Estimated Frequency of Nonrecessive Bt Resistance Genes in Bollworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Eastern North Carolina

Abstract

In summer 2000, adult female bollworm moths, Helicoverpa zea (Boddie), were collected from light-traps at four locations near the Tidewater Research Station, Plymouth, NC. Female moths were allowed to lay eggs, and at hatch, 72 larvae from each female were screened for growth rate on normal artificial diet and on diets containing 5.0 microg of either Cry1Ac or Cry2Aa Bt toxin per milliliter of diet. The growth rate bioassays were performed to isolate nonrecessive Bt resistance genes present in field populations of bollworm. We found one individual out of 583 screened that appeared to carry a major gene for resistance to Cry1Ac. Assuming four alleles per individual, the gene frequency is 1/2332 or 0.0003. Other females appeared to have minor genes for Cry1Ac resistance or major genes with lower levels of dominance. We also found one individual out of 646 screened that appeared to carry a major gene for resistance to Cry2Aa. The gene frequency for Cry2Aa resistance was estimated at 1/2584 or 0.00039. Again, other females seemed to carry additional minor resistance genes. Along with other results that indicate partially dominant inheritance of Cry1Ac resistance in bollworm, these allele frequency estimates are important for determining the rate of resistance evolution in H. zea to specific Bt toxins.

Asymmetrical cross-resistance betweenBacillus thuringiensistoxins Cry1Ac and Cry2Ab in pink bollworm

“…As in some strains of H. virescens, resistance to Cry1Ac was genetically correlated with resistance to Cry2Aa in field populations of H. zea sampled during 2000 (43). In addition, responses to Cry1Ac and Cry2Ab were genetically correlated in field populations of H. zea sampled during 2001 and 2002 (44).…”

“…However, because cotton producing both Cry1Ac and Cry2Ab was relatively rare from 2002 to 2004, the positive correlation between LC 50 values for Cry1Ac and Cry2Ab detected during this period probably resulted from crossresistance, rather than sequential resistance to the 2 toxins. Cross-resistance is more problematic in H. zea than in pink bollworm or H. virescens, because H. zea showed nonrecessive inheritance of resistance to Cry1Ac and Cry2Aa (43,46), relatively low inherent susceptibility to Cry1Ac and Cry2Ab (36), and survival in the field on cotton plants producing Cry1Ac and Cry2Ab (47).…”

Dynamically optimal strategies for managing the joint resistance of pests to Bt toxin and conventional pesticides in a developing country

“…Earlier work demonstrated that Bt cotton could control about 80-95 per cent pests on average in the Yellow River Valley (Wu, 2002;Li et al, 2004). Based on these studies (from China) and empirical studies in the United States (Caprio, 1998;Burd et al, 2001;Storer et al, 2003;Livingston et al, 2004), we assume that the mortality rate of pests with double susceptible genes to Bt toxin is 0.90 in Bt cotton fields. As assumed in Livingston et al (2004Livingston et al ( , 2002, we also assume that the mortality rate of pests with double resistant genes to Bt toxin, the so-called 'fitness cost', is 0.05.…”

Frequency and fitness cost of resistance to Bacillus thuringiensis in Chrysomela tremulae (Coleoptera: Chrysomelidae)

“…Among these factors, the frequency is certainly one of the most important: the lower the frequency, the more sustainable the strategy. The initial frequency of resistance has been estimated in only a limited number of cases, mostly in lepidopteran pests, due to practical limitations: Helicoverpa armigera (Wu et al, 2002), Heliothis virescens (Gould et al, 1997) and H. zea (Burd et al, 2003) on cotton, Ostrinia nubilalis on maize Bourguet et al, 2003), Scirpophaga incertulas (Bentur et al, 2000) on rice, and Pectinophora gossypiella (Tabashnik et al, 2000) on cotton. These studies yielded contrasting results, with the frequency of Bt resistance alleles ranging from 10 À4 to 10 À1 .…”