Bacillus thuringiensis CrylAc Resistance Frequency in Tobacco Budworm (Lepidoptera: Noctuidae)

The tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae), is one of the most important: pests of cotton, Gossypium hirsutum L., that has become resistant to a wide range of synthetic insecticides. CrylAc-expressing cotton has proven its effectiveness against this insect since its introduction in North America in 19%. However, the constant exposure of tobacco budworm to this protein toxin may result in the development of resistance to it. To estimate the frequency of alleles that confer resistance to a 1.0 mu g of Bacillus thuringiensis CrylAc diagnostic concentration in field-collected insects, the second generation (F-2) of 1,001 single-pair families from seven geographical regions representing 2,202 alleles from natural populations was screened in 2006 and 2007 without finding major resistant alleles. Neonates of 56 single-pair families were able to develop to second instar on the diagnostic concentration in the initial screen, but only seven of these lines did so again in a second confirmatory screen. Minor resistance alleles to CrylAc may be quite common in natural populations of H. virescens. Our estimated resistance allele frequencies (0.0036-0.0263) were not significantly different from a previously published estimate from 1993. There is no evidence that H. virescens populations have become more resistant to CrylAc.