INCREASING EFFICIENCY OF BIOASSAYS - EVALUATING RESISTANCE TO BACILLUS-THURINGIENSIS IN DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE)

We addressed several key questions about duration of bioassays, diagno stic concentration, and probit regression slope for response of diamon dback moth, Plutella xylostella (L.), to the microbial insecticide, Ba cillus thuringiensis Berliner. Data were obtained from 54 bioassays of larvae from seven field populations and eight laboratory colonies in Hawaii. Linear regression showed that LC50s at 96 and 120 h after expo sure to B. thuringiensis were associated with LC50s at 24 and 48 h. Co ntrol mortality increased as time increased, but time had little effec t on slope, standard error of slope, or 95% fiducial limits for LC50. A significant portion of the variation in LC50 was accounted for by va riation in mortality at a single concentration. Linear and polynomial regression showed that slope did not vary as a simple or consistent fu nction of LC50. Variaton in LC50 across all strains of diamondback mot h was >5,000-fold, yet variation in LC50 for repeated bioassays of sin gle strains was <6-fold. In contrast, variation in slope was nearly as large for single strains (2-fold) as it was across all strains (4-fol d). These results suggest that much of the variation in slope was not biologically meaningful. The results also suggest that for routine eva luation of resistance, bioassays using short time intervals and a sing le concentration may greatly increase efficiency with little loss of i nformation compared with standard bioassays.