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. This is the fourth of a series of feature articles that addresses this topic. Further information on these groups, including color photographs, can be found in North Central Regional Extension Publication 481, Biological Control of Insects and Mites.
Most of the insects in the order Neuroptera are predators in the larval stage; some also are predators as adults. The adult insects in this group are characterized by having soft, slender, bodies, well developed antennae, and large membranous wings that are richly endowed with a nerve-like network of veins ("neuro" + "ptera" = nerve wing). This group has a rather modest number of species within a relatively few families, but they are very important in natural and biological control, especially the lacewings.
Green Lacewings-Family Chrysopidae. The adult green lacewings have a soft, slender, green body, large membranous wings with green veins, and long hair-like antennae. The overall body length including wings is about 1 inch; some common species have golden eyes. Sometimes these insects come to lights at night. The eggs are small, greenish to white in color, and laid on hair-like stalks about a inch high. The larvae are somewhat elongate, up to about a inch long when fully grown, broadest in the middle and tapered toward the rear. The larvae have noticeable sickle-shaped mouthparts. When fully grown, larvae pupate within spherical, parchment-like silken cocoons. It takes 4-6 weeks to complete a generation. Green lacewings tend to specialize in feeding on aphids and usually the adults lay their distinctive eggs near aphid colonies. However, the larvae feed on many types of soft bodied insects, including mealybugs, scale insects, leafhoppers, thrips, and even small caterpillars. They can detect the larvae of leafminers actually within the mines, and will pierce the leaves in order to feed on the miners within. They are also good predators of spider mites. Different species will tend to search in different habitats; some prefer low-lying herbaceous plants or grasses whereas others tend to be more arboreal. Lacewings are very common and important naturally-occurring predators in many crops, on landscape plants, and in the home garden. However, they are also available commercially for augmentation biological control. The stage usually purchased and released is the egg, but some companies also provide partially grown larvae and adults. Ask your supplier to recommend the most appropriate species and stage for your pest management needs. Adult lacewings all require carbohydrate-rich foods such as aphid honeydew or flower nectar; the adults of some species are not predators.
Brown Lacewings-Family Hemerobiidae. The adult brown lacewings resemble green lacewings, but are generally smaller and brown. Superficially, the larvae are similar to those of green lacewings, but the mandibles are not so prominently developed. The eggs are not stalked as in green lacewings. Brown lacewings can be found in many habitats, but may be more frequently found as predators within tree canopies. Studies have shown that some brown lacewing species are restricted to certain tree types, so one species may occur on oaks while another occurs on pines in the same area. They feed primarily on various types of Homoptera, such as aphids, mealybugs, scale insects, and the nymphs of whiteflies. They are not commercially available.
Dusty Wings-Family Coniopterygidae. The dusty-wings are amongst the smallest of the Neuroptera; adults are only about 1/4 inch or less. The common name of the family is derived from the presence of a white to grayish powdery wax that covers the wing surfaces. Dustywings are not common insects but they are more numerous in the southern United States. They feed on small prey such as aphids, scale insects, and spider mites. Occasionally they are abundant when there are outbreaks of their prey.
Most other families of Neuroptera are also predatory or parasitic, especially in the larval stage. Many of these have interesting biological habits but are not important in biological control of pests. The family Myrmeleontidae are the antlions. Adults are similar to damselflies in general appearance, but have clubbed antennae and are feeble, nocturnal fliers. The larvae, or doodlebugs, have long, sickle-like jaws. Each larva hides at the bottom of a small conical pit made in sand or dust and feeds on ants or other insects that fall into the trap. They tend to be more common in the southern part of the U.S. Members of the family Ascalaphidae are called owlflies. The habits are similar to those of antlions, but the larvae only cover themselves with dust for concealment, rather than digging pits. The family Mantispidae are reminiscent of preying mantids. They have large raptorial front legs for grabbing and holding prey. The larvae are predaceous on spider egg sacs.
The order Diptera consists of the true flies and their close relatives, such as midges, gnats, and mosquitoes. The unifying characteristic of this group is that the adults have only a single pair of wings; the posterior pair has been greatly reduced and modified to function as balance organs. There are many families and species of flies, and biologically they are very diverse. Amongst those important in natural and biological control are families of both predators and parasitoids; only families with predaceous species will be discussed in this issue.
Flower Flies, Hover Flies, or Syrphid Flies-Family Syrphidae. The flower flies are by far the most important family of predatory flies. The adults are not predaceous, but the larvae of many species are. Syrphid larvae tend to prey primarily on aphids, mealybugs, scale insects, and related Homoptera. The adults, which have a yellow and black striped abdomen and look superficially like a small bee, feed on the nectar and pollen from flowers. The adults seek colonies of aphids and lay their eggs nearby. The headless, legless maggot-like larvae tend to be pale yellow to light green in color. The larger species get to be slightly bigger than 1/4 inch long when fully grown. They often pupate right where they finish feeding, forming a tan colored drop-shaped puparium. The life cycle, from egg to adult, usually takes 2-4 weeks, and several generations occur each year. Although normally thought of as aphid predators, many syrphids probably have a wider host range. In Europe, one species is known to commonly feed on small caterpillars. In Wisconsin, we have found that one very common species, generally considered to be an aphid predator, is also an important predator of cranberry tipworm which is the larva of a small midge that attacks the cranberry plant.
