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New field-corn hybrids aid in pest management

"Killing two birds with one stone" has long been a metaphor for efficiency. A 1996 IPM demonstration project on field-corn production has shown that a biopesticide known as Bt (Bacillus thuringiensis) can vastly improve the management of two pests: the insect European corn borer (ECB) and anthracnose, a fungal disease that is commonly associated with ECB presence.

Field corn is the most valuable crop in New York, with more than one million acres in production annually. It is grown as feed for livestock, both as grain and silage. Field corn is threatened each year by ECB, which weakens plants by chewing holes in their leaves and stalks. Once ECB larvae tunnel into corn plants, they are protected from insecticide treatments, making control a challenge. Management of ECB is further complicated by the presence of ECB strains that produce two generations per season. The more generations there are, the longer the potential period of injury to the corn. Not only can ECB feeding weaken plants and decrease yield and standability, but the wounds it causes provide openings for invasion by the stalk rot phase of the disease organism anthracnose. When both of these pests are present, the losses are much greater than those from either pest alone. Grain yield reductions of 12 to 46 percent have been associated with this pest duo in New York.

The good news is, first, that ECB is not a significant problem for every corn field. Second, Bt corn hybrids are now available as a new management tool for those fields that do incur serious yield losses due to ECB. These hybrids have a gene from the bacterium Bt built into their own genetic material. They produce a protein that is toxic to ECB larvae. The obvious result is less foliar feeding on Bt corn than on non-Bt plants and therefore less opportunity for invasion by anthracnose stalk rot. The toxic effect of Bt corn is very pest specific. It is toxic neither to mammals nor to the environment as a whole.

Data from the IPM-funded project show that ECB feeding injury was near zero for all of the Bt hybrids, and that symptoms of anthracnose stalk rot occurred much less frequently in the Bt hybrids than in the non-Bt's - almost a five-fold difference. The link between these two pests was verified once more by the fact that 82 percent of the plants with stalk rot symptoms also had stems wounded by ECB. Finally, silage yields were higher for three of the four Bt hybrids than for their non-Bt counterparts. Highest overall yields were obtained from a standard non-Bt hybrid, illustrating the fact that hybrids need to be selected based on factors such as production potential and standability, not just resistance to ECB.

Researchers do have a concern about Bt as well as enthusiasm for its possibilities. Given sufficient exposure to Bt, corn borers are likely to adapt to it and become immune, as a number of other insects have done. Keith Waldron, livestock and field crops IPM coordinator, points out that "The development of Bt-resistant ECB populations would be significant not only for field corn but for wheat, soybeans, snap beans, potatoes, onions, apples, and nearly 200 other plant species that are also attacked by ECB."

Cornell entomologist Elson Shields believes that the best way for growers to use Bt corn is to plant it on 70 to 75 percent of their acres. "The remaining percentage should be planted to corn that is susceptible to ECB," says Shields. "This creates a 'refuge' in which some borers are not exposed to Bt and do not develop resistance. Their populations mix with those who have survived feeding on Bt corn, and the resistance gene is diluted." Current recommendations for those using Bt hybrids are to plant non-Bt hybrids either in 20 to 30 percent of all corn fields or in the border rows of all Bt-hybrid fields.

Of the four Bt hybrids used in this study, two were commercially available as of 1996, and a third will be available sometime this year.