Submitted to: Extension Publications
Publication Type: Other
Publication Acceptance Date: 6/10/2013
Publication Date: 6/12/2013
Citation: French, B.W. 2013. Corn rootworms and Bt resistance. South Dakota State University Extension iGrow Publications. Available: http://igrow.org/agronomy/corn/corn-rootworms-and-bt-resistance/.
Technical Abstract: Corn rootworms have been a major pest of corn for many years. As their name suggests, corn rootworms damage corn plants by feeding on the roots. Western and northern corn rootworms have overcome practices farmers use to keep their population numbers down, such as insecticides and crop rotation. Corn that is genetically modified to produce a Bt toxin has been widely used as a way to reduce crop damage by insect pests. A single Bt toxin was once thought to be the cure for corn rootworm problems. However, cornfields planted with such corn hybrids year after year have been showing heavy damage in South Dakota and all across the U.S. Corn Belt. It is not known how corn rootworms overcome the Bt toxins, but the ability to cancel out the Bt toxic effects may be partly due to selective (not random) mating among adult rootworms. Corn rootworms go through an elaborate courtship sequence before mating. Our research shows that female corn rootworms will choose not to mate with up to 80% of the initial sexual advances by males. Once a female decides to mate with a male they may stay together one to four hours. We have shown that female rootworms prefer to mate with males that are about their size. Corn varieties with a single Bt toxin are being discontinued. In their place are hybrids that produce two different Bt toxins. It is expected that overcoming the dual toxins will exert more pressure on the rootworms—that it will cost them more. For example, rootworms that are resistant to both Bt toxins may be smaller than their non-resistant neighbors. We think that if resistant males and females are smaller and similar in size, they may prefer to mate with each other rather than with larger non-resistant adults. If that happens, mating of Bt-resistant rootworms will speed up the spread of Bt resistance and make new Bt corn ineffective for corn rootworm control. To better predict how rootworms will respond to Bt corn with stacked traits, we are performing research to better understand how Bt resistance affects rootworm behavior and mating. Corn rootworms have already shown they can overcome a number of strategies to control them, including a single Bt toxin. Therefore it is very important to follow rotation guidelines and refuge requirements put forth by your extension specialists.