the two sampling areas (Fisher’s exact test: P = 0.31). Sample characteristics of areas where overwintering R. pedestris were found are listed in Table 2. Overall, R. pedestris were found overwintering solitarily at the scale of our study; eight out of 10 samples contained a single overwintering individual in a 1 × 1 m grid (Table 2). In terms of the abiotic conditions of samples such as altitude of sample collected, depth of leaf litter, and moisture content of sample, there was no significant difference between samples from which R. pedestris were found and those from which R. pedestris were not found under any conditions (Fig. 3). Table 2. Sample characteristics of location where overwintering Riptortus pedestris were found during two winter seasons Year Landscapes Sitea Geographic coordinates Altitude (m) Depth (cm) Moisture contentsb (%) Detected R. pedestris 2015 Agricultural area GG 37°27′01.24″N, 127°07′ ′50.69″E 89 16.6 37.62 1 Agricultural area GG 37°27′19.70″N, 127°08′11.26″E 111 7.4 33.39 1 Agricultural area NG 37°28′43.21″N, 127°10′37.30″E 465 6.8 26.63 1 Agricultural area NG 37°28′42.78″N, 127°10′40.03″E 452 6.3 33.12 1 Agricultural area NG 37°28′36.35″N, 127°11′57.18″E 420 7.0 46.46 1 Agricultural area NG 37°28′23.32″N, 127°10′33.99″E 331 10.6 43.12 1 2016 Mountain area YP 37°54′00.31″N, 127°24′29.23″E 785 5.4 19.20 1 Agricultural area NG 37°28′39.40″N, 127°11′56.30″E 436 6.6 15.56 1 Agricultural area NG 37°28′34.30″N, 127°11′43.20″E 359 11.0 25.63 2 Agricultural area NG 37°28′23.18″N, 127°10′34.35″E 324 15.0 12.16 2 Year Landscapes Sitea Geographic coordinates Altitude (m) Depth (cm) Moisture contentsb (%) Detected R. pedestris 2015 Agricultural area GG 37°27′01.24″N, 127°07′ ′50.69″E 89 16.6 37.62 1 Agricultural area GG 37°27′19.70″N, 127°08′11.26″E 111 7.4 33.39 1 Agricultural area NG 37°28′43.21″N, 127°10′37.30″E 465 6.8 26.63 1 Agricultural area NG 37°28′42.78″N, 127°10′40.03″E 452 6.3 33.12 1 Agricultural area NG 37°28′36.35″N, 127°11′57.18″E 420 7.0 46.46 1 Agricultural area NG 37°28′23.32″N, 127°10′33.99″E 331 10.6 43.12 1 2016 Mountain area YP 37°54′00.31″N, 127°24′29.23″E 785 5.4 19.20 1 Agricultural area NG 37°28′39.40″N, 127°11′56.30″E 436 6.6 15.56 1 Agricultural area NG 37°28′34.30″N, 127°11′43.20″E 359 11.0 25.63 2 Agricultural area NG 37°28′23.18″N, 127°10′34.35″E 324 15.0 12.16 2 aGG refers to Gachon University, Gyeonggi-do; NG refers to Namhansanseong, Gyeonggi-do; YP refers to Yeonin Province Park. See Fig. 2 for geographical locations. b10% of samples in weight were subsampled and dried for 48 h at 80°C to measure moisture contents of samples. View Large Table 2. Sample characteristics of location where overwintering Riptortus pedestris were found during two winter seasons Year Landscapes Sitea Geographic coordinates Altitude (m) Depth (cm) Moisture contentsb (%) Detected R. pedestris 2015 Agricultural area GG 37°27′01.24″N, 127°07′ ′50.69″E 89 16.6 37.62 1 Agricultural area GG 37°27′19.70″N, 127°08′11.26″E 111 7.4 33.39 1 Agricultural area NG 37°28′43.21″N, 127°10′37.30″E 465 6.8 26.63 1 Agricultural area NG 37°28′42.78″N, 127°10′40.03″E 452 6.3 33.12 1 Agricultural area NG 37°28′36.35″N, 127°11′57.18″E 420 7.0 46.46 1 Agricultural area NG 37°28′23.32″N, 127°10′33.99″E 331 10.6 43.12 1 2016 Mountain area YP 37°54′00.31″N, 127°24′29.23″E 785 5.4 19.20 1 Agricultural area NG 37°28′39.40″N, 127°11′56.30″E 436 6.6 15.56 1 Agricultural area NG 37°28′34.30″N, 127°11′43.20″E 359 11.0 25.63 2 Agricultural area NG 37°28′23.18″N, 127°10′34.35″E 324 15.0 12.16 2 Year Landscapes Sitea Geographic coordinates Altitude (m) Depth (cm) Moisture contentsb (%) Detected R. pedestris 2015 Agricultural area GG 37°27′01.24″N, 127°07′ ′50.69″E 89 16.6 37.62 1 Agricultural area GG 37°27′19.70″N, 127°08′11.26″E 111 7.4 33.39 1 Agricultural area NG 37°28′43.21″N, 127°10′37.30″E 465 6.8 26.63 1 Agricultural area NG 37°28′42.78″N, 127°10′40.03″E 452 6.3 33.12 1 Agricultural area NG 37°28′36.35″N, 127°11′57.18″E 420 7.0 46.46 1 Agricultural area NG 37°28′23.32″N, 127°10′33.99″E 331 10.6 43.12 1 2016 Mountain area YP 37°54′00.31″N, 127°24′29.23″E 785 5.4 19.20 1 Agricultural area NG 37°28′39.40″N, 127°11′56.30″E 436 6.6 15.56 1 Agricultural area NG 37°28′34.30″N, 127°11′43.20″E 