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.
Generally, arthropods that eat other arthropods are considered to be either predaceous or parasitic. Predators, which have been discussed in previous issues of this series, are characterized as follows:
they are usually larger than their prey;
they may be predaceous in either the immature stage, the adult stage, or both;
they eat many prey during the course of their life;
they are generally fairly mobile so that they can find or catch their prey; and
most species tend to have a fairly broad host range.
Those insects that parasitize other insects have biological characteristics quite different than predators:
they are slightly to substantially smaller than their host insect;
all have a complex immature development that includes larval and pupal stages and it is only the larval stage that is parasitic;
each larva kills only one host during its development;
the larvae are totally restricted to parasitizing the single host insect and are often found internally in the host and therefore are not mobile in the environment; however, the adult stage is usually winged and mobile, and seeks the appropriate environment or host for its offspring; and
they tend to be more host-specific than predators.
The term "parasite" is frequently used for insects that parasitize other insects. Many biological control workers, however, point to significant biological differences between such insects and other types of parasitic animals. "True" parasites are usually much smaller than their host, have a shorter life cycle than their host, and usually do not kill their host; examples include tapeworms and ticks. Recognizing this distinction, Reuter in 1913 coined the term "parasitoid" for insects that parasitize other insects (click here for a bit more discussion on this). Some biological control workers prefer to use "parasite" whereas other use "parasitoid", so both terms can be found in MBCN.
Most agricultural and forestry pests are attacked by one or more parasitoid species. It is estimated that, on a world-wide basis, there are about 68,000 species of parasitoids that are known to science and have been given a scientific name. This constitutes a little under 10% of all known insect species. Entomologists generally believe that only about 10% of all insect species are known to science, and that possibly as many as 800,000 species of parasitoids actually exist. Most parasitoids are found within two major groups of insects, the orders Diptera (the true flies and their relatives), with about 15,000 known species of parasitoids, and Hymenoptera (bees, wasps, and ants), with about 50,000 known species. Within the Diptera, one family in particular the Tachinidae (tachinid flies), is especially important in the natural and biological control of agricultural and forestry pests. But, by far, the Hymenoptera comprise the largest group of parasitoids, and constitutes the overall most important group of natural enemies used in the biological control of insects.
However, the parasitoid way of life exists in several other groups as well, and an estimated 3,000 species exist in the other major groups of insects, most of these within the order Coleoptera (the beetles). The following families of beetles contain parasitoids.
Ground beetles--Family Carabidae. The carabids are one of the largest orders of beetles, with about 30,000 known species. Most are predators and many are important in natural and biological control. About 500 species of ground beetles are parasitoids, primarily of soil-dwelling insects. These are ectoparasites, feeding attached to the outside of their single host. Many of these are in the genus Lebia, which includes parasites of members of the leaf beetle family (Chrysomelidae). Lebia grandis parasitizes Colorado potato beetle pupae.
Rove Beetles--Family Staphylinidae. This is another large family, with about 30,000 species worldwide. Many are predators and some of these are important in biological control; about 500 species are known to be parasitoids. Most of the parasitoids attack the pupae of flies, such as filth-breeding flies (house flies, blow flies, flesh flies, etc.). One species, Aleochara bilineata, is an important parasite of cabbage maggot (see MBCN Vol. IV, No. 4).
Family Rhipiphoridae. This is a small family of about 400 species worldwide. Most are parasites of the larvae of bees and wasps, and are not of importance in biological control. A few species of Rhipidius are parasites of cockroaches. The wingless, larviform female lays eggs on the ground in areas frequented by the hosts. The first instar attaches itself to a passing cockroach and enters the body, completing its larval feeding as an endoparasite but emerging to pupate.
Family Rhipiceridae. The cedar beetles comprise a very small family, with only about a half dozen species known in the United States. Members of the genus Sandalus parasitize the nymphs of cicadas in the soil, but are not very abundant.
Blister beetles--Family Meloidae. The larvae of some blister beetles attack ground-nesting bees; others attack the egg cases of grasshoppers. Because egg cases contain many eggs, some people consider this group of blister beetles to be predators. But because each larva consumes only a single egg case, others consider these to be egg-case parasitoids.
The order Strepsiptera, or twistedwing insects, is a small group of uncommon parasites closely related to (and sometimes included in) the beetles. Bees, wasps, leafhoppers, and planthoppers are the most common hosts, but a few species attack grasshoppers or bristletails. None are important in biological control.
The order Neuroptera consists of the lacewings, antlions, and their relatives. Most neuropterans are predators; one family consists of parasitoids.
Mantidflies--Family Mantispidae. Adult mantidflies are predators and have the front legs strongly modified for catching and holding prey, similar to praying mantids. The larvae are all parasitic within the egg sacs of spiders, especially ground-dwelling spiders. Mantidflies are more common in southern states.
- Dan Mahr, University of Wisconsin - Madison
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