Know Your Friends

Rove Beetles

The family Staphylinidae, or rove beetles, is the largest family of North American beetles, with about 2900 species. Most are small and of cryptic habits and although common, the group as a whole is not well studied. Some species are predaceous as both adults and larvae; the larvae of other species are parasitoids; many others are probably scavengers. They are often found in agricultural soils and home gardens.

Adult rove beetles are generally less than 3/4 inch long. They are easily recognized by their slender, usually black or brown body, shortened front wings (elytra) that may look like pads on the abdomen, and behavior of curling the tip of the abdomen upwards when disturbed or running. Adults are usually strong fliers. The mobile larvae of nonparasitic rove beetles may be distinctly segmented.

Most rove beetles are found in association with soil or decaying organic matter. They may be seen under debris or rocks, in compost piles, or crawling on plants. Adults have even been found in sweet corn tassels and silks late in the season.

Predaceous rove beetles, depending on the species, may consume root maggot eggs and larvae, mites, small soil insects, insect eggs, or small insects on foliage. Some feed on the eggs and maggots of filth flies. Several occur in agricultural soils where they probably feed on a variety of types of prey. A few species can be found in vegetation where they feed on many types of small insects and mites.

Aleochara bilineata, a species with parasitic larvae, is a relatively well-studied natural enemy of some root maggots. This species was introduced from Europe into North America probably with one of its hosts, the cabbage maggot. The beetles are 1/8 inch in size, and they attack cabbage maggots in the pupal stage. The adults are glossy black except for the short wing covers, which are a reddish brown. They emerge in spring and deposit eggs in the soil near the roots of maggot-infested plants. When the beetle larvae hatch in 5 to 10 days they actively search out host puparia in the surrounding soil. They gnaw a hole in the shell, enter and feed externally on the maggot pupae for about 3 weeks. Beetle pupation occurs within the host puparia, and adults emerge after a month or more. Another very similar beetle, A. bipustulata, occurs in small numbers in some areas. It has the same life cycle as A. bilineata.

Aleochara adults are predators, destroying eggs or young maggots near the soil surface and maggots in the plant roots. They may consume up to 5 root maggot larvae per day, with a pair being capable of destroying 1200 eggs and 130 larvae, plus parasitizing several hundred pupae, during their lifetime.

Although they are good predators and parasitoids of root maggots, they often do not emerge early enough in the spring to have an impact on early season root maggot damage. In an extensive survey throughout Canada parasitism of cabbage maggot by A. bilineata ranged from 0% to 63% at different locations. Late season parasitism of cabbage maggot pupae may be as high as 90 to 95% in some areas.

Although not currently commercially available in North America, this species has been mass reared in Europe, the former Soviet Union, and Canada. It may someday be possible to augment natural populations of rove beetles for early season root maggot control.

The rove beetles illustrated here are primarily predators.

Platydracus is a voracious predator of armyworms in cornfields; Stenus feeds on springtails;

Tachinus attacks fly maggots; and Paederus is a generalist predator.

Siagonium, however, is thought to be a scavenger or fungus feeder.

- Susan Mahr, University of Wisconsin- Madison

Illustrations by Michelle Schwengle

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