359 11.0 25.63 2 Agricultural area NG 37°28′23.18″N, 127°10′34.35″E 324 15.0 12.16 2 aGG refers to Gachon University, Gyeonggi-do; NG refers to Namhansanseong, Gyeonggi-do; YP refers to Yeonin Province Park. See Fig. 2 for geographical locations. b10% of samples in weight were subsampled and dried for 48 h at 80°C to measure moisture contents of samples. View Large Fig. 3. View largeDownload slide Abiotic conditions of samples with or without overwintering Riptortus pedestris found. (A) Altitude of collected samples, (B) depth of samples above the soil base layer, and (C) moisture contents of the sample. Fig. 3. View largeDownload slide Abiotic conditions of samples with or without overwintering Riptortus pedestris found. (A) Altitude of collected samples, (B) depth of samples above the soil base layer, and (C) moisture contents of the sample. Discussion In general, R. pedestris adults showed overwintering behavior almost exclusively in leaf litter, which is commonly used as overwintering structure by several hemipteran species. In particular, the southern green stink bug, Nezara viridula (L.) (Hemiptera: Pentatomidae), is well known to overwinter under litter or other materials that may offer protection from low temperatures (Todd 1989, Danks 2006, Musolin 2012). Moreover, from our 2-yr survey, we found eight species belonging to seven genera of Hemipteran species overwintering in leaf litter, including Acanthosoma denticauda (Jakovlev) (Hemiptera: Acanthosomatidae), D. baccarum (Hemiptera: Pentatomidae), and Acanthosoma forficula (Jakovlev) (Hemiptera: Acanthosomatidae). Leaf litter can serve as a protective blanket and insulator that can reduce temperature variation (Mackinny 1929, Shorthouse et al. 1980). Thus, many poikilothermic animals, including insects, use leaf litter as overwintering structures (Leather et al. 1993). However, other structures, such as rotten wood and crevices in rocks, are also used by insects for overwintering. In particular, the brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), was found overwintering in the porous crevices of rotten wood (Lee et al. 2013, 2014). Interestingly, Lee et al. (2014) reported that H. halys was not found in leaf litter from a 2-yr field survey. In addition, many species in the family Coccinellidae overwinter not only in leaf litter, but also in crevices of rocks with highly aggregative behavior (Lee 1980, Obata 1986, Sakurai et al. 1993). Therefore, we cannot exclude the possibility that the insects we found may use other structures as their overwintering site. After we identified the overwintering structure of R. pedestris adults, we conducted a second laboratory experiment to analyze the spatial distributions of overwintering adults of R. pedestris. The results of the SADIE analysis indicated that R. pedestris did not form significant aggregations in their overwintering structures when they were released individually with even spacing in the experimental arena. However, it would also be worthwhile, in further study, to see if R. pedestris would maintain aggregation or disperse when released in a group. In addition, there was no detectable spatial association between overwintering males and females in the laboratory conditions. It is known that insects use various biotic and abiotic cues to select overwintering structures to increase their winter survival rate (Lee 1980; Copp 1983; Bennett and Lee 1989; Leather et al. 1993; Nalepa et al. 1996; Nalepa et al. 2005; Toyama et al. 2006, 2011; Park and Lee 2017). For instance, H. halys show high levels of aggregation behavior in their overwintering structures. It has been demonstrated that this aggregation behavior of H. halys was not affected by an aggregation pheromone, but might affected by physical cues that consequently induce negative phototaxis and thigmotaxis (Toyama et al. 2006, 2011). Moreover, some species in the family Coccinellidae can form initial overwintering aggregations using visual cues and hypsotaxis, whereas overwintering elongation is known to be maintained by releasing attractant pheromones (Lee 1980, Copp 1983, Leather et al. 1993, Nalepa et al. 1996, Nalepa et al. 2005, Wheeler and Cardé 2014). However, factors affecting overwintering