Submitted to: GM Crops
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2012
Publication Date: 7/1/2012
Citation: Luttrell, R.G., Jackson, R.E. 2012. Helicoverpa zea and Bt Cotton in the United States. GM Crops and Food Biotechnology in Agriculture and the Food Chain. 3(3):213-227.
Interpretive Summary: Commercial Bt cottons were rapidly adopted in the late 1990s because of problems with control of insecticide-resistant populations of tobacco budworm. These transgenic cottons have been highly effective against tobacco budworm and populations of tobacco budworm have declined. Bollworm is less susceptible to the Bt cottons, and bollworm has become more important as a caterpillar pest on Bt cottons. Effective management of bollworm on Bt cotton requires additional use of insecticide, and most Bt cotton in the southern U.S. is sprayed for control of bollworm on an annual basis. Bollworm populations have variable response to Bt toxins in resistance monitoring assays. Some scientists have suggested that this variability demonstrates evolution of field resistance to Bt in bollworm. Others disagree. Our summary of published information on field control indicates that bollworm populations required supplemental control with insecticides at first deployment of the commercial Bt cottons. From a practical pest management perspective, bollworm continues to be a concern for cotton production, but the intensity of the problem appears to be similar to that observed when Bt cotton was first introduced. The high variability in response to Bt toxins is a concern, and we encourage continued monitoring and observation of bollworm populations for elevated resistance to Bt toxins. Increased acreages of Bt corn may be an increased concern for Bt resistance as corn is the preferred host plant of bollworm, also known as corn earworm. Tobacco budworm populations are still present and continued susceptibility to the Bt toxins should also be routinely monitored.
Technical Abstract: Helicoverpa zea (Boddie), the bollworm or corn earworm, is the most important lepidopteran pest of Bt cotton in the United States. Corn is the preferred host, but the insect feeds on most flowering crops and wild host plants. As a cotton pest, bollworm has been closely linked to the insecticide-resistance prone Heliothis virescens, tobacco budworm. Immature stages of the two species are difficult to separate in field environments. Tobacco budworm is very susceptible to most Bt toxins, and Bt cotton is considered to be “high dose”. Bollworm is less susceptible to Bt toxins, and Bt cotton is not “high dose” for this pest. Bt cotton is routinely sprayed with traditional insecticides for bollworm control. Assays of bollworm field populations for susceptibility to Bt toxins expressed in Bt cotton have produced variable results since pre-deployment of Bt cottons in 1988 and 1992. Analyses of assay response trends have been used by others to suggest that field resistance has evolved to Bt toxins in bollworm, but disagreement exists on definitions of field resistance and confidence of variable assay results to project changes in susceptibility of field populations. Given historical variability in bollworm response to Bt toxins, erratic field control requiring supplemental insecticides since early field testing of Bt cottons, and dramatic increases in corn acreage in cotton growing areas of the southern U.S., continued vigilance and concern for resistance evolution are warranted.