Resistant Pest Management Newsletter

Susceptibility of Canadian Colorado potato beetle populations to imidacloprid and metaflumizone, 2006

G. Christopher Cutler, Cynthia D. Scott-Dupree, and Melissa L. Dugas
Department of Environmental Biology, Ontario Agricultural College,
University of Guelph,
Guelph, Ontario,
Canada, N1G 2W1

Wang Deng-Yuan
College of Agronomy,
XinJiang Agricultural University,
Urumqi, XinJiang,
P.R. China, 830052

INTRODUCTION The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is a major pest of potato and one of the most challenging agricultural insect pests in terms of insecticide resistance, having developed resistance to at least 49 different active ingredients and all major insecticide classes (Whalon et al. 2006). Since its registration in 1995, imidacloprid (Admire® 240F, Bayer CropScience) has been the insecticide of choice for CPB management throughout most of North America. Even today it continues to provide good control for the vast majority of growers. Nonetheless, imidacloprid resistance, or its onset, has been reported (Mota-Sanchez et al. 2000, Olson et al. 2000, Tolman et al. 2005, Mota-Sanchez et al. 2006). A Canadian survey of CPB susceptibility to imidacloprid was last conducted in 2003 (Tolman et al. 2005). At that time, low levels of tolerance were found but control failures in the field were not reported by growers. Since 2003, however, some Canadian growers have reported reduced efficacy of imidacloprid and concerns of future imidacloprid resistance have risen. The present study updates the current status of imidacloprid resistance in selected Canadian CPB populations from 4 provinces. Several populations were included for study based on reports by growers of reduced imidacloprid efficacy. We also document susceptibility of some of these populations to metaflumizone (BAS 320I, BASF), a novel semicarbazone insecticide with activity against key coleopteran and lepidopteran pests. Metaflumizone has favorable toxicological and environmental profiles, has exhibited low impact on beneficial arthropods (natural enemies and pollinators), and has been designated a Reduced Risk Candidate by the US EPA.

MATERIALS AND METHODS
Insects: An insecticide-susceptible CPB strain reared for over 80 generations on potato at the Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada (London, Ontario) was used as the reference strain. Fourteen field populations from four Canadian provinces - Prince Edward Island (3), Quebec (3), Ontario (6), and Manitoba (2) - were collected by research or extension personnel using standardized collection and shipping kits, forwarded to collectors several weeks before CPB adults emerged from diapause. Collections of CPB adults were sent via overnight courier to the University of Guelph. Upon receipt, 30-35 adults each were promptly transferred to oviposition cages containing potted potato plants, and held in a rearing room at 25º C and 16:8 [L:D]. Three times per week egg masses from each population were excised from plants, placed in labeled Petri dishes, and transferred to an incubator at 25º C, 16:8 [L:D], or to an incubator at 12ºC, 16:8 [L:D], if slowed development was required to synchronize hatch. Bioassays were conducted on 12-24 h old first instars from the laboratory F1 generation of each field collection.

Chemicals: Formulated imidacloprid (Admire® 240F, 240 g AI L^-1) was supplied by Bayer CropScience Canada Inc. (Calgary, Alberta, Canada). Formulated metaflumizone (BAS 320I, 220 g AI L^-1) was supplied by BASF Canada Inc. (Mississauga, Ontario, Canada).

Bioassays: Residual leaf-dip bioassays were conducted to determine the susceptibility of CPB populations to imidacloprid (14 field populations) and metaflumizone (3 field populations). Insecticides were each suspended in deionized water to give stock solutions of 1000 ppm. Serial dilutions were subsequently prepared to give concentrations ranging 0.1-300 ppm. As determined in preliminary tests, concentrations that caused 5-95% mortality were used in bioassays. Using a stainless steel cork borer, 1.5 cm diameter discs were cut from potato leaves harvested from potato plants grown in an insecticide-free greenhouse. Discs were dipped in insecticide solution for approximately 4 s and placed on a wire rack to air dry. Two dry discs were each placed on a Whatman No. 1 filter paper (1.5 cm diameter) in a well of a 32-well bioassay tray (Oliver Products Company, Grand Rapids, MI). Five first instars of a given population were transferred to each well using a clean fine-haired paintbrush. When all wells contained larvae, bioassay trays were sealed with a thin transparent polyester plastic using 3M Spray Mount™ (3M, London, Ontario, Canada), labeled and transferred to a holding room (25º C, 16:8 [L:D]). After 48 h the number of dead larvae per well was recorded.

