No. 8/May 15, 1998
Field Surveys for Bt Resistance
in European Corn Borer
As the acreage of Bt-corn planted in the Midwest increases, corn borer resistance to Bt toxins is an increasingly important concern. We consider it to be essential that "effective resistance" (the ability to survive early season field-level doses of Bt toxins), if it develops, be detected as soon as possible to provide information for resistance-management efforts. We're asking your help in identifying fields for this year's survey efforts.
1997 results. We initiated European corn borer resistance survey efforts in Illinois in 1997. In the spring, we contacted growers via this Bulletin and with their help identified Bt-cornfields in 22 counties. In June and July we walked fields to look for corn borer damage 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 susceptible corn borers to have been killed by the Bt toxin if it was present. We sampled at a time when all varieties, those derived from Monsanto's technology or those from Novartis and Mycogen, should have been producing high levels of CryIA(b) protein. Although the fields inwhich we worked totaled more than 2,200 acres in area, our crews actually covered about 370 acres in a row-by-row manner. If the plant population in those 370 acres averaged about 28,000 plants per acre and the potential level of infestation in those fields was about 0.5 larvae 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 370 acres would have been home to roughly 5.18 million corn borers (28,000 X 370 X 0.5).
We found about 200 surviving corn borers in the Bt fields. To avoid making an incorrect conclusion that all surviving larvae were Bt-resistant, we used a field assay kit to determine whether or not the infested plants were Bt-positive. Leaf tissue from infested plants was tested in the field and within a few minutes after infested plants were found. All but 2 of the 200 larvae were feeding on plants that were not producing Bt toxin. The other surviving larvae on Bt-negative plants were not necessarilyresistant to Bt; instead, they were simply fortunate that they were placed as eggs on non-Bt plants.
Of the two larvae from Bt-positive plants, one was parasitized at the time of collection, by Macrocentrus grandii, a small parasitic wasp. The other, collected as a second instar and placed on an artificial diet on June 29, died on August 27 as a fifth instar. It had not yet pupated. At a normal rate of development, it would have pupated in late July. The reasons for its failure to develop normally may be related to whatever unusual trait allowed its survival on a Bt-positive plant. It was not infected by Nosema pyrausta, a microsporidian pathogen that causes slow development and death in corn borer larvae. Our intent was to rear these larvae from Bt plants to the adult stage, mate them with each other or with lab colony moths, and then continue to rear subsequent generations for crosses and bioassays. These steps would have allowed us to investigate the nature of resistance to Bt, if in fact their survival resulted from resistance. These steps will now wait until more larvae are collected from Bt plants.
1998 plans and a request for cooperators. This year, we plan to repeat and expand the survey efforts initiated in 1997. We hope to identify and sample 600 to 800 acres of Bt-corn in June and July. We will walk fields, collect larvae, and test their host plants for the presence of Bt toxins in the same manner as we did in 1997. To do this, we need your help. If you have planted one or more Bt hybrids this spring (or will do so as soon as the rains stop), please fill out and return the form at the end of this article.
We're looking for solid plantings of at least 15 acres of any single Bt hybrid (not small test plots). We will call you to arrange any field sampling trip, so you will know in advance when we plan to come to your fields. And you're welcome to work with us if you want. If you are willing to let us check your fields, please fill out and return the form by May 28 (sooner if possible). Thanks.
Rick Weinzierl (email@example.com), Kevin Steffey (firstname.lastname@example.org), and Christopher Pierce Extension Entomology, (217)333-6651
1998 Survey for Bt-Resistant European Corn Borers
If you have Bt-cornfields (larger than 15 acres) that we can examine forsurviving European corn borers in June and early July, please complete andreturn this form.
Please include maps of field locations with this form if possible. If not, we can get this information from you later.
Please return this form (by mail, fax, or e-mail) by May 28 to
Department of Crop Sciences
University of Illinois
1102 South Goodwin Avenue
Urbana, Illinois 61801
Phone: (217) 333-6651
Fax: (217) 333-5245