Behavioral Responses and Sublethal Effects of Spruce Budworm (Lepidoptera: Tortricidae) and Fall Webworm (Lepidoptera: Arctiidae) Larvae to Bacillus thuringiensis Cry1A(a) Toxin in Diet

Abstract

The median lethal concentrations of Bacillus thuringiensis Cry1A(a) toxin (Cry1A(a) = 0.2% of total protein) for second-instar spruce bud worm and first-instar fall webworm were 6 μg engineered E. coli protein extract (EPE)/ml diet, and 0.1 μg EPE/cm2 of diet surface, respectively. The median lethal times were 8.4 and 2.8 d, respectively. Although second-instar spruce budworms were not killed rapidly, larval weight gain was decreased at sublethal concentrations and inhibited at higher concentrations of the toxin. Spruce budworm larvae preferred control diets to toxin-treated diets in choice tests. In contrast, first-instar fall webworm did not avoid toxin treated diets in choice tests. Spruce budworm larval survival was not affected significantly by most toxin concentrations when untreated diet was available (choice test), but larval weight decreased with increasing toxin concentration. Second-instar spruce budworm ingested toxic diets within 6 h of infestation, and the level of ingestion increased Significantly after 24 h. Preexposure to high concentrations of toxin-incorporated diet for 48 h did not affect ingestion of toxic diet. Spruce budworm larvae recovered from toxin upon transfer to control diet, but larval duration increased with the duration of initial exposure. Pupal weights were not affected, regardless of duration of exposure to toxic diet. These results are discussed with regard to development and deployment of trees engineered to express B. thuringiensis Cry1A(a) toxin and the evolution of behavioral and physiological resistance of insects to B. thuringiensis toxins.