Long-term selection for resistance to bacillus thuringiensis Cry1Ac endotoxin in a Minnesota population of European corn borer (Lepidoptera: Crambidae)

Transgenic corn, expressing the insecticidal delta-endotoxin of Bacillus thuringiensis Berliner, provides high levels of control of some lepidopteran pests, particularly the European corn borer, Ostrinia nubilalis (Hubner). However, resistance to B. thuringiensis has been documented recently in laboratory colonies of agronomically important Lepidoptera, including O. nubilalis. For the past 4 yr, we have selected for Cry1Ac resistance in a population of O. nubilalis from southeastern Minnesota. Increasing resistance to B. thuringiensis was noted after only 8 generations of selection, with a peak at 162-fold resistance, based on comparisons of LC(50)S to a nonselected parental strain. This resistance was found to decrease at the same rate in the absence of B. thuringiensis selection, with one selected colony becoming nearly as sensitive to the Cry1Ac toxin as the nonselected colony after 9 generations without exposure to B. thuringiensis. The most resistant of the colonies, S-I, was only marginally cross-resistant to Cry1Ab, yet another selected colony, S-IV, did demonstrate a 16-fold cross-resistance, In addition, larvae from the S-IV colony had significantly greater weight gain when feeding on diet incorporated with B. thuringiensis-transgenic corn than did larvae from the nonselected parental colony. These findings emphasize the need for careful deployment of B. thuringiensis corn to preserve this effective pest management technology.