TY - JOUR
AU1 - Carnohan,, Lucas
AU2 - Su,, Nan-Yao
AB - Abstract Workers of three termite species, Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae), Coptotermes gestroi (Wasmann) (Blattodea: Rhinotermitidae), and Reticulitermes flavipes (Kollar) (Blattodea: Rhinotermitidae), were force-fed with cellulose media pads treated with 20-hydroxyecdysone (20E) alone and in combination with noviflumuron for 3 d to examine their mortality time trends up to 14 d (including 3-d exposure time). Termites exhibited symptoms of hyperecdysonism before the onset of death. The times required for 20E to fully express its effects were 10–13 d for C. formosanus, 13–14 d for C. gestroi, and 11–13 d for R. flavipes. Higher 20E concentrations resulted in higher mortalities at 14 d, and the addition of noviflumuron generally yielded higher mortalities. The lethal time of 20E was similar to chitin synthesis inhibitors such as noviflumuron than metabolic inhibitors, and the 10–14 d lethal time may be sufficient for 20E-affected termites to return to the central nest before the onset of hyperecdysonism and ultimate death. As an active ingredient in baiting systems, 20E has shown potential to reduce the colony elimination time. termite bait, hyperecdysonism, Coptotermes formosanus, Coptotermes gestroi, Reticulitermes flavipes ‘One of the’ contributing factors to the success of chitin synthesis inhibitors (CSIs) in colony elimination of subterranean termites is that their lethal times are dose-independent (Rust and Su 2012). When CSIs are used in bait stations around a structure, individual termites may consume various quantities of a CSI, but those that consume larger quantities of the CSI are affected in the same way as termites consuming lesser amounts, i.e., they behave normally until attempting to molt, which may take 45 d or more (Kakkar et al. 2016a). A recent study showed that termites must return to the central nest to molt (Kakkar et al. 2017), and CSI-affected termites die near the eggs, queen and king (Kakkar et al. 2018). To avoid the decomposing corpses, the royal pair moved to other locations only to be surrounded by dead termites days later, and this cycle repeated until the colony collapsed. The dose-independent lethal time of CSI baits, coupled with termite molting behavior account for the capability of CSI baits to eliminate termite colonies. Other compounds were previously thought to have similar colony elimination potential because of their slow mode of action and non-repellency (Su et al. 1982), but ultimately failed to produce results because their lethal times were dose-dependent. For example, sulfluramid is a slow-acting metabolic inhibitor that is used in baiting systems such as FirstLine (FMC Corp., Princeton, NJ) (Glenn and Gold 2002). In Texas, the sulfluramid systems were shown to result in the persistence of the Formosan subterranean termite, Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae) around structures, while the noviflumuron-containing Sentricon (Dow AgroSciences, Indianapolis, IN) system resulted in the elimination of colonies (Glenn and Gold 2002). For houses treated with FirstLine, bait stations were abandoned by the colonies, suggesting that the sulfluramid was deterrent to the termites (Glenn and Gold 2002). Delayed termite deterrence to sulfluramid after an initial period of uninhibited feeding has been observed in field trials (Su et al. 1995). It has been suggested that this delayed deterrence response is a result of the dose-dependent nature of sulfluramid. Specifically, those termites that acquire sub-lethal doses of the compound are able to employ associative learning to avoid future contact with the toxicant (Su et al. 1995). It is therefore important that a compound be slow-acting and dose-independent to be useful in achieving colony elimination. 