No. 22/August 29, 1997
Preliminary Results of 1997 Summer Survey for Bt-Resistant European Corn Borers
Earlier this summer we asked growers for information on their Bt-corn fields and for access to those fields so that we might begin what should be periodic surveys of corn borer resistance to Bt. We received plenty of responses and were able to get in several days of sampling in the latter part of June and early July. Here's a quick explanation of what we were trying to do and a preliminary report of our findings.
First, let's make clear that in the 1997 summer survey we did not try to address the development of resistance as a result of repeated, low-level doses of exposure to Bt. Resistance to Bt, if it develops, may result from several generations of repeated selection of individual corn borers that are able to survive slightly higher doses of Bt than "average" corn borers. These "slightly resistant corn borers" (really just the upper end of the mortality curve in an existing population) might survive in corn as Bt concentrations drop in late season or in specific plant tissues that have low levels of Bt toxins. As time and corn borer generations pass, these slightly resistant corn borers will be more and more likely to mate with each other because the more susceptible insects that might otherwise be mates with them will have been killed. The results of this selective breeding program would be a "concentration" of resistance genes in their offspring and perhaps an increased level of resistance that might cause control failures. Although we have started some research on this path, and other researchers around the Midwest have done much more, this scenario was not the topic of our summer survey efforts.
Another possibility that might lead to resistance problems is that extremely rare individual corn borers are already present and able to survive even the very high doses of Bt toxins that are in Bt-corn plants. If this is true, field problems with resistance might develop from the increasing prevalence or frequency of such individuals instead of or in addition to the gradual selection process described previously. It is this possibility (that borers able to survive very high doses of Bt toxins in Bt-corn plants already exist) that led us to the "needle-in-the-haystack" survey this summer.
With the help of more than 30 growers, we identified and surveyed Bt-corn fields in 23 counties. We walked each field to look for corn borer injury and surviving corn borers at a time when most of the first-generation larvae should have reached second or third instar; this timing was meant to allow them to be killed by the Bt if it was present and the larvae were susceptible. To conduct the survey, we walked fields, examining four to six rows per sampler in each pass through the field. Although the area of the fields we worked in totaled more than 2,200 acres, our crews actually covered about 325 acres in this row-by-row manner. Let's assume for a moment that the plant population in those 325 acres averaged about 28,000 plants per acre and that the average first-generation infestation in those fields would have been about 0.5 larva per plant had the borers not been killed by Bt (an infestation level based on surveys of nearby non-Bt fields on the same days). The 325 acres that we surveyed should have been home to roughly 4.5 million corn borers (28,000 x 325 x 0.5). So what we were able to do was to examine the results of a vast screening program that provided a chance to detect rare, resistant corn borers, even if they were present at a one-in-a-million frequency.
Did we find corn borers surviving in the Bt-corn fields? Yes, more than 200. However, all but two were on plants that were not producing Bt toxin. Based on data from the seed companies and previous observations, we expected that at least a small portion of the plants in a Bt-corn field would not be producing Bt toxins. To avoid making an incorrect conclusion that any surviving larvae were Bt-resistant, we used a Gene-Check strip (ELISA assay) supplied by Monsanto to determine whether or not the infested plants were Bt-positive. All but two of the infested plants were Bt-negative: The surviving larvae were not necessarily resistant to Bt; they were simply lucky that their mothers placed them (as eggs) on Bt-negative plants. Note that the frequency of Bt-negative plants in Bt-corn fields was extremely low, at least based on the number of infested plants that we were able to detect.
So what about the two larvae from Bt positive plants? One was parasitized at the time of collection by Macrocentrus grandii ... now a dead end for our interest in resistance. The other, collected as a second instar and placed on artificial diet on June 29, is now, on August 22, a fifth instar. Our intent is to rear this insect to the adult stage, mate it with a lab-colony moth, and then continue to rear subsequent generations for crosses and bioassays. These steps will allow us to determine whether its survival is the result of Bt resistance or another factor: It could have moved onto the Bt-positive plant just before we collected it; therefore, it may not have fed on the foliage expressing the Bt toxin. (We don't think this is true, but it's possible.)
Those who are familiar with corn borer biology probably noted an inconsistency in the preceding paragraph. The prize, needle-in-the-haystack larva that we collected on June 29 still has not pupated and reached the adult stage. It should have done so in late July. The reasons for its failure to develop at a normal rate may be related to whatever unusual trait allowed its survival on a Bt-positive plant; its delayed development may instead be related to infection by Nosema pyrausta, the microsporidian pathogen that helps regulate corn borer populations in the Midwest.
The bottom line: We found a needle in the haystack, but we don't know yet if it's really the needle we were looking for. We'll keep you updated on our progress.
Rick Weinzierl, Christopher Pierce, and Kevin Steffey, Extension Entomology, (217)333-6651