Major Groups of Natural Enemies: Parasitoids, Part III

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 order Hymenoptera includes the sawflies, bees, wasps, and ants. The term "wasp" usually conjures up a negative image by the lay person because of such creatures as hornets and yellowjackets that can sting, and can be somewhat aggressive when they or their nests are disturbed or threatened. But the group known as wasps by entomologists is much larger in scope, and the vast majority of wasp species are incapable of stinging. Many of these stingless wasps appear to have an external stinger, which, however, is the egg-laying structure (ovipositor). In the stinging bees and wasps this organ is no longer used as an ovipositor, but has become modified for defense.

The stingless wasps comprise the single largest and most important group of natural enemies of insect pests. It has been said that every species of insect that attacks plants is itself attacked by one (or usually more) species of parasitic wasp. I'm not sure that this statement has ever been validated, but at least its essence is correct. There are about 18,000 species of Hymenoptera in the United States and Canada; the great majority of the these are wasps that parasitize other insects. Parasitic wasps are very diverse in their size, biologies, life cycles, and types of hosts attacked. The smallest known insects, less than 0.5 mm, are the fairyflies in the family Mymaridae, which spend their entire parasitic life at the expense of the egg stage of another insect. At the opposite extreme, some of the wasps that parasitize large grubs or wood borers may be more than 3 inches long.

Because of their abundance and importance, we can not cover the entire group in one article; therefore I will spread the information out in a continuation of this series. As in past articles, I will take a taxonomic approach. The parasitic wasps fall into several superfamilies, which are then subdivided into families; these will be the taxonomic groupings that I will use. In this article, we will start by discussing the wasps in the superfamily Ichneumonoidea.

The Ichneumonoidea is a very large and important group; all of the species are parasitoids. There are two major families, the Braconidae and Ichneumonidae; both are very important in natural and biological control.

Braconid wasps comprise a large group of over 1700 North American species; all are parasites of insects or other arthropods. Most are small wasps, ranging in size from 2-3 mm up to about 15 mm (1/2 inch) in length. They are generally black or brown in color, but some have yellow, orange, or red markings. They have long and noticeable antennae, and the ovipositor may also be long and readily apparent.

Caterpillars constitute the largest group attacked by braconids. Other common hosts include flies, sawflies, wood boring beetles, weevils, leafmining insects, true bugs, and ants. One important subfamily, the Aphidiinae (sometimes considered to be a separate family, the Aphidiidae) are very small wasps that parasitize aphids. Braconids usually parasitize the immature stages (larvae, pupae, or nymphs) of their hosts. A few parasitize adult insects, such as Microctonus aethipoides, which parasitizes and sterilizes adult alfalfa weevils.

The generation time of most braconids is relatively short, often in the range of 10-30 days. Many braconid larvae leave their host just prior to pupation, which usually occurs within a silken cocoon. Some species produce only one parasite per host insect; others may have several-even a dozen or more-if the size of the host can support them all.

Braconids are important in the natural and biological control of many serious pests of the farm, forest, and garden. Several exotic species have been introduced and permanently established for the control of alien pests. A few species are available commercially for augmentation biological control.

Some Important Species of Braconid Wasps:

Species Host
Aphaereta pallipes root maggots
Aphidius spp. aphids
Bracon hebetor moth larvae in stored grain
Cotesia glomerata imported cabbageworm
Cotesia melanoscela gypsy moth
Dacnusa sibirica leafminers
Diaeretiella rapae aphids
Lysiphlebus testiceipes aphids
Macrocentrus ancylivorus leafrollers/caterpillars
Macrocentrus grandii European corn borer
Microctonus aethiopoides adult alfalfa weevil
Microctonus vittatae striped flea beetle
Microplitis croceipes corn earworm
Peristenus digoneutis tarnished plant bug
Pholetesor ornigis spotted tentiform leafminer

Ichneumonid wasps, or ichneumons, constitute one of the largest groups of insects, with over 3100 species in North America. Adults vary a lot in size, form and color, but generally they are larger than braconids. Size is somewhat host-dependent, and some that parasitize large hosts may be 1-1/2 inches long. The ovipositor is often quite long, sometimes longer than the body. As with the braconids, the antennae are long and noticeable. Most are black, brown, or tan, but some are more brightly colored.

Many types of insects are attacked by ichneumons, though most individual species of wasp attack a relatively narrow range of hosts. Common types of hosts include caterpillars and the larvae of beetles and sawflies.

The larval stage of ichneumons varies from about 10 days to several weeks. Many species have a single generation per year but others have two, three, or more generations annually.

Although ichneumons are very important in the natural control of many plant pests, surprisingly few have been actively used in biological control programs.

Some Important Species of Ichneumonid Wasps:

Species Host
Bathyplectes spp. alfalfa weevil
Collyria coxator wheat stem sawfly
Diadegma insulare diamondback moth
Eriborus terebrans European corn borer
Lathrolestes nigricollis birch leafminer
Scambus applopapi nantucket pine tip moth

- Dan Mahr, University of Wisconsin - Madison

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