A filter paper technique for larvae and a dip technique for adults were two bioassay methods used to determine insecticide susceptibility in populations of Colorado potato beetle, Leptinotarsa decemlineata (Say), on the Eastern Shore of Virginia. Bioassay results indicated a wide variation in Colorado potato beetle insecticide susceptibility among sample sites. LC50 values were generally highest from specimens collected in fields that received frequent insecticide applications for > 10 consecutive growing seasons. Larvae from all but one sample site were more resistant to all insecticides tested than larvae from a susceptible colony. Larvae exhibited least susceptibility to azinphosmethyl and greatest susceptibility to fenvalerate. Lower LC50 values obtained from Colorado potato beetle in southern sample sites indicated that insecticide resistance was not as severe in earlier-planted fields. Differences in LC50 ranking among fields between larvae and adults indicated a differential susceptibility to insecticides between life stages. The filter paper method may be most suitable for on-farm use because of lower variability in LC50 estimates and reduced time cost compared with the dip technique.
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Tisler et al. (1990) studied this question.