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 virescen s (F.) beet armyworm. Spodoptera exigua (Hubner), Colorado potato beet le, Leptinotarsa decemlineata (Say), and the twospotted spider mite, T etranychus urticae Koch. Methods varied in amounts of test solutions d elivered and included a moderate pressure rotating spray tower that sp rayed 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 tile diet. Bioasssays were compare d using 2 avermectin insecticides. abamectin and emamectin benzoate, 2 pyrethroid insecticides. fenvalerate and lambda-cyhalothrin, 2 organo phosphorous insecticides, mevinphos and trichlorfon, and 1 ecdysone ag onist, tebufenozide. All 3 methods were comparable at estimating the s ensitivity of arthropods to these compounds. Differences in lethal con centration values among assays were species specific. Variation among assays was more apparent with less potent compounds (e.g., organophosp horous 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 bioa ssay types for most compounds tested. These data indicate that low vol ume bioassays were comparable to high volume bioassays at detecting an d estimating insecticidal activity against a select panel of arthropod pests. The potential of miniature volume bioassays for use in insecti cide discovery programs is discussed, with particular reference to scr eening programs that rely on sample collections that are available in limited quantities, such as natural product and combinatorial chemistr y sources.