Resistant Pest Management Newsletter

Susceptibility to Insecticides in Representative Canadian Populations of Colorado potato beetle, Leptinotarsa decemlineata (Say).

J.H. Tolman, S.A. Hilton, J.W. Whistlecraft and J.R. McNeil
Agriculture and Agri-Food Canada
Southern Crop Protection and Food Research Centre,
1391 Sandford St., London, Ontario, Canada, N5V 4T3

ABSTRACT Second instar larvae from representative Canadian populations of Colorado potato beetle were exposed by residual leaf-disc bioassay to a diagnostic dose (DD98) for several insecticides. Three days after treatment, mortality was 75% for 41.7%, 97.9%, 72.3% and 80.9% of populations treated respectively with imidacloprid, lambda-cyhalothrin, azinphos-methyl or spinosad.

KEYWORDS Colorado potato beetle, Canada, susceptibility, imidacloprid, lambda-cyhalothrin, spinosad, azinphos-methyl.

INTRODUCTION With a farm value exceeding $882 million in 2003, potato continues as an extremely important field vegetable crop in Canada. For a number of years the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), has been the most damaging and recalcitrant foliage-feeding, insect pest of potatoes in Canada. Infestation and defoliation of plants in the field could sharply reduce potato yields (Tolman et al. 1986; Noronha et al. 2002). If the problem was not addressed promptly, repeated applications of insecticides often proved necessary. Adequate control of this pest has been a major concern for potato growers due to repeated development of resistance of CPB populations to conventional insecticides. By 1995 only foliar insecticides containing the - endotoxin of the bacterium Bacillus thuringiensis var. tenebrionis remained widely effective in eastern Canada. In 1995, imidacloprid (Admire 240F), a chloronicotinyl insecticide with a new mode of action, was registered in eastern Canada. Management programs utilizing Admire 240F have subsequently generally provided excellent CPB control. Localized resistance to imidacloprid has, however, developed. By 1999 as much as 90-fold resistance had been demonstrated on Long Island, NY (Moyer 1999). Subsequent research on two CPB strains from Long Island confirmed resistance of as much as 150-fold in adults (Zhao et al. 2000; Hollingsworth et al. 2002) and 13-fold in larvae (Zhao et al. 2000).

In Canada in 2002, less than 75% mortality at an imidacloprid Diagnostic Dose lethal within 3 days to 98% of the reference insecticide-susceptible population (DD98) was observed in 13 of 35 populations of larvae reared from CPB collected in Ontario, Québec, New Brunswick and Nova Scotia (Tolman et al. 2003). Control-failures were not, however, associated with these populations. The survey was expanded in 2003 to further study possible developing tolerance to imidacloprid. In addition, to identify possible reversion (Kristensen et al. 2000) of previously identified tolerance to lambda-cyhalothrin and azinphos-methyl, CPB larvae were exposed to the DD98 for each insecticide. Finally, in order to determine variation in baseline susceptibility to spinosad, registered in 2003 for CPB control in Canada, additional larvae were also exposed to the DD98 for spinosad. Results of the 2003 Canadian survey of larval susceptibility to these insecticides are herein summarized.

MATERIALS AND METHODS
Insects - All bioassays were undertaken using second instar larvae. An insecticide susceptible (LAB-S) strain, reared on potato plants in the laboratory (Harris and Svec 1976) for over 45 generations served as the reference strain. Depending on the insecticide, 47 or 48 populations collected from six Canadian provinces were surveyed for susceptibility (Prince Edward Island - 6; Nova Scotia - 1; New Brunswick - 12/13; Québec - 9; Ontario - 17; Manitoba - 2). Approximately 300-500 adult CPB from each population were collected directly from infested potato fields and shipped to the laboratory. For each population, approximately 20-25 pairs of field-collected adults were held for oviposition on potted potato plants in the laboratory. Eggs were collected daily to produce second instars for bioassay.

Insecticides - Formulated imidacloprid (Admire 240F - 240 g a.i./L), spinosad (Tracer 480SC - 480 g a.i./L), azinphos-methyl (Guthion 240SC - 240 g a.i./L) or lambda-cyhalothrin (Matador 120EC - 120 g a.i./L) was suspended in reverse-osmosis (RO)-water to give stock solutions containing 2.0 ppm active insecticide; dilutions were subsequently made as required with RO-water.