20-hydroxyecdysone (20E) is an insect molting hormone that can cause mortality by inducing hyperecdysonism when ingested by insects. Because 20E induces termite molting and mortality that may lead to avoidance of corpses by royal pairs as observed in colonies baited with CSIs (Kakkar et al. 2018), and because 20E-induced mortality occurred in 12 d, Su and Monteagudo (2017) suggested that 20E baits may reduce colony elimination time due to its shorter lethal time (12 d) than a CSI (45 d). However, it is unknown if the lethal time of 20E is dose-independent. One objective of this study was to examine time trends in mortality of subterranean termites after being exposed to concentrations of 20E alone and in combination with noviflumuron. We also observed molting activity in termites that ingested 20E. Materials and Methods Termites C. formosanus and the eastern subterranean termite, Reticulitermes flavipes (Kollar) (Blattodea: Rhinotermitidae) were collected from field colonies in south Florida using a modified method described by Tamashiro et al. (1973) and Su and Scheffrahn (1986). Samples of the Asian subterranean termite, Coptotermes gestroi (Wasmann) (Blattodea: Rhinotermitidae) were collected from incipient colonies. C. gestroi alates used for the formation of incipient colonies were collected during the swarming season between March and April in South Florida (Scheffrahn 2013). Alates were collected using a light trap and transported to the laboratory for processing. Alates were sexed, paired and placed into plastic snap-cap vials (37 ml; IntraPac, Plattsburgh, NY). Each vial contained ≈10 ml of organic garden soil (Miracle-Gro Nature’s Care, The Scotts Co., Marysville, OH), 15 ml of yellow pine sticks, with the remaining space filled in with 0.3% agar. The soil and wood provided nutrition for the young colonies, and the agar maintained moist conditions. Termite colonies were transferred to larger containers as they grew and required more space. The 3-yr-old C. gestroi colonies used in this study were contained in plastic boxes (30.5 × 45.7 × 15.2 cm) and consisted of 30,000–50,000 termites. No-Choice Bioassay Cellulose media pads (45 mm diameter) (Millipore Corp., Billerica, MA) were treated with 20E, noviflumuron, or both. For C. formosanus and C. gestroi, 500, 1,000 and 2,000 ppm were used for 20E alone treatments, and 2,500 ppm noviflumuron was added to these three concentrations to yield 20E plus noviflumuron treatments. For R. flavipes, 500 and 1,000 ppm were used for 20E alone treatments, and 2,500 ppm noviflumuron was added to these two concentrations to yield the combined treatments. For noviflumuron alone treatment, 2,500 ppm was used. Solutions were prepared by dissolving technical grade 20E (97%, AdipoGen Int., San Diego, CA), noviflumuron (98%, Dow Agrosciences) or both into methanol. Control pads were treated with methanol alone. The methanol was allowed to evaporate from the pads, and then they were placed into Petri dishes (60 mm diameter and 15 mm high). All media pads were also impregnated with 0.025% (wt/wt) Nile Blue A dye dissolved in deionized (DI) water to confirm termite feeding, and moistened with deionized water. Each Petri dish contained half of a treated media pad, and the bottom of each dish was scratched to provide termites with traction. Thirty termite workers (undifferentiated larvae of at least the third instar) each were placed into a dish. To maintain appropriate worker to soldier ratios, three soldiers were placed in replicates containing Coptotermes species, and one soldier was placed in replicates containing R. flavipes. Petri dishes containing termites were placed on a bed of moist paper towel inside plastic boxes (30 × 22 × 10 cm) at ≈28°C and >90% RH for 3 d. Sheets of black plastic were used to cover the boxes to provide a dark environment for the termites. After 3 d of no-choice exposure to treatments, the treated and dyed media pad half in each Petri dish was replaced with an untreated, dye-free media pad moistened with DI water. Termites displayed varying degrees of blue coloration of the dye, confirming their consumption of the treated substrate. Digital images of termites in Petri dishes were taken daily to count the numbers of live termites. Recently molted workers (<36 h) as recognized by orange-colored mandibles (Kakkar et al. 2016b) were also recorded as the successfully molted individuals. Observations were continuous for another 11 d, totaling 14 d. The assay was replicated four times for each of three different colonies for each species, for a total of 12 replicates for each species and each treatment. For C. formosanus and C. gestroi, there were eight different treatments, for a total of 96 experimental units; 32 for each colony. For R. flavipes, there were six different treatments, for a total of 72 experimental units, 24 for each colony. Data Analysis Mortality for each termite species at 14 d were arcsine root transformed and subjected to an analysis of variance (ANOVA) by using a completely randomized block design with colony origin as the block factor. Significant differences (α = 0.05) among treatments for