Effect of Bacillus Thuringiensis Subsp. Kurstaki Toxin on the Mortality and Development of the Larval Stages of the Banded Sunflower Moth (Lepidoptera: Cochylidae)

A 66-kDa subunit of the protoxin from Bacillus Thuringiensis subsp. Kurstaki was isolated and incorporated into an insect diet to study its effect on mortality and development of the larval stages of the banded sunflower moth, Cochylis hospes Walsingham. Intact B. thuringiensis caused death in ≍48 h but the purified toxin had a range of effects depending on the dosage and the instar. Feeding stopped almost immediately when toxin was present in the diet, but death occurred over times that ranged up to 60 d or more. One microgram of toxin per ml of diet was sufficient to cause 100% mortality of 1st, 2nd, 3rd, and 4th instars and >80% of 5th instars. The total development time from the beginning of the 1st first instar to the pupal stage was 21.4 ±0.1 (mean ± SEM) and 33.8 ±1.1 d (P < 0.05) d for the controls and surviving larvae that were fed a sublethal dosage of B. thuringiemis toxin, respectively. The LD50 of the 3rd instar was estimated to be 0.152 ± 0.0l6 µg toxin per milliliter of diet. Fifty percent of 1st instars and ≥80% of the later instars that were exposed to a lethal dosage of 1 µg toxin per milliliter of diet recovered and resumed development if they were transferred to control diet without the toxin. The ability to recover after exposure to a lethal dosage may be helpful in strategies to delay development of resistance to the toxin.