Residual Bioassay - Leaf discs (43 mm diameter), punched from fresh potato leaves, were immersed in the desired concentration of insecticide and allowed to dry. Dry, treated discs were individually transferred to labelled, sterile Gelman® 47 mm microbiological dishes. Five second instars were placed on the leaf disc. The dish was then covered and transferred to a holding room at 27°C and 65% RH under continuous light. Mortality was counted three days after treatment (3 DAT) and corrected for natural mortality using Abbott's correction (Abbott 1925). For each series of bioassays for each population, control insects were placed on leaf-discs dipped in RO-water.

Diagnostic Dose - For each insecticide, using the LAB-S strain, at least 3 separate series of bioassays were run at each of 7 concentrations with a minimum of 60 larvae (3 bioassays x 4 replicates/bioassay x 5 larvae/replicate) for each concentration tested. Probit analysis (SAS Institute 2001) of the data generated for each insecticide was then completed to develop dose-mortality regression lines. For each insecticide the LC98, lethal to 98% of the LAB-S population 3 DAT was selected as the Diagnostic Dose (DD) for the susceptibility survey. To complete the survey for each population, two replicates of 5 second instars were treated on each of 3 separate days and mortality counted 3 DAT. Results for each population were then averaged for presentation. Possible correlation in susceptibility between insecticides was also determined (MSTAT Development Team 1991).

OBSERVATIONS AND DISCUSSION The average responses of second instars from 47-48 populations exposed in 2003 to the DD98 for the studied insecticides are illustrated in Figure 1 and summarized in Tables 1 and 2. For imidacloprid, mortality at the DD98 was 90% for only 16 (33.3%) populations exposed by residual contact/ingestion to 0.36 ppm imidacloprid. Larval mortality 50% was recorded for 5 (10.47%) collected populations. For one population collected in Québec, mortality barely exceeded 25%, a response level considered to represent a resistant population (Ellis 1989, in Olson et al. 2000). While susceptibility to imidacloprid of Canadian CPB in 2003 appeared lower than that recently reported in a European survey (Nauen and Denholme 2005), Canadian growers have not yet reported wide-spread control failures in the field. Admire 240F remains the dominant insecticide applied for CPB control in Canada.

Mortality of second instars exposed to residual deposits of 0.125 ppm lambda-cyhalothrin was 75% for 46 of 47 tested populations and below 25% for 7 (14.8%) collections. Permethrin, the first of the pyrethroids, was registered in Canada in 1978. By 1982 significant resistance to several pyrethroids, including permethrin, had been documented in CPB collected in Québec (Harris and Turnbull 1986). Resistance to pyrethroids continued to spread in Canadian CPB populations to the extent that by the time Matador was registered in Canada, it was not recommended for CPB control in Ontario (OMAFRA 1997). While the spray-histories of individual collections is not fully known, it would appear that there has been little reversion of tolerance to lambda-cyhalothrin in any tested population.

Mortality of second instars exposed to residual deposits of 25.0 ppm of azinphos-methyl was 75% for 38 of 47 tested populations and 50% for 19 (40.4%) collections. Larval mortality 90% at the DD98 was, however, recorded for 2 populations. First registered in Canada for CPB control in 1962, azinphos-methyl remained effective for a number of years. Not until 1979 was resistance to Guthion 2SC confirmed in CPB collected near Sherbrooke, Québec (Harris and Svec 1981). In this study, in contrast with results reported for lambda-cyhalothrin where larval mortality at the DD98 remained below 75% for all but 1 population, larval mortality for azinphos-methyl at the DD98 was 75% for 20% of 47 populations. Thus while resistance to azinphos-methyl remains a very serious problem, we recommend growers undertake a dip test (Banks and Squire 1992) to determine insecticide resistance of early instar larvae in their fields. Use of azinphos-methyl against susceptible larvae would reduce selection for imidacloprid-resistant CPB, preserving effectiveness of imidacloprid for CPB control.

As was the case for imidacloprid prior to its introduction and widespread use (Olson et al. 1996), there was considerable variation in susceptibility to spinosad among CPB populations never exposed to the insecticide. For second instars, only 5 (10.6%) populations exhibited 90% mortality when exposed to residual deposits of 0.20 ppm, the DD98 for spinosad; for 34 (72.3%) collections of larvae exposed to the DD98 mortality was 75%. Representative Canadian CPB populations have also shown significant variation in susceptibility to a DD98 for novaluron, an insect growth regulator not yet registered for CPB control in Canada (Cutler et al. 2005).

Potential correlation in larval susceptibility at the DD98 was compared across the four insecticides in the study for 47 different populations (Table 3). In contrast to Olsen et al. (2000) who found that CPB populations were generally less susceptible to imidacloprid if they were resistant to other classes of insecticides, we identified no significant correlations between susceptibility of second instars to imidacloprid and susceptibility to any other tested insecticide. There was, however, positive correlation between susceptibility to lambda-cyhalothrin and susceptibility to azinphos-methyl (Table 3).

