Resistant Pest Management Newsletter

Baseline susceptibility to abamectin in thirty-four Colorado potato beetle (CPB) populations was determined by diet-incorporated assays. Also, filter-paper exposure assays quantified esfenvalerate and azinphosmethyl resistance. LC50 values ranged from 0.09 to 2.69 ppm abamectin and differed by 9fold in 1993 and 21-fold in 1994. Based on the composite response of this assay, the predicted LC50, LC95, and LC99 of 0.62, 15.8, and 60.3 ppm, respectively, represent the best estimate of the overall baseline susceptibility in CPB populations before field exposure to abamectin. Susceptibility to abamectin significantly decreased as the levels of resistance to esfenvalerate and azinphosmethyl increased. INTRODUCTION: Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), has developed resistance to virtually every class of insecticides. Because of this resistance problem, insecticides with novel modes of toxic action are needed to deploy in pesticide rotations to minimize selection pressure to any one pesticide. Abamectin exhibits a uniquely different toxic action and is highly active on CPB. Because registration of this insecticide for CPB control may be forthcoming, we determined baseline susceptibility in CPB populations to abamectin as a reference to detect shifts in susceptibility following commercial use. We also determined if abamectin showed any potential cross resistance to esfenvalerate and azinphosmethyl to provide insight into how resistance may occur and how the product should be used to minimize the risks of resistance. MATERIALS and METHODS: Thirty-two geographically distinct populations of post-diapause CPB adults were collected by cooperators from nine states in the U.S. and one province in Canada in 1993 and 1994. During May through July, roughly 200~300 adults from each population were shipped to a bioassay facility at the University of Maryland Wye Research and Education Center, Queenstown, MD. Questionnaires documented the insecticide use patterns used by the grower at each collection site. In 1994, three laboratory colonies of CPB were also tested: 1) a standard New Jersey (NJ) strain from the NJ Department of Agriculture Beneficial Insects Rearing Laboratory (Trenton, NJ) that was maintained continuously for 12 years without introgression of new adults. This population was never exposed to abamectin and served as a standard for future comparisons with field populations and as a measure of variation in our bioassay technique; 2) an abamectin-resistant strain (~25-fold) from the University of Massachusetts induced field selection in large cages over two growing seasons (Argentine and Clark 1990); and 3) a susceptible strain that was used to generate the resistant one. Adults from each population were segregated and reared in large field cages on potato plants. Initially, a limited number of adults were available in the abamectin-resistant strain, so this population was reared through two generations and tests were performed on the F2 progeny. Egg masses were collected daily, surface-sterilized in a 5% chlorox solution, and stored attached to leaf discs at 13°C to synchronize development. Several days prior to each test, eggs were removed from storage and reared at 24°C until eclosion. Neonates were allowed to feed on the remaining chorion then transferred directly