Modeling insect resistance to transgenic corn and cowpea

Insect resistance management (IRM) is implemented to protect the efficacy of commercial transgenic crops expressing Bacillus thuringiensis toxins because the benefits of the pest susceptibility to Bt toxins is considered as a common good. For my dissertation research, I built four species-specific, simulation models to study the evolution of resistance by four insect pests of corn or cowpea to transgenic crops to 1) forecast the durability of traits in transgenic crops, 2) study the effects of biological, ecological factors on resistance evolution, 3) investigate potential concerns in the use of transgenic cowpea for controlling an insect pest of stored products, and 4) provide simulation models for future research in IRM.;A mathematical model with processes reflecting larval mortality resulting from feeding on cross-pollinated ears or Bt ears of corn was used to analyze the risk of evolution of Cry-toxin resistance in Ostrinia nubilalis. In the simulations, evolution of resistance was delayed equally well by both seed-blend and block refuges with the same proportion of refuge. The results showed that Bt-pollen drift has little impact on the evolution of Bt resistance in O. nubilalis. The study suggested that stakeholders should examine toxin-survival rates for insect pests and take into account that instars may feed on different parts of Bt corn.;An emergence delay and female-skewed-sex-ratios among adults of Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) from Bt corn have been reported in field studies. A simulation model was used to study the effect of a maturation delay and a female-skewed-sexratio for D. v. virgifera emerging from Bt corn on the evolution of Bt resistance. Early emergence of resistant beetles from Bt corn accelerates evolution of Bt-resistance.;The cowpea weevil, Callosobruchus maculatus F. (Coleoptera: Bruchidae), can cause up to 100% yield loss of stored cowpea seeds, Vigna unguiculata (L.) Walp, in a few months in West Africa. Genes expressing toxins delaying insect maturation (MDTs) are available for genetic engineering. A simulation model was used to investigate the possibility of the use of MDTs to manage C. maculatus. Specifically, I studied the effect of transgenic cowpea expressing a MDT, an insecticide, or both, on the evolution of resistance by C. maculatus at constant temperature. Transgenic cowpea expressing a non-lethal MDT causing 50% maturation delay did not prevent 98% yield loss by C. maculatus for one year. Transgenic cowpea expressing a lethal MDT causing 50% maturation delay and 90% mortality protected 80% of yield for one year at 25°C, but its durability was only three years. Mortality caused by a maturation delay improves the efficacy of transgenic cowpea expressing only a lethal MDT. I concluded that transgenic cowpea expressing only a MDT has little value for managing C. maculatus. The resistance by C. maculatus to transgenic cowpea expressing only an insecticide rapidly evolves. Stacking a gene expressing a non-lethal MDT and a gene expressing an insecticide in transgenic cowpea did not significantly improve the durability of an insecticide, but stacking a gene expressing a lethal MDT and a gene expressing an insecticide in transgenic cowpea significantly improved the durability of an insecticide and a MDT.;Major resistance alleles to Cry1Ab corn in the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), were first detected in a field population in northeast Louisiana in 2004, and the estimated frequency of the resistance allele was 0.0177 in populations in Louisiana in 2009. I used a demographic model to study the evolution of resistance by D. saccharalis to transgenic corn expressing one or two Bt proteins. An increase in the proportion of refuge did not always improve the durability of Bt corn if there were sugarcane, sorghum, or rice in the agro-ecosystem. A low frequency of inter-field movement of moths delayed the evolution of resistance to single-protein or pyramided-Bt corn when the proportion of refuge was 20%, but it accelerated Bt-resistance evolution when the proportion of refuge was 50%.;The three major challenges were 1) low susceptibility of insect pests to Bt toxins expressed in singletoxin- Bt corn, 2) lack of alternative IRM strategies, and 3) lack of knowledge about the biology and behavior of an insect pest. IRM and integrated pest management (IPM) should be considered together because robust IRM strategies are complemented by good IPM. Implementing principles of IPM will reduce unnecessary exposure of PIPs in insects, which will improve the management of the pest susceptibility to PIPs.