Gall Midges-Family Cecidomyiidae. As the name implies, the larvae of most gall midges feed within plant tissue, creating abnormal plant growths called galls. However, the larvae of a large number of species are predaceous, and some are even reported as parasitoids. These insects are very tiny, usually only 2-3 mm in length. Probably the most common prey are aphids and spider mites, followed by scale insects, then other small prey such as whiteflies and thrips; many eat the eggs of insects or mites. The adults, which are very tiny, fragile midges, locate colonies of appropriate prey and lay their eggs in these locations. Because the small, maggot-like larvae are incapable of moving considerable distances, there usually has to be a fair population of prey present before the adults will lay eggs. These insects can frequently be seen where there are colonies of aphids or outbreaks of spider mites, and are usually considered to be important natural enemies of these pests. In a recent survey of the predators of aphids in an organic apple orchard in Wisconsin, we found these to be the most numerous predators, especially during the summer months. At least one species, Aphidoletes aphidimyza is commercially available, although rather expensive to use.
There are many additional species of Diptera that are predatory. In some cases, all members of a family may share this trait; in other cases, predation is considered somewhat out of the ordinary when looking at the majority of species in a family. In most cases, these flies have little impact on agricultural pest control. However, a few of the more common or interesting groups are mentioned here.
The larvae of some mosquitoes (family Culicidae) are highly predaceous and frequently feed on the larvae of other mosquito species that may be of more importance as health pests or nuisances. There have been some cases of successful introduction of predaceous, nonpest species into areas to reduce the populations of serious mosquito pests. The longlegged flies (family Dolichopodidae) are small but beautiful flies, brightly colored with metallic green, blue or copper. They are particularly abundant near swamps, streams, woodlands, and in meadows. The adults are predaceous on smaller insects, and some can be found feeding on aphids. The larvae of many are predaceous, living in moist habitats such as decaying logs, or moist organic soil. The larvae of the genus Medetera live under bark and feed on bark beetles. Robber flies (family Asilidae) are large insects and often quite numerous, although they tend to occur more commonly in the southern states. Many are stout-bodied and very hairy; some resemble bumble bees. Adults are predaceous and attack a variety of insects including wasps, bees, dragonflies, grasshoppers and whatever else they can capture, usually in flight. Larvae live primarily in soil or decaying wood and prey on the larvae of other insects. Marsh flies (family Sciomyzidae) are small to medium-sized insects, often yellowish or brownish; many have spotted or patterned wings. They are common along the banks of streams and ponds, in marshes, swamps and woods. Larvae feed on snails, snail eggs and slugs.
The order Hymenoptera consists of bees, wasps, ants, and their relatives. The vast majority of species in this group are parasitic on other insects; these parasitoids will be discussed in future issues of MBCN. However, some families are predaceous. Although such predators play a very important role in the overall food web, their importance in pest control is somewhat limited except for a few specific cases. The paper wasps and their relatives (family Vespidae) are primarily social, living in colonies with a caste system that includes a queen and workers. This is a notorious group because it includes some of the more potent stinging wasps such as yellowjackets. However, these wasps feed the larvae in their nests with animal material, including insect prey. Some studies have shown that there is a correlation between the number of wasps in an area and the number of certain types of caterpillars. Research at the University of Wisconsin has shown that increasing the numbers of colonies by providing suitable nesting sites resulted in a decrease in the number of the cabbageworm complex. The threadwaisted wasps (some are called muddaubers) and their relatives (family Sphecidae) are also large wasps and capable of stinging, but they are not social species; that is, they do not live in colonies, therefore their numbers in an area are usually lower than the yellowjackets. Individual females provision individual cells, each of which contains a single larva. Each species uses a somewhat different type of prey; such as grasshoppers, katydids, cockroaches, or caterpillars. Some use spiders, and one uses exclusively the black widow spider. Ants (family Formicidae) also have somewhat of a negative reputation because some species can be a nuisance in dwellings while others are capable of stinging. Ants are amongst the most numerous of all types of animals, and many species are predaceous. Because of their abundance, they can have a significant impact on the populations of other insects, including pest species. Some of the earliest records of biological control, going back many centuries, involve the use of ant colonies in date and citrus orchards for controlling a variety of pests. More recently, studies in the southern United States have shown that fire ants are very important in controlling certain types of agricultural pests. It should be noted however, that fire ants also consume other beneficial insects.
- Dan Mahr, University of Wisconsin - Madison
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