For each insecticide and CPB population, at least 3 bioassays (one bioassay was one tray) were conducted, each on a separate day. Each bioassay consisted of at least 7 concentrations with 3 replicates per concentration. Concentration-mortality regression lines were generated for each population using PROC PROBIT (SAS Institute 2001). Concentrations lethal to 50 percent of the CPB larvae (LC₅₀), 95% confidence limits, regression line slopes and chi-squared goodness-of-fit test results are reported.

RESULTS AND DISCUSSION Limited utility of alternative control options for growers dictates heavy reliance on chemical insecticides to control CPB. For most of the past decade growers have effectively managed CPB with foliar or in-furrow/seed treatment applications of imidacloprid. And while imidacloprid continues to provide good CPB control for most growers, imidacloprid resistant CPB populations have developed in the United States (Zhao et al. 2000, Mota-Sanchez et al. 2006) and grower accounts of reduced efficacy have been reported in Ontario and Quebec (A. Dornan, personal communication)Field Development Representative, Bayer CropScience, Rockwood, Ontario, Canada. The present study therefore sought to determine the current status of imidacloprid susceptibility in selected Canadian CPB populations. We also assessed the susceptibility of several of these populations to metaflumizone to determine cross-resistance potential with imidacloprid-resistant beetles.

CPB populations were highly variable in their susceptibility to imidacloprid (Table 1). Whereas the LC₅₀ of several populations was less than 4-fold that of the insecticide-susceptible strain, other populations displayed decreased susceptibility to imidacloprid with LC₅₀ values up to 22-fold that of the insecticide-susceptible strain, indicating hotspots where future control failures are foreseeable. Most populations from Prince Edward Island and Ontario were quite susceptible to imidacloprid, although some were tolerant of lower concentrations. Ontario populations ON4 and ON5 were from farms where growers reported decreased CPB sensitivity to imidacloprid or shorter-lived residual activity compared to previous years. As expected, these populations had LC₅₀ values significantly higher than the insecticide-susceptible population and the other Ontario populations tested. All tested populations from Quebec exhibited greatly reduced sensitivity to imidacloprid compared to the laboratory-susceptible strain. Conversely, both populations from Manitoba were highly susceptible (Table 1). These types of variable responses are not unusual since rather than resistance evolving in isolated areas and spreading geographically, discrete CPB populations respond to local selection pressures (Bishop and Grafius 1996). While resistance in CPB is most strongly correlated with history of insecticide application (Roush et al. 1990, Tisler and Zehnder 1990, Dively et al. 1992, Huang et al. 1995), crop rotation is also thought to be crucial for effective, long-term management (Wright 1984, Bishop and Grafius 1996). Indeed, populations with the lowest susceptibilities to imidacloprid in the present study were generally from farms practicing inadequate crop rotation (A. Dornan, personal communication)Manager, Research & Commercial Development, BASF Canada, Inc., Mississauga, Ontario, Canada.

CPB field populations displaying up to 13-fold reduced susceptibility to imidacloprid were very susceptible to metaflumizone (Table 2). This was not unexpected given the unique mode of action of metaflumizone. Unfortunately bioassays with metaflumizone were possible only near the end of the summer, limiting the number of populations tested. Nonetheless, along with field trials confirming good efficacy against CPB (W. Barton, personal communication), the lack of cross-resistance with imidacloprid suggests metaflumizone could be an effective control and resistance management option for potato growers. The biocide spinosad and chitin synthesis inhibitor novaluron are also recently developed compounds with insecticidal activity against CPB and modes of action different from that of neonicotinoids. While cross-resistance and future resistance development to these compounds is of some concern (Cutler et al. 2005, Tolman et al. 2005, Mota-Sanchez et al. 2006), along with metaflumizone they constitute a suite of reduced-risk alternatives that should be very useful in CPB resistance management programs.

CPB continues to be a formidable challenge for insecticide resistance management researchers. Work in the United States indicates that CPB tolerance or resistance to imidacloprid is becoming widespread and that alternation with more recently developed neonicotinoids, such as thiamethoxam, is likely to be a inadequate long-term pest and resistance management strategy (Mota-Sanchez et al. 2006). Although imidacloprid continues to provide good CPB control for most Canadian potato growers, the present study highlights the potential for future control failures if they rely solely on imidacloprid for CPB management. Our work also demonstrates that metaflumizone may be an effective option for growers encountering imidacloprid-resistant beetles.

ACKNOWLEDGEMENTS We thank J. Whistlecraft (Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada, London, Ontario) for the insecticide-susceptible CPB stain, as well as the numerous growers and government/extension personnel for CPB collections. We also thank Bayer CropScience Canada and BASF Canada Inc. for their contributions to this research.

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