Analyses of resistance to Bacillus thuringiensis delta-endotoxins in the European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae)

The first instars of the Kansas Dipel-resistant strain of Ostrinia nubilalis had 170, 200, 510, and >640-fold resistance to four Cry protoxins in Dipel, Cry1Aa, Cry1Ab, Cry1Ac, and Cry2Aa, respectively. However, first, second, and third instars were not able to survive on tissues from event MON810 and Bt11 Bt corn plants. Nevertheless, a high proportion of 22.5--55.2% of third-instar survivors previously exposed to Bt corn for 5 days were able to develop to the pupal stage when transferred to non-Bt corn tissues. These data argue against planting mixed Bt and non-13t corn seeds as a refuge in resistance management. There was no significant difference in the survival of resistant and susceptible strains on Bt corn tissues. The ∼200-fold resistance to Cry1Ab protoxin in the resistant strain was not observed when assayed on Bt corn tissues. Mechanism studies indicated that resistance to Cry1Ab and Cry1Ac protoxins was not associated with alterations in receptor binding in the resistant strain. However, the activity of soluble trypsin-like proteinases was reduced by 56% in the resistant strain, significantly lower than in the susceptible strain, and the reduced proteinase activity resulted in decreased Cry1Ab protoxin processing in the resistant strain. Reduced mRNA expression of at least one trypsin-like gene, OnT23, was predicted to result in reduced trypsin-like proteinase activity in Dipel-resistant O. nubilalis larvae. All data indicated that reduced trypsin-like proteinase activity is the major mechanism of resistance to Cry1Ab protoxin in the Kansas Dipel-resistant O. nubilalis strain. One implication of these results is that transgenic Bt corn events expressing fully activated Cry1Ab protein should overcome Bt-resistance in O. nubilalis that is mediated by reduced proteinase activity.