Field Crops News

Habitat Selection in Three Closely Related Parasitoids of Stem Borers

One strategy of biological control is to use natural enemies of pests collected from areas where the pest does not occur ("novel association" biological control). Careful study of the proposed natural enemy and its target pest species must be conducted to ensure that it will be both effective against the pest insect and not harmful to non-target insects. Recent research at the University of Illinois has examined three braconid wasp species in the genus Cotesia, native to the Old World, to evaluate their potential to provide biological control of grass-inhabiting stem borers. All three species are specialized as parasitoids of grass-inhabiting stem boring caterpillars. One species, Cotesia flavipes, has been widely used against a novel host, the sugarcane borer, Diatraea saccharalis. Although all three Cotesia species are closely related and have nearly identical life histories (all are gregarious, internal parasitoids of pyralid and noctuid caterpillars), they are found on hosts in different grass species in their native ranges.

One aspect of parasitoid host selection is to find the habitat of a potential prey individual. A Y-tube olfactometer was used to evaluate habitat preference of wasps exposed to airborne odors of different grass species. When given a choice between a grass species and air, all three wasp species preferred the grasses most closely related to their original host plants. When given a choice between two grass species, parasitoids sometimes exhibited a preference. Thus far, it seems likely that these parasitoids would have different habitat ranges if released as novel biological control agents.

The divergent habitat preferences shown by these closely related species reemphasizes the need for pre-release testing of novel biological control agents. Future studies will look at additional grass species and at selected non-grass species, and the effect of plant damage on parasitoid habitat preference.

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