Last December, a seed company representative and I were in the farm office of a large dairy farm, far from my northern N.Y. home base, discussing corn hybrid selection with the farm owner and his crop manager. The seed company rep was going over the various hybrid options, including the insect refuge requirements when using Bt hybrids. The farmer then asked, "What's a refuge?" - much to the shock of the seed company representative and I. We turned to the crop manager, who shrugged and said he didn't know anything about refuge requirements either.
Now, this farm grows over 2,000 acres of corn, and had almost certainly planted Bt hybrids in the past, so there's no excuse for their ignorance of insect refuge requirements. (Note the word: "requirements," not "suggestions.") The farm had been growing hybrids from a different seed company, so I'd like to give the farmer - and the seed company - the benefit of the doubt and assume that the farm planted the required acreage of non-Bt corn hybrids even if he wasn't aware of doing so. But we're getting ahead of ourselves.
Corn rootworm control options
A big advantage of Bt events for corn rootworm control is that when used correctly they provide a higher level of control than pesticides. Granular insecticides as well as the highest rates of company-applied seed treatments provide some protection against rootworms, but they only protect a portion of the corn root system. That's why even with the use of insecticides, corn yield decreases as the population of rootworms increases. On the plus side, entomologists say that there's never been a confirmed case of corn rootworms developing resistance to any of the several soil insecticides registered for soil insect control in corn. The toxin produced by the Bt-rootworm event is in every part of the corn plant's root system, so higher levels of rootworm control are now possible. However, with this comes the need to leave some rootworms to complete their life cycle; that's the best way (probably the only way) to prevent the development of Bt-resistant rootworms.
Insect refuges are an essential part of insect resistance management (IRM). Planting a refuge is required by law through the U.S. Environmental Protection Agency (EPA) as a condition of Bt corn hybrid registration. At the time of purchase, farmers enter into a contractual agreement with the company supplying the Bt corn hybrid. This agreement obligates the farmer to plant the appropriate insect refuge. The size of the refuge depends on the particular Bt "event" - for control of corn rootworms, corn borers or both. It also depends on whether cotton is grown in the region, with much stricter refuge requirements in cotton country. The specifics of refuge plantings are too complex to discuss here; this information is provided by the seed company supplying the Bt corn hybrids. The objective of insect refuges: The number of the "target" insect pests capable of surviving the Bt toxin within a treated field is very small, while the number surviving in the untreated refuge is very large. The number of target insects surviving in the refuge overwhelms the very few insects - in this case, corn rootworms - that may have survived the Bt toxin by mating with them and producing Bt-susceptible offspring. This is why the Bt field and the refuge area must be planted in close proximity, with the maximum allowable distances and planting particulars clearly stated in the IRM agreement.
The temptation may be for some farmers to assume that their neighbors will plant enough non-Bt corn that they can ignore the IRM requirements. Although not right, this may have worked when Bt corn hybrids were much less common, but as farmer use of Bt hybrids increased this has become an ineffective means of providing an insect refuge. You may be planting all Bt hybrids assuming that your neighbor will be the responsible party, while he's thinking exactly the same thing.
To date, there haven't been any confirmed cases of Bt-resistant corn rootworms (either the Northern or Western species) or European corn borers in the eastern U.S. There are, however, confirmed instances of Bt-resistant corn rootworms in the Corn Belt, with the first cases in Iowa. Insect resistance to Bt corn hybrids isn't a theory, it's a fact. There also have been some reports of Bt-resistant corn borers in the Midwest, but these occurrences seem to come and go and aren't as pronounced as the Bt-resistant corn rootworms. Corn borers aren't nearly as serious a problem in the East as they are in the Corn Belt. The threshold for corn borers is one per plant, something we seldom see in our cornfields. Often the problem appears worse than it really is: Some years ago we did a trial at Miner Institute (Chazy, N.Y.) comparing a Bt hybrid and its isoline. (The isoline was the same hybrid minus the Bt-corn borer gene.) Corn borers appeared to be a serious problem, as individual plant examinations of hundreds of plants determined that 46 percent of the plants had been infested by at least one corn borer. Meanwhile, absolutely no plants were infested in the Bt hybrid. However, when we measured silage yields there was no significant yield difference between the two hybrids. In looking at the two plots (which were replicated several times) you'd certainly have thought there would be since there were a number of broken stalks in the non-Bt hybrid. While it's possible that this level of corn borer feeding could eventually have had an adverse impact on the crop through lodging losses and/or increased mold leading to possible mycotoxin problems - we didn't measure these - we were surprised that there was no difference in yield.
Refuge in a Bag
The latest attempt to provide insect control while avoiding resistance problems is "Refuge in a Bag." A small amount of non-Bt seed, normally 5 to 10 percent, is included in each unit of Bt seed corn. This may not fit every field situation, but the idea of "fill it and forget it" will appeal to a lot of farmers. Farmers interested in this technology should discuss it with their seed company representatives, including the possibility that a separate refuge may still be needed in some cases. Insect resistance management is serious stuff and should not be taken lightly by farmers planting Bt corn hybrids.
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has a written our Forage column for over 13 years and has been an expert contributor on a number of other topics.