Major Groups of Natural Enemies: Parasitoids, Part IV

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, in this series we are giving a broader overview of the major groups. The five previous issues in this series of feature articles covered the predators and a portion of the parasitic insects. Further information on many of these groups, including color photographs, can be found in North Central Regional Extension Publication 481, Biological Control of Insects and Mites.

The chalcid wasps (Superfamily Chalcidoidea) comprise a large group with over 2000 North American species. Nearly all species are very small; some are the smallest of all known insects. There are a great number of pests attacked by members of this group and different parasite species attack different host stages from egg to adult. Because of their small size, the wing venation is greatly reduced and generally distinctive. Also because of their small size, their presence and importance are often overlooked, but they are considered to be one of the most important groups in both natural control and applied biological control.

Two interesting biological trends occur within this group. First, there is a tendency toward plant-feeding, even in the larval stage, and some species are agricultural pests (even though others in the same family or even genus are insect-parasitic). One family known as the fig wasps are responsible for pollinating figs, and their larvae feed within the developing fig fruits. Second, there is a tendency toward hyperparasitism. Hyperparasitism is the parasitism of another parasite larva that is already associated with the original host. For example, one species, called the primary parasite, may parasitize a caterpillar. The hyperparasite parasitizes the primary parasite. Sometimes this is a facultative association, where the hyperparasite may attack either the original host or the primary parasite. In many cases, hyperparasitism is obligatory. In extreme cases, called autoparasitism, male parasite larvae parasitize female larvae of their own species.

Some taxonomic relationships in the Chalcidoidea are unclear, and therefore the classification of the superfamily is unsettled. About 20 North American families are recognized. Of these, the following six are very important in natural and biological control.

Fairyflies--Family Mymaridae. Some of the fairyflies are the tiniest of all insects, being 0.5-1 mm in length. The smallest ones known are about 0.2 mm (0.008 inch!). They are distinctive because the wings (at least the hind wings) are edged by a fringe of long hairs, but a microscope is needed to see this. All known species are parasites of the eggs of other insects. Common host groups include the flies, beetles, booklice, and leafhoppers. Many species important in biological control are in the genus Anaphes. A. flavipes is an introduced parasitoid of the eggs of cereal leaf beetle, a serious pest of small grains. A. iole is being investigated as a natural enemy of tarnished plant bug. A. luna contributes to the overall biological control of alfalfa weevil. Anagrus is another important genus. For example, A. epos parasitizes the eggs of white apple leafhopper and A. nigriventris parasitizes the eggs of potato leafhopper.

Trichogramma wasps--Family Trichogrammatidae. Like the fairyflies, the trichogrammatid wasps all are parasites of the eggs of other arthropods and therefore all are very small, mostly less than 1 mm in size. They have been used much more often in biological control than have fairyflies, and constitute the single most widely used genus of arthropods in augmentation biological control programs. Several species are commercially available. Common species in the United States include Trichogramma minutum, T. platneri, and T. pretiosum, but there are about 40 other species native to the U.S. and Canada. Some foreign species are being evaluated for augmentation biological control, such as T. ostriniae, T. nubilale, and T. brassicae for control of European corn borer. Many Trichogramma species have a wide host range and will attack the eggs of many species and even many families of insect hosts. Common hosts are the eggs of moths and butterflies, beetles, flies, wasps, and true bugs. Several species are aquatic, attacking the eggs of aquatic insects such as diving beetles and dragonflies. Some aquatic trichogrammatids use their wings as oars, to swim through the water in search of host eggs.

Eulophids--Family Eulophidae. This is one of the largest families of the Chalcidoidea, with about 3400 species worldwide and more than 600 in North America. The wasps themselves are rather small (1-3 mm), and often are a brilliant metallic color (commonly metallic blue or green). This is a biologically diverse group, with some species attacking mites (as predators), spider egg cases, scale insects, and thrips, but most attack beetle or moth larvae or pupae, flies or other wasps. Many parasitize leaf-mining and wood-boring insects. Many major crop pests are attacked by eulophids. The following are some examples from here in the Midwest. Sympiesis marylandensis is an important parasite of spotted tentiform leafminer of apple. Diglyphus isaea is an important parasite of agromyzid leafminers and is available from commercial suppliers of natural enemies; it is used primarily in commercial greenhouses. Edovum puttleri is a parasite of the eggs of Colorado potato beetle. Pediobius foveolatus is commercially available and parasitizes larvae of Mexican bean beetle; it was originally introduced from India. Tetrastichus is a genus with several parasites of plant pests; for example, T. asparagi parasitizes the asparagus beetle. Sometimes several species of eulophid will attack one type of host. For example, Elachertus cacoeciae, Horismenus microgaster, Pediobius crassicornis, Pediobius facialis, and Sympiesis bimaculatipennis are all parasitoids of obliquebanded leafroller, a fruit tree pest. Ceranisus sp. is being investigated as a biocontrol agent of western flower thrips larvae.

Pteromalid wasps--Family Pteromalidae. This is another large family, with about 350 known species in the U.S. and Canada. It is a very diverse group and many types of insects serve as hosts to wasps in this family. Common types of hosts include the larvae of moths, flies, beetles, and wasps. A few parasitize scale insects and mealybugs. Many are hyperparasitoids, parasitizing other parasitoids within a variety of hosts. Several have been used in classical biological control. The following are some examples of pteromalids important in natural and biological control in the upper Midwest. Pteromalus puparum is a parasite of the pupae of the imported cabbage worm. This is a gregarious species, and as many as 100 or more parasites can develop from one host pupa. Anisopteromalus calandrae is a parasite of the larvae of beetles that infest stored grain; this species is commercially available for release in grain storage and handling facilities. Several types of pteromalids parasitize the pupae of filth-breeding flies, such as flesh flies, house flies, blow flies, and stable flies. Examples include parasites in the genera Nasonia, Muscidifurax, and Spalangia. Some species occur naturally in our area, and some species are commercially available for release on dairy farms or at cattle feedlots.

