Induction and transmission of Bacillus thuringiensis tolerance in the flour moth Ephestia kuehniella

Abstract

The use of Bacillus thuringiensis (Bt) endotoxins to control insect vectors of human diseases and agricultural pests is threatened by the possible evolution of resistance in major pest species. In addition to high levels of resistance produced by receptor insensitivity (5, 16, 17), several cases of tolerance to low to medium levels of toxin have been reported in laboratory colonies of lepidopteran species (3, 18). Because the molecular basis of some of these cases of tolerance to the toxin are not known, we explored alternative mechanisms. Here, we present evidence that tolerance to a Bt formulation in a laboratory colony of the flour moth Ephestia kuehniella can be induced by preexposure to a low concentration of the Bt formulation and that the tolerance correlates with an elevated immune response. The data also indicate that both immune induction and Bt tolerance can be transmitted to offspring by a maternal effect and that their magnitudes are determined by more than one gene.

Fig. 1.

Melanization assay of cell-free hemolymph from tolerant and susceptible larvae. Bars represent SD. •, hemolymph from tolerant larvae; ○, hemolymph from susceptible larvae.

Fig. 2.

Relationship between the rate of the melanization reaction [as the slope of the plot of absorbance vs. time (arbitrary units)] of cell-free hemolymph and the LC₅₀ values of individual cohorts of 28-day-old T×S larvae. For one replicate, a melanization rate of 7.92 was recorded, but an LC₅₀ value could not be calculated because mortality in the 8,000 ppm condition was <10%. Note that the rates of the melanization reactions ranged from 0.32 to 7.92. Dashed line represents fit in linear regression (r² = 0.966). Bars represent 95% confidence intervals.