Bt-Corn and European Corn Borer

Bt-Corn for Control of European Corn Borer
Following is an explanation of Bt-corn and some suggestions for managing its use. Please consider these suggestions seriously before you decide to plant or recommend the sale of Bt-corn.

What is Bt-corn?
The "active ingredient" of Bt-corn is derived from a naturally occurring soilborne bacterium, Bacillus thuringiensis (Bt), that is found worldwide. Bt produces a crystalline protein (endotoxin) that is toxic to specific groups of insects. The endotoxin is a stomach poison that must be ingested by the insect, after which the insect dies. The Bt subspecies kurstaki is toxic to certain caterpillars, including the European corn borer. After a caterpillar ingests the protein, the crystal breaks down and releases a toxin that attacks the gut lining. The insect stops feeding within a few hours, and it dies within a couple of days.

Modern gene transfer techniques have been used to develop corn plants that contain the endotoxin-producing gene taken from Bacillus thuringiensis. The Bt gene has been inserted directly into the corn ge-nome. When a young European corn borer larva eats plant tissue that contains the Bt endotoxin, it dies. The presence of the toxin produced by Bt in corn provides nearly season-long protection against European corn borers. Bt-corn offers an opportunity to control one of our most economically damaging corn insect pests without the use of conventional insecticides.

Is Bt-corn effective against European corn borers?
Research trials to test the efficacy of Bt-corn against European corn borers have been conducted for several years in many corn-producing states, including Illinois. Some Bt-corn hybrids have provided greater than 95 percent reduction in stalk damage caused by corn bor-ers. However, the effectiveness of Bt-corn varies among hybrids. Because the level of expression of the Bt protein declines in Ciba’s and Mycogen’s hybrids after pollination, some second-generation borers may survive and tunnel in the stalks, shank, and ears. The YieldGard Bt gene protects the plant from corn borers for a longer period of time. Bt-corn also has yielded well, both in the presence and absence of corn borers.

Will Bt-corn control other corn insect pests?
Research results from the western Corn Belt states indicate that several Bt-corn hybrids offer excellent control of the southwestern corn borer, an insect that occurs occasionally in southern Illinois. Limited research suggests that although corn earworms are affected by Bt-corn, the level of control may not be acceptable. However, Bt-corn affects the growth and development of corn earworms, so the overall impact of Bt-corn on corn earworms may be population reduction.

Currently available Bt-corn hybrids are not effective against cutworms and stalk borers, caterpillars that may be controlled by other Bt-corn hybrids developed in the future. Additionally, Bt-corn developed from the subspecies Bt kurstaki does not control aphids, corn rootworms, or other pests that are not caterpillars.

Does Bt-corn affect beneficial insects?
Bt-corn developed from the subspecies Bt kurstaki is highly selective against caterpillars, so it does not directly affect the predators found in cornfields. In studies conducted thus far, Bt-corn has had no direct effects on lady beetles, green lacewing larvae, spiders, minute pirate bugs, and parasitic wasps. Any indirect effects on populations of beneficial insects caused by the removal of corn borers as a food source for predators and parasitoids remain unknown.

Is Bt-corn safe, or does it pose any hazards to the environment?
Bacillus thuringiensis is not toxic to humans, other mammals, birds, fish, or bees. The use of Bt in formulated insecticides (for example, DiPel) has been registered for many years. Microbial insecticides that contain Bt are suggested for control of European corn borers in seed-corn fields where detasselers will be working. Because of its safety and the rapid degradation of Bt toxins on plant surfaces and in Bt-corn as the plants die, Bt is exempt from the need to establish residue tolerances for all raw agricultural commodities.

Economics of Bt-corn
Loss estimates suggest that European corn borers cause 5 to 7.5 percent yield reductions in the United States annually. Bt-corn should prevent these yield losses. However, because the occurrence of European corn borers cannot be predicted accurately from year to year, an investment in Bt-corn is an economic decision that each grower must make. In some years, the premium paid for Bt-corn seed will not be returned in yield benefits if corn borers are not present. However, in years when corn borer infestations are moderate to heavy, the grower who plants Bt-corn will more than recover his or her investment.

Corn growers should not plant Bt-corn on all acres. A grower may choose to plant Bt-corn in fields that are planted early or late to offset the risk posed by either first- or second-generation corn borers, respectively.

Resistance Management and Bt-corn
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. Intense selection pressure by any insect-killing agent often results in the development of an insect population that is resistant to that killing agent. By repeatedly exposing a lab colony of European corn borers to Bt, researchers at the University of Minnesota already have created a population of corn borers that is resistant to Bt.

If Bt-corn hybrids are planted widely, European corn borer populations eventually will develop resistance to this very specific insect toxin. Consequently, producers who grow Bt-corn should implement a resistance management plan to slow down the potential onset of resistance. Maintaining "refuges" where corn borers 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 percent 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 refuges, 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 or a designated percentage of rows of non-Bt-corn throughout the field.

The amount of corn that should be used as a refuge within a field or area is not known. However, a managed refuge of 5 to 40 percent may be necessary, depending upon geographical location and the presence or absence of other refuges (wild hosts and non-Bt crops). Throughout most of the Midwest, a 25 percent refuge of non-Bt-corn likely will be necessary to delay the onset of corn borer resistance to the Bt toxin.

Monitoring Bt-corn fields
Any grower who plants Bt-corn should monitor the fields for evidence of injury caused by European corn borers. Remember, some leaf feeding is expected; the borers must eat the leaf tissue before they die. However, any evidence of unacceptable levels of injury should be reported immediately to the seed-corn company from which the seed was purchased. Representatives from these companies are committed to sampling any fields that show evidence of corn borer injury greater than expected. They should be able to determine whether the plants affected are expressing the Bt gene or whether the corn borers have developed some level of resistance to the Bt gene. If corn borer resistance to Bt occurs, alternate management tactics will be suggested and all growers in the area surrounding the initial detection will be notified.

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