No. 17/July 18, 1997
European Corn Borers in Bt-Corn
During the past couple of weeks, several of us have been walking through fields of Bt-corn and looking for evidence of injury caused by first-generation European corn borers. For the most part, whorl-feeding injury has been difficult to find. However, in some fields, plants with whorl-feeding injury and living, breathing corn borers have been found. The Bt endotoxin kills young instars but is not effective against older instars. However, some of the corn borer larvae found in Bt-corn had reached their third instar of development.
Does this mean that the Bt-corn is not working? The answer to this question probably is "no." Representatives with the seed corn companies that have sold Bt-corn hybrids will tell you that expression of the Bt endotoxin probably will not occur in 100 percent of the plants. I have heard from some sources that a lack of expression of the endotoxin may occur in as many as 5 percent of the plants. This obviously would not result in an economic problem. However, growers may be expecting to observe 100 percent control.
As folks walk through Bt-corn, maybe to assess its performance, finding a few injured plants should not be alarming. However, if live larvae are found, the seed company should be contacted. The seed company representatives should be able to determine whether the plant(s) on which the borers survived are expressing the endotoxin. When we have tested plants on which borers survived in Bt-corn fields, we have found that the plants were "negative" (not expressing the endotoxin). Thus far we have found only one corn borer larva that survived on a Bt-expressing plant, and we have not deduced the reason for this.
I have been very impressed with the performance of Bt-corn against first-generation corn borers this year. However, we urge anyone who has planted Bt-corn to scout the field for assessing the level of performance. Let us know what you find.
Kevin Steffey, Extension Entomology, (217)333-6652