Influence of destruction of male corn plants on movement of Ostrinia nubilalis (Hübner) larvae in Zea mays L. hybrid seed production fields
Genetically-modified corn hybrids that contain a cry gene from the bacterium Bacillus thuringiensis (Bt) are gaining popularity for controlling the corn pest, Ostrinia nubilalis. Continuous use of Bt corn, however, could select for O. nubilalis that are resistant to Bt corn. Monitoring for insect resistance is important because it could help maintain the Bt technology. A monitoring method is needed that is cost effective, not too labor intensive, and will allow sampling of large areas. A proposed method of collecting resistant insects in seed-corn drying bins, i.e., bin monitoring, could overcome these barriers. This method takes advantage of the existing system used in hybrid seed production. Seed production fields are planted in alternating strips of two rows of non-Bt corn (male) and four rows of Bt corn (female). After anthesis, male rows are destroyed. Ears of female plants are harvested and dried in bins. Some O. nubilalis larvae in the ears may move during the drying and fall underneath a partition to the bin floor, where they are collected. These larvae are brought into the laboratory and their progeny undergo a diagnostic-dose bioassay. The objectives of this study were to first find which combination of planting date and destroy date produces the least number of larvae that move from non-Bt male plants to Bt female plants; and then determine how far larvae move from downed male plants into Bt female plants. These questions are important because larvae that are captured in drying bins and escape exposure to Bt toxin (called false positives) reduce monitoring efficiency. Field studies in 2002 and 2003 were designed to simulate a hybrid seed production field. Results suggest that movement of O. nubilalis larvae from male corn are minimized when corn is planted early and male plants are destroyed by two weeks post-anthesis. This reduces the likelihood of false-positives by reducing the number of susceptible larvae moving from the non-Bt male plants to female Bt plants. Also larvae will move to all four female rows, but there were significantly more larvae found in row one than all other rows. These results can be used in developing a monitoring program to find O. nubilalis larvae with resistance to Bt corn in field populations of European corn borer.