University of Illinois

No. 1/March 21, 1997

Planting Bt-Corn for Corn Borer Management: What Do We Need To Know?

We've talked about it, we've written about it, and now producers are ready to experience it for themselves. "It," of course, is transgenic Bt-corn, the corn hybrid s that have been genetically altered with a gene from the bacterium Bacillus thuringiensis, a gene responsible for the production of the toxic protein that kills some caterpillar pests. In 1996, Ciba and Mycogen introduced their Bt-corn hybr ids; they are joined in 1997 by Cargill, Golden Harvest, Northrup King, and Pioneer. Other companies, like DeKalb, are not far behind. This exciting new tactic for managing corn borers will change the way we think about insect management.

Many quest ions about Bt-corn remain to be answered. Throughout the winter meeting season, I have stated repeatedly that the technology is ahead of the research. The technology was registered so quickly (not necessarily a bad thing, in my opinion) that scient ists have not had enough time or space to address some burning issues associated with transgenic crops. To answer the questions "Will Bt-corn work?" and "Will Bt-corn show yield drag?", we have focused primarily upon the efficacy of B t-corn against European corn borers and the subsequent yield produced by the Bt hybrids. We shared some of our data with you in issues no. 24 and 25 of last year's Bulletin (November 8 and December 6, 1996, respectively). Undoubtedly you hav e been exposed to considerably more efficacy data from many other sources. However, two things became clear after 1996: Bt-corn works (it controls corn borers effectively), and the yields of Bt-corn hybrids do not always reflect the corn bor er control data. In several university plots throughout the Corn Belt in 1996, some Bt-corn hybrids were out-yielded by "conventional" hybrids that possess some tolerance to corn borer injury. We suggest that growers who are considering planting Bt-corn should evaluate a lot of data and ask a lot of questions before making a decision.

Growers also should realize that the premium ($) they pay for Bt-corn will be returned in yield benefits only if corn borers reach economic levels in the area. If populations of corn borers are low this year, the investment in Bt-corn will not pay for itself. At this time, we are uncertain whether or not economic levels of corn borers will develop in 1997. The data from last year's fall surv ey (issue no. 24 of this Bulletin, November 8, 1996) revealed a moderate level of infestation statewide (averages of 64% of the plants infested and 1.4 borers per plant). However, some areas, like northeastern Illinois, had a higher level of infest ation (averages of 84% of the plants infested and 1.8 borers per plant). Unfortunately, predicting this year's potential for corn borer infestations based upon last year's fall survey information is guesswork at best. We don't know when growers will plant corn this year, and we don't know what the weather will be like when corn borer moths begin laying eggs in June. Early planting and nice weather in June increase the potential for establishment of first-generation corn borers, but who knows what our seas on will be like.

Maybe more importantly, we don't know what level of mortality will be caused by the disease organism, Nosema pyrausta, which "overwinters" in corn borer larvae. Corn borers that were infected with this disease last year will survive the disease initially, but the organism stays with the insect as it develops from larva to pupa to adult, and the infected female infects her eggs. Consequently, the young instars that hatch also are infected, and they usually die. So from one inf ected borer, several borers are killed. (Not bad for a microorganism, huh?) We have some evidence that borers in some parts of Illinois might be heavily infected with Nosema. My graduate student, Maria Venditti, collected corn borer larvae from two fields in Sangamon County in mid-March, and her preliminary examinations revealed that a large percent of the borers were infected. When she completes her work, we'll report what we find.

The gist of this discussion is that the use of Bt-corn shoul d be a long-term investment rather than a one-shot experiment. Growers in areas that have economically damaginng levels of corn borers frequently (for example, 6 years in 10) might be inclined to plant Bt-corn to reduce the impact of corn borers. B y contrast, growers in areas where corn borers cause problems infrequently (maybe 1 year in 10) may not be quite as excited about paying for Bt-corn. Growers will be the best judges of the long-term benefits of Bt-corn.

What about the resistance issue? Although several entomologists and modelers have addressed the questions associated with the potential for corn borers developing resistance to the Bt endotoxin, field testing to verify the theories being generated has been limite d. Nevertheless, corn growers must accept the very real possibility that European corn borers may become resistant to Bt if Bt-corn is planted widely and resistance management tactics are not implemented. Consequently, producers who grow Bt-corn should implement a resistance management plan to slow down the potential onset of resistance. Maintaining "refuges" where corn bores are not exposed to the Bt toxin may be the most practical resistance management tactic. In theory, high doses of the Bt toxin in Bt-corn kill virtually 100% of the corn borers. However, if any borers survive, it is highly desirable to increase the odds that surviving moths (possibly resistant individuals) mate with moths emerging from refuges (susceptible moths).

Refuges include all fields of non-Bt-corn and many other species of plants (including several crops and weeds on which corn borers can develop. However, resistance management will almost certainly require management of r efuges, including entire fields of non-Bt-corn planted adjacent to Bt-corn specifically to provide a refuge for (and source of) susceptible corn borers. Other types of management of refuges may be a block of non-Bt-corn planted within a field of Bt-corn.

The amount of corn that should be used as a refuge within a field or area is not known. However, a managed refuge of at least 25% is recommended in Illinois and other Corn Belt states. Current thinking is that a larger re fuge could be treated with an insecticide (but not a Bt-based insecticide like DiPel) if corn borers reach outbreak proportions. We would discourage insecticide applications for corn borer control in smaller refuges.

As new data accumulate, w e'll keep you up-to-date. Entomologists and their graduate students throughout the country are working on many aspects of Bt-corn and corn borer management. Stay tuned as more information about this management unfolds.

Kevin Steffey, Extensio n Entomology, (217)333-6652