site selection by R. pedestris are rarely understood, including the role of aggregation pheromones. Although adult males stop producing aggregation pheromones when they go into reproductive diapause, they still respond to aggregation pheromones after they are in reproductive diapause (Mizutani et al. 2008a,b; Rahman and Lim 2016). Therefore, we cannot rule out potential effects of aggregation pheromones on overwintering site selection by R. pedestris. Further study is needed to address what factors determine the overwintering behaviors of R. pedestris and their spatial distributions observed in this study. Our field survey over two winter seasons revealed that leaf litter was the main overwintering structure used by R. pedestris. In addition, R. pedestris showed no significant aggregation behavior at the scale of the samples used in this study. In the survey, at most two overwintering individuals were found from the same sample. Such scattered distribution may complicate applications of effective monitoring and management tactics against R. pedestris, because it is difficult to identify pest hotspots that could serve as a source of pest populations invading agricultural fields in the spring (Kim et al. 2014, Tabuchi et al. 2014, Park et al. 2016). At the landscape level, almost all overwintering R. pedestris were detected in agricultural areas adjacent to soybean fields. This result indicates that R. pedestris adults tend to make a short dispersal from crop fields to surrounding wooded areas in order to select overwintering sites. Therefore, it might be possible to effectively intercept R. pedestris that seek overwintering sites in proximity to agricultural fields in the fall by establishing pheromone traps at the borders of fields or in neighboring woods. Although 11 out of 12 overwintering individuals were found in agricultural areas with altitudes lower than 470 m, we could not rule out that R. pedestris could overwinter at higher elevations. Indeed, one individual was found overwintering at 785 m in the mountain sampling site. In this study, we examined what abiotic conditions would affect the selection of overwintering sites by R. pedestris in agricultural areas, where most overwintering individuals were found in this study. Although statistical analysis indicated that none of the abiotic factors (altitude, depth, and moisture content) significantly affected the choice of overwintering sites, our data suggest some general patterns in the preference for overwintering sites by R. pedestris. First, it seems that R. pedestris can overwinter in a wide range of altitudes within agricultural landscapes: overwintering individuals were found in areas with altitudes ranging from 89 to 465 m. Second, there might be a minimum threshold in the depth of leaf litter to serve as a proper insulator against harsh winter temperatures: no overwintering individual was found in leaf litter when its depth was less than 5 cm. Finally, our data suggest that R. pedestris may avoid high levels of moisture in the leaf litter as an overwintering site. In our study, no overwintering individual was found when the moisture content of leaf litter was higher than 46%. To confirm the potential effect of abiotic conditions on the selection of overwintering microhabitats by R. pedestris, controlled laboratory or semi-field experiments are warranted in future studies. In summary, the results of this study revealed that adult R. pedestris use leaf litter as their overwintering structure, and they are randomly distributed in their overwintering structures without forming significant aggregations. Furthermore, a 2-yr field survey indicated that R. pedestris tend to make short dispersals from crop fields to surrounding wooded areas in order to select overwintering sites. 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TI - Characterization of Overwintering Behaviors and Sites of Bean Bug, Riptortus pedestris (Hemiptera: Alydidae), Under Laboratory and Field Conditions
JO - Environmental Entomology
DO - 10.1093/ee/nvy123
DA - 2018-10-03
UR - https://www.deepdyve.com/lp/oxford-university-press/characterization-of-overwintering-behaviors-and-sites-of-bean-bug-pOu50KFKKK
SP - 1280
VL - 47
IS - 5
DP - DeepDyve
ER -