Evaluation of miniature and high-volume bioassays for screening insecticides

Three methods for screening insecticides and acaricides were evaluated against select arthropod targets: tobacco budworm, Heliothis virescens (F.) beet armyworm. Spodoptera exigua (Hubner), Colorado potato beetle, Leptinotarsa decemlineata (Say), and the twospotted spider mite, Tetranychus urticae Koch. Methods varied in amounts of test solutions delivered and included a moderate pressure rotating spray tower that sprayed 50 ml of solution to runoff on 3 plant species. and 2 miniature volume assays; a low pressure, airbrush applicator that delivered 1 mi to both surf,ices of individual leaves; and an agar-based artificial diet assay (Lepidoptera only) that delivered small aliquots (50 mu l) of test solutions to the surface of the diet. Bioasssays were compared using 2 avermectin insecticides. abamectin and emamectin benzoate, 2 pyrethroid insecticides. fenvalerate and lambda-cyhalothrin, 2 organophosphorous insecticides, mevinphos and trichlorfon, and 1 ecdysone agonist, tebufenozide. All 3 methods were comparable at estimating the sensitivity of arthropods to these compounds. Differences in lethal concentration values among assays were species specific. Variation among assays was more apparent with less potent compounds (e.g., organophosphorous compounds and tebufenozide) compared with more potent compounds (pyrethroids and avermectins) for H. virescens, whereas in S. exigua and T. urticae, lethal concentration values were comparable among bioassay types for most compounds tested. These data indicate that low volume bioassays were comparable to high volume bioassays at detecting and estimating insecticidal activity against a select panel of arthropod pests. The potential of miniature volume bioassays for use in insecticide discovery programs is discussed, with particular reference to screening programs that rely on sample collections that are available in limited quantities, such as natural product and combinatorial chemistry sources.