Toxicity of Bacillus Thuringiensis Var. San Diego and Var. Kurstaki HD290I to Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract

Bacillus thuringiensis (Bt) is a soil dwelling bacterium that produces various insecticidal crystal proteins (ICPs) (Bauer 1990). It has been demonstrated that some beetles can develop a resistance to Bt isolates (Whalon, et al. 1992). A research project is underway that will attempt to discover which mode of ICP action: ingestion, processing, or binding, most greatly controls resistance. I was responsible for developing a reproducible bioassay and calculating the median lethal dose (LD50) for Cottonwood Leaf Beetles (CLB), Chrysomela scripta and Colorado Potato Beetles (CPB), Leptinotarsa decemlineata (Say) to Bt var. san diego (sd) and Bt var. kurstaki (k) HD290i. The objectives of these experiments were three-fold: (1) to experimentally determine if a CLB colony could become Bt-resistant through graduall y increasing exposure to Bt-sd toxin, (2) to see if cross resistance to another Bt isolate (Bt-k) occurs in Bt-sd resistant strains, and (3) to calculate the effectiveness of crystal Bt-k vs. solubilized Bt-k. The results show that CLB do form a significant level of Bt-sd resistance, some cross resistance occurs in CLB but not in CPB, and solubilized toxin appears to be more lethal than crystallized.