Vegetable Crops News

A Tachinid Fly Parasite of Colorado Potato Beetle

Myiopharus doryphorae is a small (5/16"), bristly, grey and black tachinid fly that parasitizes larvae of the Colorado potato beetle. Female flies produce live larvae (instead of eggs), which they insert into 2nd, 3rd, or 4th instar Colorado potato beetle larvae. The maggots complete their development inside the larvae after the beetles enter the soil to pupate.

This fly has two generations per year. They overwinter inside dormant Colorado potato beetles and emerge as adults in late spring or early summer. They may be almost absent early in the season. The first generation is usually small, parasitizing midsummer larvae. The larger second generation generally emerges after the potato beetle population has already peaked. In some parts of the United States parasitism by this fly is very high, up to 75% of the beetle population. Unfortunately this high parasitism usually occurs after the potato crop has already been damaged. High mortality may occur late in the season, but generally after many of the Colorado potato beetles have already entered diapause and dug into the soil. However, in Colorado (the beetle's original range - before the introduction of potatoes), this fly is quite numerous early in the season and the rarity of damaging populations of the Colorado potato beetle may be due in part to high rates of parasitism early in the season. Researchers are not sure why high rates of early season parasitism occur here and not elsewhere.

This parasitic fly has limited potential as a lone biological control agent of Colorado potato beetle. To be effective it would have to parasitize first generation beetles at a much higher rate than it currently does in most areas. However, it does contribute to beetle mortality, and may be useful as one of a group of natural enemies that together can keep Colorado potato beetle populations below damaging levels.

- Susan Mahr, University of Wisconsin

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