each species were separated by Fisher’s least significant difference (LSD) test (SAS Institute 1985). Cumulative numbers of successfully molted termites during the 14-d period were also subjected to ANOVA and significant differences among treatments were separated by LSD test. Results C. formosanus treated with 20E, with or without noviflumuron, began to die on the sixth day (including 3 d exposure time) from the start of the test, and affected termites exhibited hyperecdysonism (Fig. 1) before the onset of death as reported by Su and Monteagudo (2017). Effects of 20E were not fully expressed until 10, 12, and 13 d, for 2,000, 1,000, and 500 ppm, respectively (Fig. 2). Treatments with higher concentrations of 20E generally yielded significantly higher mortalities for C. formosanus at 14 d, and the addition of 2,500 ppm noviflumuron to 20E appeared to increase the lethality, especially at 1,000 ppm (Table 1). The lethal time of a CSI such as noviflumuron may extend for months, and as expected, mortality of those fed on noviflumuron alone was not significantly different from the untreated control at 14 d. Table 1. Mean mortality (% ± SE) for C. formosanus, C. gestroi, and R. flavipes 14 d after being force-fed with various concentrations of 20E, alone and in combination with noviflumurona Treatment concentration (ppm) % Mortality 20E Noviflumuron C. formosanus C. gestroi R. flavipes 2,000 2,500 100.0 ± 0.0a 74.2 ± 6.7a - 1,000 2,500 94.2 ± 1.8b 62.8 ± 6.2b 91.7 ± 3.0a 500 2,500 73.6 ± 5.3cd 51.4 ± 5.1bc 78.6 ± 7.5ab 2,000 0 97.8 ± 1.0ab 78.1 ± 5.8a - 1,000 0 80.3 ± 6.0c 48.9 ± 6.0bc 85.8 ± 4.7a 500 0 67.5 ± 6.7d 42.5 ± 6.6c 70.0 ± 8.0b 0 2,500 11.9 ± 3.3e 10.8 ± 1.6d 46.9 ± 3.5c 0 0 6.1 ± 1.5e 2.8 ± 1.1e 14.4 ± 2.0d Treatment concentration (ppm) % Mortality 20E Noviflumuron C. formosanus C. gestroi R. flavipes 2,000 2,500 100.0 ± 0.0a 74.2 ± 6.7a - 1,000 2,500 94.2 ± 1.8b 62.8 ± 6.2b 91.7 ± 3.0a 500 2,500 73.6 ± 5.3cd 51.4 ± 5.1bc 78.6 ± 7.5ab 2,000 0 97.8 ± 1.0ab 78.1 ± 5.8a - 1,000 0 80.3 ± 6.0c 48.9 ± 6.0bc 85.8 ± 4.7a 500 0 67.5 ± 6.7d 42.5 ± 6.6c 70.0 ± 8.0b 0 2,500 11.9 ± 3.3e 10.8 ± 1.6d 46.9 ± 3.5c 0 0 6.1 ± 1.5e 2.8 ± 1.1e 14.4 ± 2.0d aMeans followed by the same letter within a column are not significantly different (α = 0.05) according to LSD test (SAS Institute 1985). Open in new tab Table 1. Mean mortality (% ± SE) for C. formosanus, C. gestroi, and R. flavipes 14 d after being force-fed with various concentrations of 20E, alone and in combination with noviflumurona Treatment concentration (ppm) % Mortality 20E Noviflumuron C. formosanus C. gestroi R. flavipes 2,000 2,500 100.0 ± 0.0a 74.2 ± 6.7a - 1,000 2,500 94.2 ± 1.8b 62.8 ± 6.2b 91.7 ± 3.0a 500 2,500 73.6 ± 5.3cd 51.4 ± 5.1bc 78.6 ± 7.5ab 2,000 0 97.8 ± 1.0ab 78.1 ± 5.8a - 1,000 0 80.3 ± 6.0c 48.9 ± 6.0bc 85.8 ± 4.7a 500 0 67.5 ± 6.7d 42.5 ± 6.6c 70.0 ± 8.0b 0 2,500 11.9 ± 3.3e 10.8 ± 1.6d 46.9 ± 3.5c 0 0 6.1 ± 1.5e 2.8 ± 1.1e 14.4 ± 2.0d Treatment concentration (ppm) % Mortality 20E Noviflumuron C. formosanus C. gestroi R. flavipes 2,000 2,500 100.0 ± 0.0a 74.2 ± 6.7a - 1,000 2,500 94.2 ± 1.8b 62.8 ± 6.2b 91.7 ± 3.0a 500 2,500 73.6 ± 5.3cd 51.4 ± 5.1bc 78.6 ± 7.5ab 2,000 0 97.8 ± 1.0ab 78.1 ± 5.8a - 1,000 0 80.3 ± 6.0c 48.9 ± 6.0bc 85.8 ± 4.7a 500 0 67.5 ± 6.7d 42.5 ± 6.6c 70.0 ± 8.0b 0 2,500 11.9 ± 3.3e 10.8 ± 1.6d 46.9 ± 3.5c 0 0 6.1 ± 1.5e 2.8 ± 1.1e 14.4 ± 2.0d aMeans followed by the same letter within a column are not significantly different (α = 0.05) according to LSD test (SAS Institute 1985). Open in new tab Fig. 1. Open in new tabDownload slide Coptotermes formosanus workers that have undergone hyperecdy sonism after ingesting a lethal dose of 20-hydroxyecdysone. Fig. 1. Open in new tabDownload slide Coptotermes formosanus workers that have undergone hyperecdy sonism after ingesting a lethal dose of 20-hydroxyecdysone. Fig. 2. Open in new tabDownload slide Average time trend in mortality of C. formosanus after exposure to 20E alone (500, 1,000 and 2,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Fig. 2. Open in new tabDownload slide Average time trend in mortality of C. formosanus after exposure to 20E alone (500, 1,000 and 2,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Akin to C. formosanus, mortalities of C. gestroi exposed to 20E were not observed until 6 d, but mortality at 2,000 ppm plateaued at 12 d (vs. 10 d for C. formosanus), and those exposed to 1,000 ppm and 500 ppm plateaued at 13 d (Fig. 3). The overall mortalities of C. gestroi