Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2008
Publication Date: 8/1/2008
Citation: Blanco, C.A., Perera, O.P., Gould, F., Sumerford, D.V., Hernanderz, G., Abel, C.A. 2008. An Empirical Test of the F2 Screen for Detection of Bacillus thuringiensis-Resistance Alleles in Tobacco Budworm (Lepidoptera: Noctuidae). Journal of Economic Entomology. 101(4):1406-1414. Interpretive Summary: The preservation of transgenic Bacillus thuringiensis (Bt) expressing cotton use depends on its effectiveness against important pests such as tobacco budworm. This pest has developed resistance to almost all the insecticides aimed for its control, therefore, it is expected that it might also develop resistance to Bt. Since 1995, the Southern Insect Management Research Unit in Stoneville, MS has conducted annual screens to assess the susceptibility of this insect to Bt. Numerous methodologies have been implemented with the aim of producing the most reliable results. The currently-implemented second generation (F2) screen is the most reliable method to detect resistant alleles in feral populations. However, this test has not been validated in tobacco budworm. This study shows that the F2 screen is a feasible method for detecting Cry1Ac resistance and it shows important logistical considerations for its implementation.
Technical Abstract: Insects exposed to genetically-modified crops expressing Bacillus thuringiensis (Bt) toxins are under intense selection pressure that could result in widespread Bt resistance. Screening for early indications of Bt resistance developing in targeted Lepidoptera is conducted in many of the regions where genetically-modified cotton and corn have been commercialized. A few insect species under artificial selection pressure have developed Bt resistance based on single recessive alleles. Availability of laboratory-maintained homozygous lines with such resistance offers the possibility of testing theoretical predictions related to detection of recessive Bt-resistant alleles in field populations based on a second generation (F2) screen. In this study we took advantage of a known mutation that confers Cry1Ac-resistance to Heliothis virescens. By creating single-pair families from mating a heterozygous Cry1Ac-resistant moth with a Cry1Ac-susceptible moth, we simulated the most common genotype when Bt-resistance alleles are at low frequency in the field. The second generation (F2) neonates of single-pair families were screened daily with diagnostic concentration bioassays. Cry1Ac-resistant homozygous larvae were detected, but the proportion of resistant larvae was generally below the theoretical expectation and was influenced by the moth F1 sib mating density and by the day of oviposition of F2 eggs. Logistical considerations such as F1 sib mating density and F2 neonate screening are important for the successful implementation of a reliable method.