ECOLOGICALLY-BASED MANAGEMENT OF INSECT PESTS OF CORN, WITH EMPHASIS ON CORN BORERS, ROOTWORMS, AND CUTWORMS
Location: Corn Insects and Crop Genetics Research
Title: Modeling the impact of cross-pollination and low toxin expression in corn kernels on adaptation of European corn borer (Lepidoptera: Crambidae) to transgenic insecticidal corn
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 25, 2011
Publication Date: February 1, 2012
Citation: Kang, J., Onstad, D.W., Hellmich II, R.L., Moser, S.E., Hutchison, W.D., Prasifka, J.R. 2012. Modeling the impact of cross-pollination and low toxin expression in corn kernels on adaptation of European corn borer (Lepidoptera: Crambidae) to transgenic insecticidal corn. Environmental Entomology. 41(1):200-211.
Interpretive Summary: Successful control of insects by Bt (Bacillus thuringiensis) corn has many scientists concerned that without adequate resistance management implementation, insects may become resistant to Bt toxins. A resistance management strategy has been developed that uses high-dose corn and refuge to produce insects that are not exposed to Bt corn. The keys to this strategy are that rare resistant insects mate with plentiful susceptible insects (produced from the refuge) and their progeny (i.e., heterozygous insects) do not survive on high-dose Bt corn. This process essentially dilutes the resistance genes and maintains a population of susceptible insects. This strategy could be compromised, however, if larvae move among Bt and non-Bt corn plants, which may occur when Bt and non-Bt seed are blended in the same field, or if larvae feed on low-dose tissues from non-Bt ears pollinated with Bt pollen. In this study a mathematical model was used to compare different refuge strategies for seed mixtures and incorporates the effect of pollen drift for the important corn pest, European corn borer. One-toxin and two-toxin Bt corn (a.k.a. pyramided corn) were modeled. Pyramided corn has redundant insect control properties and is predicted to delay corn borer resistance to Bt corn. Results show that Bt-pollen drift (i.e., non-Bt ears pollinated with Bt pollen) has little impact on corn borer resistance to Bt corn. However, low-toxin expression in two-toxin Bt corn ears can reduce Bt corn durability. In many cases a 5% mixed-seed refuge for the two-toxin corn was as or more effective than a 20% block refuge for the one-toxin corn. In addition, the toxin-survival rate of heterozygous larvae in Bt corn ears has more impact on evolution of Bt resistance in corn borer than parameters related to larval movement. Thus, stakeholders should focus attention on toxin-survival rates for insect feeding, which may differ on different parts of Bt corn and may change with plant age or growth stages. This information is useful for industry, government, and academic stakeholders interested in delaying insect evolution of resistance to Bt crops using insect resistance management plans.
We used a mathematical model with processes reflecting mortality of larval feeding on cross-pollinated ears in the refuge or on ears of Bacillus thuringiensis (Bt) corn to analyze the risk of evolution of Cry-toxin resistance in European corn borer, Ostrinia nubilalis. Our results showed that Bt-pollen drift has little impact on the Bt resistance in European corn borer. However, low-toxin expression in ears of pyramided transgenic corn can reduce the durability of pyramided transgenic corn. In some modeled scenarios with seed mixtures, the predicted durability of a 5% refuge and two-toxin transgenic corn was reduced to that of a 20% refuge and single-toxin transgenic corn. In addition, the model determined that the toxin-survival rate of heterozygous larvae in Bt ears has more impact on evolution of Bt resistance than the parameters related to larval movement to Bt ears or the toxin-survival rate of the homozygous susceptible larvae in Bt ears. Therefore, the toxin expression in ears of insecticidal corn hybrids should be high enough to kill most heterozygous larvae on Bt ears to delay resistance evolution in European corn borer. We suggest that stakeholders focus more attention on toxin-survival rates for insect pests feeding on different parts of Bt corn at different growth stages when selecting the minimum percentage of refuge corn.