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.
C. E. Rutledge, Dept. of Entomology, University of Illinois, Urbana IL 61801 and R. N. Wiedenmann,
Center for Economic Entomology, Illinois Natural History Survey, Champaign IL 61820. Based on a paper
presented at the 1996 North Central Branch meeting of the Entomological Society of America, Omaha, NE.
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