REFERENCES

Abbott, W.S. 1925. A method for computing the effectiveness of an insecticide. J. Econ. Entomol. 18: 265-267.

Banks, E., and S. Squire. 1992. Colorado potato beetle dip test - on farm test to determine resistance to insecticides. OMAF Fact Sheet No. 92-028. AGDEX 257/621. 4 pp.

Cutler, G.C., J.H. Tolman, C.D. Scott-Dupree, and C.R. Harris. 2005. Resistance potential of Colorado potato beetle (Coleoptera: Chrysomelidae) to novaluron. J. Econ. Entomol. 98: 1685-1693.

Harris, C.R., and H.J. Svec. 1976. Susceptibility of the Colorado potato beetle in Ontario to insecticides. J. Econ. Entomol. 69: 625-629.

Harris, C.R., and H.J. Svec. 1981. Colorado potato beetle resistance to carbofuran and several other insecticides in Québec. J. Econ. Entomol. 74: 914-921.

Harris, C.R., and S.A. Turnbull. 1986. Contact toxicity of some pyrethroid insecticides, alone and in combination with piperonyl butoxide, to insecticide-susceptible and pyrethroid-resistant strains of the Colorado potato beetle. (Coleoptera: Chrysomelidae). Can. Entomol. 118: 1173-1176.

Hollingsworth, R.M., D. Mota-Sanchez, M.E. Whalon, and E.J. Grafius. 2002. Comparative pharmokinetics of imidacloprid in susceptible and resistant Colorado potato beetles. Proc. 10th IUPAC Int. Congr. on the Chemistry of Crop Prot. Basel, Switzerland 2002. 1: 312.

Kristensen, M., M. Knorr, A.G. Spencer and J.B. Jespersen. 2000. Selection and reversion of azamethiphos-resistance in a field population of the housefly Musca domestica (Diptera: Muscidae), and the underlying biochemical mechanisms. J. Econ. Entomol. 93: 1788-1795.

Moyer, D. 1999. Resistance to Admire: the Long Island experience. In: Proc. 1999 Ont. Hort. Crops Conf., Toronto. Pp. 90-93. MSTAT Development Team. 1991.

MSTAT-C: A microcomputer program for the design, management and analysis of agronomic research experiments. (Revised 1991). Michigan State Univ., East Lansing, MI.

Nauen, R., and I. Denholme. 2005. Resistance of insect pests to neonicotinoid insecticides: current status and future prospects. Arch. Insect Biochem. Physiol. 58: 200-215.

Noronha, C., G.M. Duke, and M.S. Goettel. 2002. Damage potential and phenology of the Colorado potato beetle (Coleoptera: Chrysomelidae) on potato in southern Alberta. Phytoprotection 83: 89-98.

Olson, E.R., G.P. Dively, and J.O. Nelson. 1996. Survey of susceptibility to imidacloprid (Admire) in Colorado potato beetle (Coleoptera: Chrysomelidae). Resistant Pest Manag. Newsl. 8(1): 39-41.

Olson, E.R., G.P. Dively, and J.O. Nelson. 2000. Baseline susceptibility to imidacloprid and cross resistance patterns in Colorado potato beetle (Coleoptera: Chrysomelidae) populations. J. Econ. Entomol. 93: 447-458.

OMAFRA. 1997. Publication 363 (Supplement) Vegetable Production Recommendations. Toronto, ON. 12 pp. SAS Institute. 2001.

SAS System for Windows Release 8.2. SAS Institute, Cary, NC.

Tolman, J.H., D.G.R. McLeod, and C.R. Harris. 1986. Yield losses in potatoes, onions and rutabagas in southwestern Ontario, Canada - the case for pest control. Crop Prot. 5: 227-237.

Tolman, J.H., S.A. Hilton, J.W. Whistlecraft, and D.C. MacArthur. 2003. Survey of Susceptibility of Representative Canadian Populations of Colorado Potato Beetle (CPB), Leptinotarsa decemlineata (Say) to Selected Insecticides: i - Admire 240F (imidacloprid); ii - Matador 120EC (lambda-cyhalothrin); iii - Success 480SC (spinosad). Annual Progress Report (FY2002-2003) to Ontario Potato Board (AAFC M.I.I. Project No. A03027). 47 pp.

Zhao, J.-Z., B.A. Bishop, and E.J. Grafius. 2000. Inheritance and synergism of resistance to imidacloprid in the Colorado potato beetle (Coleoptera: Chrysomelidae). J. Econ. Entomol. 93: 1508-1514.