Encyrtid wasps--Family Encyrtidae. This family, with over 3000 known species in the world, and about 400 in the U.S. and Canada, is one of the most important chalcidoid families for biological control. Again, there is a great diversity in types of hosts. Many are parasites of soft scales, armored scales, and mealybugs, and many of the very successful cases of classical biological control of scale and mealybug pests of fruit trees have involved encyrtid wasps. Other hosts include the eggs or larvae of insects in about 15 families of beetles, 10 families of flies, and 20 families of moths and butterflies. Less common hosts include eggs, nymphs, or adults of the true bugs (such as stink bugs), katydid eggs, and the nymphs of certain ticks. A couple important examples of encyrtids include Ooencyrtus kuwanae, an introduced gypsy moth egg parasite (see MBCN Vol. IV, No. 6) and Copidosoma floridanum, a native parasite of cabbage looper larvae. Several species of encyrtids are commercially available for control of scale insects and mealybugs in greenhouse, conservatories, and interior plantscapes. Two examples are Leptomastix dactylopii which is a specialized parasite of citrus mealybug, and Metaphycus helvolus which is a general parasite of many species of soft scale, including black, hemispherical, brown soft, nigra, and citricola scales. As I sit working at my desk, it is not at all unusual to see a tiny parasitic wasp walking or hopping across my papers--Comperia merceti, an egg parasite of cockroaches. Our building has a permanent infestation of the German cockroach, and a naturalized population of this little parasite contributes to the reduction of cockroach populations.

Aphelinid wasps--Family Aphelinidae. Because of its importance in the control of scale insects, this family is considered one of the most important in all of biological control. However, there is confusion over the proper classification of this group, and in various references it may be included within the Encyrtidae or Eulophidae. One reason for the taxonomic difficulties is the very small size of these insects; most are under 1 mm. The aphelinids are most commonly associated with various families of Homoptera, including scale insects, mealybugs, whiteflies, and aphids. Some are egg parasites and additional hosts occur in various insect groups. One of the most important members of this group is Encarsia formosa, a parasite of greenhouse whitefly that is commercially produced and widely used. Members of the genus Eretmocerus are also whitefly parasites and are important in the control of silverleaf whitefly. Various Aphelinus species are aphid parasites, such as A. varipes, a parasite of greenbug, and A. mali, a parasite of woolly apple aphid. A complex of foreign Aphelinus species have been introduced into the U.S. for control of Russian wheat aphid.

The above six families of the Chalcidoidea are the most important in biological control. The following summarizes the remaining families.

The small family Leucospidae has only a half dozen species in the U.S. and Canada. Relatively little is known about the biology of this family, but all known species are parasites of the stinging bees and wasps. Members of the family Chalcididae can be recognized by their enlarged hind legs. There are somewhat over 100 species in the U.S. and Canada. Most attack the pupae of flies or moths. Brachymeria intermedia is a parasite of the pupae of gypsy moth.

The family Eurytomidae is of intermediate size, with about 250 known species in North America north of Mexico. Some are plant feeders including gall-makers and seed-feeders. There are a few important agricultural pests, such as the alfalfa seed chalcid. The parasitic species of eurytomids largely attack host insects found within plant tissues, such as stems, seeds, or galls. There has been little if any usage of eurytomids in biological control of insects, but some of the seed-feeders have been used in biological control of weeds.

The family Torymidae, with about 170 species in the U.S. and Canada, is biologically similar to the eurytomids in that many are plant feeders while others are parasitic on other insects. At least one species, the apple seed chalcid, can be an agricultural pest. Torymid wasps are often brightly colored, especially metallic blue or green. Those that lay their eggs in plant tissue have very long ovipositors. Many species are parasitic on gall insects. Other hosts include the eggs and larvae of moths, beetles, and flies. Praying mantid eggs are often parasitized by torymids.

The family Eupelmidae is of moderate size, with about 100 species in North America. Many eupelmids attack hosts within plant tissues, such as gall-makers or the larvae of wood-boring beetles. Other eupelmids are egg parasites. Anastatus disparis, an egg parasite of gypsy moth, was introduced from Europe into the U.S. for biological control and is now established here.

All members of the fig wasp family, Agaonidae, are pollinators of figs, with the larvae feeding within the fruit receptacle; there are very few species in North America. Another very small family is the Ormyridae, with all known North American species parasitic on the larvae of gall wasps on oak trees. All members of the small family Eucharitidae are parasitic on the larvae and pupae of ants. The small family Perilampidae consists mostly of hyperparasites, parasitizing the larvae of fly and wasp parasites of primarily moth hosts; a few species are primary parasites of beetle and lacewing larvae. The very small family Tanaostigmatidae are all believed to be gall-formers. The small family Signiphoridae consists mostly of hyperparasites, although some are primary parasites of scale insects. Another small family, Elasmidae, also consists of both primary and hyperparasites.

In summary, the superfamily Chalcidoidea is quite large and of diverse biological habits. Although there are many families, relatively few contribute significantly to natural and biological control of pests. However, the contributions from these important families has resulted in many noteworthy successes in biological control.

- Dan Mahr, University of Wisconsin - Madison

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