Initial frequency of alleles for resistance to Bacillus thuringiensis toxins in field populations of Heliothis virescens

Abstract

The risk of rapid pest adaptation to an insecticide is highly dependent on the initial frequency of resistance alleles in field populations. Because we have lacked empirical estimates of these frequencies, population-genetic models of resistance evolution have relied on a wide range of theoretical estimates. The recent commercialization of genetically engineered cotton that constitutively produces an insecticidal protein derived from the biocontrol agent, Bacillus thuringiensis (Bt) has raised concern that we lack data needed to quantify the risk of insect pests such as Heliothis virescens rapidly adapting to this ecologically valuable class of toxins. By individually mating over 2,000 male H. virescens moths collected in four states to females of a Bt toxin-resistant laboratory strain, and screening F1 and F2 offspring for tolerance of the toxic protein, we were able to directly estimate the field frequency of alleles for resistance as 1.5 x 10(-3). This high initial frequency underscores the need for caution in deploying transgenic cotton to control insect pests. Our single-pair mating technique greatly increases the efficiency of detecting recessive resistance alleles. Because alleles that decrease target site sensitivity to Bt toxins and other insecticides are often recessive, this technique could be useful in estimating resistance allele frequencies in other insects exposed to transgenic insecticidal crops or conventional insecticides.

Figure 1

Histogram of the percentage of larvae from each of the first 262 successful single pair crosses that grew beyond second instar in 7 days of rearing on an artificial diet containing 0.064 μg/ml CryIA(c). Depicted only by arrows are the three families from the later 763 crosses with atypically high percentages of large larvae.

Figure 2

Histograms depicting the frequency of F₂ larvae in given size classes (natural log) after rearing on 0.064 μg/ml CryIA(c). (A) F₂ larvae resulting from all single pair crosses of F₁ larvae that had grown well on Bt-containing diet. (B) F₂ larvae resulting from all single pair crosses of F₁ larvae that had grown poorly on Bt-containing diet.