Cabbage loopers’ path to Bt resistance

Although insects becoming resistant to Bt is not new, recent news coverage of the first Bt resistance in corn rootworms showing up in the Midwest has spurred research into understanding how insects develop resistance to the natural toxin. Researchers have been trying to identify what mechanism causes the resistance in insects. For the first time, researchers have discovered how cabbage looper caterpillars develop resistance to Bt. This discovery may have implications for research with other insects.

According to an article from Cornell University, “Under normal circumstances, the Bt toxin Cry1Ac, which is a caterpillar-specific toxin, binds to an enzyme called APN 1 along the wall of the insect's gut, where the toxin destroys the gut lining. But when cabbage loopers develop resistance, APN 1 significantly decreases while another aminopeptidase, APN 6, which does not bind to Bt, significantly increases, allowing the insect to properly digest food and Bt without harm.”

"If an insect loses an aminopeptidase N, you will expect to see a negative effect on the physiology of the insect gut," said Ping Wang, associate professor of entomology and senior author of the paper published online in the Aug. 15 issue of the Proceedings of the National Academy of Sciences. Kasorn Tiewsiri, a postdoctoral associate in Wang's lab, is the paper's lead author.

"To compensate for the loss of the enzyme APN 1, the activity of APN 6 jumps up high, and that allows the insect to perform a normal digestive process, where Bt no longer binds to the gut," Wang added.

Next, the researchers plan on trying to identify which gene mutates in the Bt-resistant insects, how that gene controls the expression of targeted proteins, and uncover resistance mechanisms to other Bt toxins, as many varieties are used in agriculture. The researchers hope their studies will lead to new management strategies for Bt-resistant insects, Wang said.

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