at 14 d were lower than those of C. formosanus, but as with C. formosanus, higher 20E concentrations resulted in higher C. gestroi mortalities (Table 1). At 1,000 ppm, 20E plus noviflumuron treatment resulted in significantly higher mortality than 20E alone. Again, as expected no significant mortality was recorded for C. gestroi fed on noviflumuron alone at 14 d. Fig. 3. Open in new tabDownload slide Average time trend in mortality of C. gestroi after exposure to 20E alone (500, 1,000 and 2,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Fig. 3. Open in new tabDownload slide Average time trend in mortality of C. gestroi after exposure to 20E alone (500, 1,000 and 2,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Effects of 20E on R. flavipes were observed at the third day of exposure, 3 d earlier that the two Coptotermes species, but the lethal time was more protracted, with mortality of those exposed to 1,000 and 500 ppm plateauing at 11 and 13 d, respectively (Fig. 4). As with the two Coptotermes species, higher 20E concentrations also resulted in higher mortalities at 14 d, but the addition of noviflumuron did not appear to have a significant impact (Table 1). Mortality of R. flavipes exposed to 2,500 ppm noviflumuron alone, however, was significantly higher than that of untreated control at 14 d. Fig. 4. Open in new tabDownload slide Average time trend in mortality of R. flavipes after exposure to 20E alone (500 and 1,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Fig. 4. Open in new tabDownload slide Average time trend in mortality of R. flavipes after exposure to 20E alone (500 and 1,000 ppm) and in combination with noviflumuron (2,500 ppm) for 3 d in a no-choice test. Of the three termite species exposed to 20E, only some C. formosanus successfully molted (Fig. 5). Significantly larger numbers of C. formosanus workers completed ecdysis after fed on 20E at 500 ppm and 1,000 ppm than the control and those exposed to 2,000 ppm. Approximately half of the survivors in the 500 ppm treatment completed ecdysis, and all of the survivors at 1,000 ppm treatment were successfully molted individuals, but a small number of successfully molted termites in 1,000 ppm treatment eventually died by 14 d. None of the C. formosanus workers exposed to noviflumuron and none of the tested C. gestroi or R. flavipes successfully molted. Fig. 5. Open in new tabDownload slide Total number of successful molts (mean ± SE) and mortality (mean ± SE) during a 14-d period by C. formosanus that ingested 20E alone and in combination with noviflumuron. Mean number of successful molts followed by the same letter are not significantly different (α = 0.05) according to LSD test (SAS Institute 1985). Fig. 5. Open in new tabDownload slide Total number of successful molts (mean ± SE) and mortality (mean ± SE) during a 14-d period by C. formosanus that ingested 20E alone and in combination with noviflumuron. Mean number of successful molts followed by the same letter are not significantly different (α = 0.05) according to LSD test (SAS Institute 1985). Discussion The lethal time of CSIs is considered dose-independent because the time required for CSIs to fully express their effects are more or less the same regardless of the doses ingested by insects (Su and Lees 2009). In this study, termites were confined for 3 d in a Petri dish containing a media pad impregnated with pre-determined concentrations of 20E, and it was assumed that termites in a Petri dish ingested roughly the same dose of 20E. Our results showed that it took 1–3 d longer for 20E to fully express its effects at lower concentrations (≈doses) for each termite species (Figs. 2–4). But in the scheme of a baiting program that may take weeks or months to complete, the 1–3 d difference is negligible, and at the concentrations tested, we consider the lethal time of 20E to be relatively dose-independent. Another distinction with a CSI is that its lethal time typically ranges from weeks to months instead of days for metabolic inhibitors (at slow-acting low doses), or hours for fast-acting insecticides (Su and Lees 2009). At the highest concentrations used in this study (2,000 ppm for C. formosanus and C. gestroi, and 1,000 ppm for R. flavipes), the lethal time of 20E (≈2 wk) was within the range of a CSI. Termites exposed to lower 20E concentrations, however, plateaued at significantly lower mortalities. Further studies are needed to determine if higher concentrations of 20E, or a longer