Submitted to: Environmental Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/23/2009
Publication Date: 8/1/2009
Citation: Kim, K.S., Bagley, M.J., Coates, B.S., Hellmich II, R.L., Sappington, T.W. 2009. Spatial and Temporal Genetic Analyses Show High Gene Flow Among European Corn Borer (Lepidoptera: Crambidae) Populations Across the Central U.S. Corn Belt. Environmental Entomology. 38(4):1312-1323. Interpretive Summary: The European corn borer (ECB) is a major pest of corn in the U.S. and is the main target of transgenic Bacillus thuringiensis (Bt)-corn expressing the Cry1Ab toxin. Because of the high use of Bt-corn, there is worry that this insect will develop resistance to it, so scientists must continually monitor populations across the U.S. for resistance. However, the distance between sites needed for adequate monitoring is unknown, because the distance European corn borer moths disperse per generation is unknown. We used genetic markers to estimate gene flow, an index of dispersal, along two 720-km long transects using both spatial and temporal (change over time) analyses. We discovered that there is no genetic structuring even at the large spatial scale tested, indicating that the moths must fly very long distances. This was confirmed by the temporal analyses that calculated high migration rates. The results of this study suggest that the geographic dimensions of ECB populations are quite large, which means that monitoring for resistance to Bt-corn can be continued at widely separated distances in the Midwest Corn Belt without risking the missing of resistance development. Other important implications are 1) that resistance to Bt corn will be slow to develop, and 2) if it does develop, it will spread geographically so quickly that corrective strategies may be ineffective. This information will be of use to officials in regulatory agencies responsible for resistance management regulations for Bt crops. It also will be useful to university, government, and industry scientists and modelers trying to understand and predict rates of resistance development in European corn borer to Bt-corn.
Technical Abstract: European corn borer, Ostrinia nubilalis, were sampled at 13 sites along two perpendicular 720 km transects intersecting in central Iowa, and two generations later at 4 of the same sites separated by 150-km in the cardinal directions. More than 50 moths from each sample location and time were genotyped at nine microsatellite loci. Spatial analyses indicated that there is no spatial genetic structuring between European corn borer (ECB) populations sampled 720 km apart at the extremes of the transects, and therefore no pattern of genetic isolation by distance at that geographic scale. Although this result suggests high gene flow over the spatial scale tested, it is possible that populations have not had time to diverge since the central Corn Belt was invaded by this insect about 60 years ago. However, temporal analyses of genetic changes in single locations over time suggest that the rate of migration is indeed very high. The results of this study suggest that the geographic dimensions of ECB populations are quite large, which means that monitoring for resistance to Bacillus thuringiensis (Bt)-corn can be continued at widely separated distances in the Midwest Corn Belt without risking the missing of resistance development. Other important implications are 1) that resistance to Bt corn will be slow to develop, and 2) if it does develop, it will spread geographically so quickly that mitigation strategies may be ineffective.