Date: 3/31/99

Time: 8:30

Type: Symposium

Number: 230

Value of in-field monitoring of Bt corn for insect resistance: Practical and statistical considerations

*R.C. Venette, W.D. Hutchison and D.A. Andow, Department of Entomology, University of Minnesota, St. Paul, MN 55108
Contact e-mail: venet001@tc.umn.edu

Selection for resistant individuals in populations of targeted pests threatens to undermine the utility of transgenic insectidal crops, especially crops expressing toxins from the bacterium Bacillus thuringiensis (Berliner). Effective resistance management programs depend on early detection of changes in the frequency of resistant individuals. A novel in-field screen based on sentinel plots of Bt- and isogenic, non-Bt sweet corn can be used to estimate resistance frequencies in populations of Lepidopteran pests. The in-field screen has several advantages including exposure to a large number of feral insects, discrimination of resistant individuals based on Bt dosages encountered in the field, incorporation of natural and Bt-induced mortality factors, simultaneous monitoring for more than one insect species, and ease of use. Frequencies of resistant individuals are determined by comparing the density of larvae on Bt plants to the density of larvae on non-Bt plants. If fewer than 1 in 1,000 individuals are resistant, binomial probability theory indicates that /3,000 larvae must be screened to have a 95% chance of finding at least one resistant individual. As the density of larvae on non-Bt corn increases, the number of samples that must be collected decreases. Bayesian statistics provide expected frequencies of resistance (±95% credibility intervals), even if no resistant larvae are found. Sentinel plots established in 1997 and 1998 in southern Minnesota indicate that the expected frequency of resistant individuals among populations of Ostrinia nubilalis (Hübner) is ~6x10-4.

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