Sex Life Of Corn Rootworm May Be Trouble For Stacked Bt Trait

Size matters to corn root worm beetles, says a USDA researcher in South Dakota, and stacked traits may affect size.

Published on: Jun 21, 2013

A USDA researcher in South Dakota is warning that new hybrids with stacked Bt traits may speed -- not slow down -- development of Bt resistant corn rootworm populations.

Bryan Wade French, a research entomologist with the USDA-ARS North Central Agricultural Research Laboratory in Brookings, S.D., says the unexpected result stems from corn root worm beetles mating habits.

"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," he writes in a recent post on igrow.com, South Dakota State University's ag news website.

A female corn rootworm beetle looks for a male of the same size to mate with. Bt resistant beetles are smaller than susceptible beetles, which increases the chances that two resistant beetles will get together and reproduce.
A female corn rootworm beetle looks for a male of the same size to mate with. Bt resistant beetles are smaller than susceptible beetles, which increases the chances that two resistant beetles will get together and reproduce.

"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…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.

Corn rootworms have been a major pest of corn for many years, French says.

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, French says.

Corn varieties with a single Bt toxin are being discontinued. In their place are hybrids that produce two different Bt toxins.

"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," French says.

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, French says.