Major Groups of Natural Enemies: Parasitoids, Part II

Editor's Note: As we have written in past issues of MBCN, there are three general groups of natural enemies of pest insects and mites: predators, parasitic insects, and insect pathogens (some would include entomogenous nematodes as a separate fourth group). Although we have highlighted specific natural enemies, or specific groups, in our monthly Know Your Friends column, we would like to give a broader overview of the major groups. The four previous issues in this series of feature articles covered the predators; this issue begins coverage of parasitic insects. Further information on these groups, including color photographs, can be found in North Central Regional Extension Publication 481, Biological Control of Insects and Mites.

The true flies (insect order Diptera) are characterized by having only one pair of wings that are functional for flight, with the hind pair of wings greatly reduced to tiny, knob-like structures, called halteres, that are involved in balance and orientation. Although the Diptera is a very large and important group of insects, relatively few families of flies are parasitoids. However, all of the species of one large family, the tachinid flies, are parasitic, and many species parasitize important agricultural pests.

Bee Flies--Family Bombyliidae. Many of these flies are stout with densely hairy bodies and a long, slender proboscis which is used for extracting nectar from flowers; their overall appearance and behavior gives rise to the common name for the family. The adults frequently hover over or rest on flowers or the ground in sunny places. When at rest, they tend to hold their wings outstretched, perpendicular to the body. This is a large and widely distributed group, with about 800 species in the U.S. and Canada. They occur throughout the United States, but are more common in the arid areas of the Southwest. The larvae are parasitic on cutworms, beetle grubs, bee and wasp larvae (including some parasitoids), or grasshopper egg pods. Those that attack grasshopper eggs are sometimes considered predators rather than parasites because several host individuals (in the egg stage) are killed. But functionally, the entire egg pod can be considered as a single host that is parasitized by the bee fly larva. In the few studies that have been conducted, the degree of parasitism of egg pods can be in the range of 50-80%, but in other locations, there is much less bee fly activity. Overall the family is considered beneficial, even though some species may be detrimental because they parasitize predators, other parasitoids, or pollinator bees. The few attempts to use bee flies in classical (importation) biological control of grasshoppers have been unsuccessful.

Tangle Veined Flies--Family Nemestrinidae. This family is related to the bee flies. There are only about six North American species, and most occur in the western United States. In most locations, these are rather rare flies, but they can be locally common, especially where appropriate hosts are abundant. The adult flies are of medium size and are stout-bodied; some are noticeably hairy and with a bee-like appearance. They are often seen hovering in open fields, or taking nectar from flowers. They primarily parasitize grasshoppers, but some are known to be parasites of the soil-dwelling grubs of scarab beetles.

Bigheaded Flies--Family Pipunculidae. These tiny, usually black flies have a very large head covered by huge eyes. The larvae are parasites of various species of leafhoppers and planthoppers. Although not uncommon, they are rarely abundant, and their role in biological control of pest leafhoppers is probably limited. There are about 100 species in the U.S. and Canada.

Thickheaded Flies--Family Conopidae. These are medium sized flies, with the head wider than the thorax, hence the common name. They are usually brownish in color, and often the base of the abdomen is slender, so they somewhat resemble wasps. The adults are often seen on flowers. This group, with about 70 species, is widely distributed in the United States and Canada, but nowhere are they abundant. Although there are some records of larvae parasitizing grasshoppers, some species are considered to be detrimental because they parasitize important pollinators, such as honey bees and bumble bees, but generally they are not sufficiently common to seriously reduce the populations of pollinators. This family has not been used in biological control.

Flesh Flies--Family Sarcophagidae. This is a large and common family, with well over 300 species in North America. Flesh flies are large grey, black, and white flies, related to blow flies. Many are scavengers of animal matter, with the larvae feeding on carrion, manure or dead insects, but a good proportion of the family are parasitoids of other insects, especially beetles, grasshoppers, or caterpillars. Some parasitoid species are sufficiently common to be considered important for biological control. For example, Sarcophaga aldrichi is an important parasite of the forest tent caterpillar, an important forest pest in much of the U.S. and southern Canada. In some locations, especially in the area of the Great Lakes, forest tent caterpillar goes through cyclic outbreaks causing considerable defoliation to hardwood forests. S. aldrichi responds to these outbreaks, building up in large numbers due to the abundance of hosts, and ultimately the tent caterpillar population crashes. The adult flies, which are large, droning, active fliers, can become incredibly abundant. Because they are attracted to sugars, they can be a considerable nuisance.

Tachinid Flies--Family Tachinidae. All species of this family are parasitic on insects and other arthropods. Some scientists consider this to be the largest family of flies, with about 1300 species known from the U.S. and Canada. Many are very important natural enemies of a variety of plant pests, especially caterpillars, but also beetles, grasshoppers, and sawflies. The tachinid flies have already been reviewed in this newsletter.

- Dan Mahr, University of Wisconsin - Madison

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