EPA approves a new type of refuge for a Bt crop

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

A few people contacted me to ask about a recent EPA decision to allow a “natural” refuge for resistance management in Bt cotton. Because there may be some confusion about how this relates to Bt corn, here are some details of the story.

Bollgard II is a stacked, transgenic cotton that produces two different Bt toxins, Cry2Ab2 and Cry1Ac. These Bt toxins control two important cotton pests, bollworm and tobacco budworm. In the past, producers using Bollgard II cotton were required to plant a certain percentage of acres to non-Bt cotton to manage insect resistance. In “government-speak,” this type of man-made refuge is called a “structured” refuge. Most of you are familiar with structured refuges, because they are also required when you plant Bt corn for European corn borer or corn rootworm control.

The Bollgard II cotton situation is a bit different from Bt corn. Since Bollgard II produces two Bt toxins, its efficacy (kill off pest caterpillars) is very high. If a caterpillar survives one Bt toxin, it is presumably killed by the other toxin. The assumption is that the chance of producing a resistant moth is extremely low, less than for transgenic cottons producing only a single Bt toxin. Also, bollworm and budworm have many alternate host plants, other crops as well as weeds. The assumption is that caterpillars growing on the other hosts are not exposed to Bt, thus the alternate crops and weeds act as a “natural” refuge for susceptible insects. Based on the high efficacy and alternate hosts, Monsanto petitioned EPA to drop the structured refuge requirement in favor of a natural refuge. EPA approved this change for Bollgard II cotton planted from east Texas into the mid-Atlantic region. In other words, cotton growers in these states do not have to plant a refuge. However, the structured refuge remains in effect for Bollgard II planted in the west.

What does this mean for Bt corn refuges? Nothing. The “natural” refuge only applies to one type of cotton planted in the eastern United States. Structured refuges (20 percent non-Bt corn in Michigan) are still required for corn borer and rootworm corn to manage insect resistance. The refuge needs to be planted adjacent to (rootworm) or within a half mile (corn borer) of the Bt field. Ideally, refuge and Bt acres should be planted at the same time and managed in a similar fashion. However, the use of a natural refuge in cotton may spur discussion and research into natural refuges for other crops.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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