Baseline Susceptibility and Changes in Susceptibility to Bacillus thuringiensis subsp. kurstaki Under Selection Pressure in European Corn Borer (Lepidoptera: Pyralidae)
Baseline susceptibility of European corn borer larvae, Ostrinia nubilalis (Hübner), to a commercial formulation of Bacillus thuringiensis Berliner Subsp. kurstaki (Dipel ES), was determined for colonies from 3 different geographic locations. The KS-NE colony, collected near Manhattan in northeast Kansas, was more tolerant to Dipel than those collected from near St. John in south central Kansas (KS-SC colony) and colonies provided by the USDA Corn Insects Laboratory in Ames, IA (Iowa colony). Five colonies of European corn borers were exposed to laboratory selection using a meridic diet incorporating Dipel ES. Decreasing susceptibility of European corn borer to Dipel ES was observed in each colony selected. In the colony collected in south central Kansas (KS-SC), the LC50 and survival of larvae at specific concentrations increased more rapidly than it did in the other colonies. After only 3 selected generations, the LC50 of this colony increased 35.8-fold, and after 7 selected generations, it reached 72.9-fold. The LC50s of the 3 Iowa colonies increased 25.0- to 35.2-fold after 9-14 selected generations. The LC50 of the colony collected in northeast Kansas increased 16.2-fold after 4 selected generations and it reached 35-fold after 6 selected generations. This indicates that European corn borer populations can respond rapidly to intense selection pressure with a commercial formulation of Bacillus thuringiensis. The LC90s of the 5 colonies also increased significantly in a pattern similar to that observed in the LC50s, but the increases in LC90s were not as large as the increases for the LC50s. The bioassay data suggest that a dose of ≈2.4 ml Dipel per kilogram of diet could be used as a preliminary discriminating dose. Neonates from field-collected insects could be placed on diets incorporating this concentration of Dipel to monitor for changes and regional differences in susceptibility among field populations of European corn borer. Studies on the genetic basis, the mechanism of action, and the stability over time (in the absence of selection pressure) of this change in susceptibility remain to be carried out.