Feeding disruption bioassays: The application of novel diets to assessments of insecticide toxicity and resistance

Bioassay methodology was developed for species diagnosis of Heliothis virescens (F.) compared to Helicoverpa zea (Boddie) in cotton and to detect H. virescens larvae with significant levels of resistance to the biopesticide, Bacillus thuringiensis . The assay end-point is feeding disruption which is measured by a lack of fecal production by larvae exposed to a diagnostic dose of CryIAc in a blue indicator diet. In laboratory tests, the bioassay accurately distinguished neonates of H. zea from H. virescens and was able to detect B. thuringiensis resistance in H. virescens. The assay is rapid compared with mortality assays and should be inexpensive. The assay should also be adaptable to current cotton IPM programs and sampling techniques and detect most physiological mechanisms of B. thuringiensis resistance. The potential utility of the feeding disruption assay in cotton IPM and with other crops, insect pests and insecticides is discussed. The initial studies were conducted on laboratory strains of B. thuringiensis susceptible H. virescens and H. zea and a highly B. thuringiensis -resistant laboratory strain of H. virescens (YHD2) originally collected in North Carolina.;Validation of the feeding disruption bioassay for the detection of resistance to B. thuringiensis toxins and species identification was conducted using field strains of H. virescens and H. zea collected from the southern US in 1998. The bioassay provided rapid (24 h) diagnosis of the species composition of larvae tested and simultaneously monitored for the presence of resistance in H. virescens, with an additional diagnostic dose established for resistance monitoring in H. zea. A model was designed to aid in the interpretation of future results from field trials based on the variability in bioassay response among the field strains tested. Integration of the feeding disruption bioassay into integrated pest management programs is discussed.;In order to investigate the utility of using lyophilized meal pads as substitutes for fresh diet, fresh and freeze-dried lepidopteran artificial diets were compared. Trichoplusia ni (Hubner), H. virescens and H. zea performed at least as well on freeze-dried diets (hydratable meal pads) as on fresh diets in growth and development comparisons. Rearing T. ni for seven generations on freeze-dried diet did not negatively affect growth potential in comparison to T. ni reared exclusively on fresh diet. Meal pads were found to absorb water in a consistent manner over time and distribute solutes through the matrix more efficiently with increasing water solubility. Hydratable meal pads perform equally as well as fresh diets in oral toxicity bioassays while providing multiple advantages which are fully discussed. The substitution of hydratable meal pads for fresh diets had no effect on the feeding disruption bioassay for B. thuringiensis resistance in H. virescens and H. zea and a new bioassay for the field monitoring of spinosad resistance in H. virescens was developed using meal pads.