No. 7/May 9, 1997
Summer Survey for European Corn Borer Resistance to Bt
Last fall we published a note in the Pest Management &;Crop Development Bulletin asking for permission to sample Bt cornfields for European corn borer larvae, and now we would like to offer a brief update on our research. Overall, we managed to check for corn borers in 44 Bt corn- fields and collected at least a few larvae from most of those. Our goals last fall were to (1) determine, at least in a preliminary manner, the extent of late-season corn borer infestation in Bt hybrids, and (2) collect corn borer larvae from infested plants to initiate colonies for studies of resistance. Following are our findings.
Most of the fields we sampled last year in October and November were located in the northern half of the state; most contained Ciba and Mycogen Bt hybrids; and almost all of them were infested to some degree with European corn borer larvae. You may already know that this result (borers infesting Bt corn in late season) was quite common around the Midwest last year, and that it is fairly well understood.
In general, the combination of transgenic technology and hybrids used by Mycogen and Ciba resulted in plants that slow and then stop their production of Bt toxin after flowering begins. As corn plants matured in late summer and fall last year, Bt toxin levels dropped, eventually reaching levels at which corn borer larvae were able to survive. So when we surveyed fields looking for larvae that had developed on and in Bt plants, we found some. However, the fact that Bt toxin levels drop (or disappear) in late season means that these larvae need not have been more resistant than normal to have survived on these plants. Nonetheless, it is possible that even the reduced levels of Bt toxins remaining in plants by late season resulted in some selection for low levels of resistance, and that the larvae we collected were among the most resistant in the overall population.
With that possibility in mind, we kept those larvae, "overwintered" them in refrigeration, and have begun the process of establishing lab colonies that we can bioassay to determine their "profiles" in terms of Bt susceptibility. (In bioassays, corn borer larvae are exposed to particular doses or concentrations of Bt toxin to determine how much is required to kill them.) Rearing and bioassay efforts will be ongoing for several months. The prevalence of Nosema pyrausta, a disease organism that infects and sometimes kills corn borers, in last fall's collections is complicating our efforts at rearing, because we need to establish a disease-free colony that can be used for reliable bioassays. In sum, the larvae we collected last fall have given us starts for laboratory colonies, and we are working toward the bioassays that will allow us to be sure that late-season infestations were not the result of resistance.
Last fall's field sampling and the bioassays that are to follow are funded in large part by a C-FAR grant. (C-FAR is the Illinois Council for Food and Agricultural Research.) As part of the research funded by that grant, our work in 1997 will include small plot evaluations of Bt hybrids and continued research on Bt resistance in the European corn borer. Our small plot evaluations will be straightforward assessments of the yield and insectcontrol performance of several companies' hybrids. We will not attempt to include all the possible Bt hybrids that are on the market or soon to reach the market (there are too many), but we will plant a range of Bt corns and publish our findings in the fall.
Although we see good reasons to establish and experiment on laboratory colonies to study resistance, we think that the most important step in resistance monitoring at this stage in Bt corn development is to search for the "needle-in-the-haystack" corn borer that might survive a full dose of Bt. To conduct this hunt, we plan to identify lots of Bt cornfields (up to 200) and scout them for any surviving larvae. Several scouts will walk fields during the period when first-generation larvae are mostly at the third and fourth instars in given areas. When (if) we find larvae in fields of Bt corn, we will determine whether or not the infested plants are producing Bt toxin. If Bt toxin is not found in the infested plants, the larvae will be discarded, because they likely will be Bt-susceptible. In contrast, if we find larvae surviving on "good" plants, the larvae might be resistant individuals that will give rise to future populations resistant to Bt. We think that looking for such "needles-in-the-haystack" is a worthwhile step to take as Bt corn is planted on a significant acreage this year.
To conduct this field-sampling project for detecting any existing resistant corn borers, once again we are asking growers to help us identify fields and supply information about hybrids, planting dates, etc. We would like to sample fields that are at least 10 to 15 acres in size. If you have planted Bt corn (or will do so as the weather allows) and are willing to let us walk your fields this year, please fill out the attached form (below) and return it to us as soon as possible. We will contact as many growers as we can to arrange field-sampling trips at the time when first-generation corn borer larvae have reached the third and fourth instars in specific areas.
We hope this update is useful to you, and we look forward to working with you this year. Please let us know if you have specific questions or concerns.
Rick Weinzierl, (217)333-6651, Kevin Steffey, (217)333-6652, and Christopher Pierce, (217)333-7512
1997 Summer Survey for European Corn Borer Resistance
Please complete and return this form to:
Department of Crop Sciences
AW-101 Turner Hall
1102 S. Goodwin Avenue
Urbana, IL 61820
(Or fax your information to (217)333-5245 or send e-mail to firstname.lastname@example.org)
No. of acres of Bt corn:
Hybrids (company and numbers):
Although we will contact you before working on your farm, it would help us to have a map showing the precise location of your field(s) in advance. If possible, please draw a map on the back of this sheet or attach a copy of a plat map that shows each field location. Please note county road numbers or names and any other information that will help us to identify the correct field(s).