Issue No. 22, Article 4/September 1, 2006
Behavior Bioassay Capable of Discriminating Between Variant and Nonvariant Western Corn Rootworm Populations
For many years, entomologists have been unable to accurately discriminate between variant and nonvariant western corn rootworm populations. In the most recent issue of Environmental Entomology (August 2006, Vol. 35, pages 1049-1057), Lisa Knolhoff (graduate student in the Department of Entomology, University of Illinois) and David Onstad, Joe Spencer, and Eli Levine (entomologists with the University of Illinois and Illinois Natural History Survey) reported how they could separate variant from nonvariant western corn rootworms utilizing a behavioral bioassay. As the authors indicate in their paper, this is the "first step in determining the genetic basis of rotation resistance." By measuring the time it took for western corn rootworm adults to exit behavioral assay arenas, the investigators began to observe some interesting differences among beetles that had been collected from different areas (Ames, Iowa, and three Illinois locations: Monmouth, Perry, and Urbana).
The arenas were cylindrical and made of plastic mesh screen, with an opening in an inverted cone at the top. The bottom of the arena was made of plastic, with an entry hole (1.8 cm in diameter) into which beetles were introduced. In 2004, the arenas were 16 cm tall by 11 cm in diameter. The following year, the height of the arenas was extended to 42 cm. The authors made the following observation: "Results from these assays indicate that D. v. virgifera females from regions where crop rotation is no longer effective are more active than females from regions where rotation remains effective." Differences among variant and nonvariant populations were more evident when the bioassay was performed in the field as compared with the laboratory.
During 2004, beetles collected from Urbana were "consistently more active" than specimens obtained from Monmouth. Urbana is much nearer the geographic epicenter (Piper City, Ford County) of where it is believed the variant western corn rootworm had its origin. In 2005, western corn rootworm females collected near Ames, Iowa, were the "least active" as compared with the beetles collected from Urbana and Perry. The researchers made these concluding statements in the abstract of their paper: "Results were consistent with the hypothesis that a loss of fidelity to corn rather than any particular attractant is the cause of rotation resistance. Behavioral differences between populations of beetles in similar environments suggest that there is a genetic difference between rotation-resistant and wild-type D. v. virgifera, although no specific gene or genes have yet been identified."
The pdf version of this article is available here.
Knolhoff, L.M., D.W. Onstad, J.L. Spencer, and E. Levine. 2006. Behavioral differences between rotation-resistant and wild-type Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae). Environmental Entomology 35(4): 10491057.