Regional and Temporal Variation in Susceptibility to λ- Cyhalothrin in Onion Thrips, Thrips tabaci (Thysanoptera: Thripidae), in Onion Fields in New York

Populations of onion thrips, Thrips tabaci Lindeman, from commercial onion Þelds in New York were evaluated for their susceptibility to the commonly used pyrethroid, -cyhalothrin (Warrior T), using a novel system called the Thrips Insecticide Bioassay System (TIBS). To use TIBS, thrips are collecteddirectly from theplant into an insecticide-treated 0.5-mlmicrocentrifuge tube that has a ßexible plastic cap with a small well into which 0.08 ml of a 10% sugar-water solution with food colorant is deposited. The solution is sealed into the well with a small piece of stretched paraÞlm throughwhich the thrips can feed on the solution. Thrips mortality is assessed after 24 hwith the help of a dissecting stereoscope. In 2001, onion thrips populations were collected from 16 different sites and resistance ratios were 1,000 in Þve populations. Percent mortality at 100 ppm, a recommended Þeld rate, varied from9 to100%, indicatinghigh levels of variation in susceptibility. Particular instances of resistance appeared to be the result of practices within an individual Þeld rather than a regional phenomenon. In 2002, we also observed large differences in onion thrips susceptibility, not only between individual Þelds but also between thrips collected in a single Þeld at mid season and late season, again suggesting that insecticide-use practices within an individual Þeld caused differences in susceptibility.Additional tests indicatednodifferences in susceptibilitybetweenadult and larval onion thrips populations and only relativelyminor differences between populations collected fromdifferent parts of the same Þeld. Using TIBS, several populations of onion thrips with different susceptibilities to -cyhalothrin were identiÞed and then subjected to -cyhalothrin-treated onion plants. There was a highly signiÞcant positive relationship between percent mortality of thrips from TIBS and percent mortality fromthe treatedonionplants, indicating that results fromTIBScouldbeused topredict spray performance.Thesedata suggest that useofTIBS for evaluating susceptibility toparticular insecticides could be instrumental for developing a resistance management strategy for onion thrips.