exposure time than 3 d, may produce ≈100% mortality with the same lethal time. One unexpected outcome of this study was that some C. formosanus exposed to 20E alone successfully molted (Fig. 5). The presence of 20E in insects is known to activate molt-related genes to induce a series of events such as epidermal cell division, apolysis of old cuticles, secretion of molting fluid, and production of new cuticle (Nation 2016), but after the initial spike, 20E titers in an insect must decline back to basal levels to allow the expression of down-regulatory genes to complete the molt (Retnakaran et al. 2003). Hyperecdysonism results from the persistence of high titers of ecdysteroids such as 20E, which is often fatal (Williams 1968). Our results showed that at 500 and 1,000 ppm, 20E induced hyperecdysonism and mortality of most individuals, but some survived. These concentrations were not high enough to reliably induce hyperecdysonism in all individuals. This may be because, despite equal exposure time, individual termites did not acquire the exact same dose. It could also be related to the physiological state each termite was in when it ingested the 20E. For C. formosanus, some of the workers probably ingested enough 20E to initiate the molting process, but not enough to cause hyperecdysonism, allowing these individuals to molt successfully. This observation was most pronounced at 1,000 ppm, and less pronounced at the lower (500 ppm) and higher (2,000 ppm) concentrations, suggesting that termites must ingest a relatively specific quantity of 20E to successfully molt. At 2,000 ppm hyperecdysonism was more likely, and at 500 ppm, a lack of discernable effects occurred more frequently. It is unclear as to why none of the C. gestroi or R. flavipes successfully molted, even after similar exposure to 20 E. It was also interesting that C. gestroi did not exhibit similar levels of mortality to C. formosanus. This could indicate that 20E is metabolized more quickly by C. gestroi, or it could be that, despite the no-choice feeding, C. gestroi was deterred from feeding on the treated media pad to a greater extent than C. formosanus. This study shows that the time-trend in mortality induced by 20E was more in line with CSIs than metabolic inhibitors. The 10–14 d lethal time is similar to slow-acting toxicants such as chlorantraniliprole, but may be sufficient for 20E-affected termites to return to the central nest in the attempt to molt and die of hyperecdysonism because the intoxicated termites consistently survived for several days post-ingestion of 20E, regardless of the treatment concentration. Unlike metabolic inhibitors, which cause mortality through interference with energy production, 20E is similar to noviflumuron in that it causes mortality by affecting the molting process. Additional studies on whole colonies that add a spatial component are warranted to investigate this issue further. If baits containing 20E kill termites more quickly than noviflumuron and termites die near the central nest like CSI-baited colonies (Kakkar et al. 2018), the process of colony elimination could be shortened. Acknowledgments We greatly appreciate T. Chouvenc (University of Florida) for providing insightful suggestions during the development of the experiments, and A. Mullins (University of Florida) for review of this article. We would also like to thank the members of the Su laboratory (University of Florida) for their help with this project. This work was supported in part by the USDA National Institute of Food and Agriculture, Hatch project number FLA-FTL-005342. References Cited Glenn , G. J. , and R. E. Gold . 2002 . Evaluation of commercial termite baiting systems for pest management of the Formosan subterranean termite (Isoptera: Rhinotermitidae) , pp. 325 – 334 . In S. Jones , J. Zhai , and W. Robinson (eds.), Proceedings, 4th International Conference on Urban Pests , Pocahontas Press, Inc ., Blacksburg, VA . Google Preview WorldCat COPAC Kakkar , G. , T. Chouvenc , W. Osbrink , and N. Y. Su . 2016a . 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TI - Mortality Time Trends of Three Subterranean Termite Species When Exposed to 20-Hydroxyecdysone, Alone and in Combination With Noviflumuron
JF - Journal of Economic Entomology
DO - 10.1093/jee/toy186
DA - 2018-09-26
UR - https://www.deepdyve.com/lp/oxford-university-press/mortality-time-trends-of-three-subterranean-termite-species-when-5l8S7QNaoc
SP - 2312
VL - 111
IS - 5
DP - DeepDyve
ER -