ARS | Publication request: EFFECTS OF BACILLUS THURINGIENSIS KURSTAKI AND SODIUM BICARBONATE IN COLEOPTERAN AND LEPIDOPTERAN LARVAL DIETS

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2006
Publication Date: March 1, 2006
Citation: Skrivanek, S., Ripple, B., Lopez, J., Harris, M. 2006. Effects of Bacillus thuringiensis kurstaki and sodium bicarbonate in coleopteran and lepidopteran larval diets. Southwestern Entomologist. 31:55-58.

Interpretive Summary: The common soil-dwelling bacterium, Bacillus thuringiensis, produces toxic materials that, when eaten, kill many different insect species but do not have an adverse impact on man. This is significant because some of these insects are agriculturally- and medically-important pests. However, the materials produced by different subspecies of the bacterium vary in toxic effects toward different kinds of insects. A factor found to be important in this differential toxicity of the subspecies kurstaki is midgut pH. Insects with a basic pH such as larval stages of some moth pests (bollworm) are susceptible while some beetle pests (meal worm) that have an acid pH are not. By including sodium bicarbonate (antacid) and toxin in the diet, the beetle larvae became susceptible while the moth larvae became even more susceptible to the toxin. These results show how relatively simple manipulations in an insect diet can have major effects on efforts to control different insect pests with materials that are very safe to man. A major problem still exists in making this dietary manipulation practical for use by farmers, ranchers, and pest control operators.

Technical Abstract: We tested the hypothesis that Bacillus thuringiensis subspecies kurstaki (Btk) would adversely affect coleopteran larvae if the mid-gut pH is adjusted to the alkaline pH of lepidopteran larvae, which are susceptible to Btk. Bollworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), and mealworm, Tenebrio molitor L. (Coleoptera: Tenebrionidae), larvae in TRT 1 were fed Instant Soybean-Wheat Germ Insect Diet (SWD); TRT 2, SWD + sodium bicarbonate (to alter the mid-gut pH); TRT 3, SWD + Btk; TRT 4, SWD + sodium bicarbonate + Btk. Mealworms in TRT 1, 2, and 3 all lived and had a comparable weight gain. Mealworms in TRT 4 had some mortality and no weight gain. Bollworms in TRT 1 all lived and gained weight. Bollworms in TRT 2 had low mortality and gained weight. Bollworms in TRT 3 and 4 had high mortality and no weight gain. TRT 2 and 4 containing sodium bicarbonate significantly increased mid-gut pH in bollworm and mealworm, changing the latter from acidic to alkaline. Our results supported the hypothesis that coleopteran larvae with an alkaline mid-gut would be adversely affected when fed Btk and also suggest that increasing the alkalinity of bollworm mid-gut increases susceptibility to Btk.