These biotic agents help control injurious insect species by greatly 
limiting the potential increase of the pest. The biologically curtailed pest 
may be much reduced in numbers, or in other cases be left relatively 
abundant. The resultant population may be sodecreasedthat it is ineffectual. 
But often the surviving pest population is large enough to cause considerable 
economic damage. 
Agents of Biological Control 
Parasites and predators are the two kinds of agents most frequently 
referred to in biological control. 
The parasites! discussed in this report are insects that live at the ex- 
pense of insect pests and their relatives. The insect parasite usually makes 
its initial attack as an adult laying eggs in or on the body of the victim. 
Larvae from these eggs then develop inside or outside the host and feed on 
it. In either event the host ultimately dies from the attack, although it may 
live and feed for some time after being parasitized. 
A predator kills the host rather quickly by direct attack and usually 
devours several or many individuals. The predator is often larger than the 
prey and frequently both adults and immature stages of a predator feed on 
the same insect pest. 
Parasites have a restricted host range, often being specific for one 
insect. This characteristic may add to their effectiveness in the control of 
a particular pest. Predators, on the other hand, are generally less discrim- 
inating and may therefore be less effective against a specific pest insect. 
There are, of course, exceptions. 
The insectivorous habit is found in all the major orders of insects 
although it is more common in some than in others. 
The Hymenoptera (bees, wasps, and ants) is chief among the insec- 
tivorous orders. This order predominates not only in the number of species 
which feed on insects but also in the frequency and effectiveness of the 
members' attacks on noxious insects. Four families of parasitic wasps-- 
Ichneumonidae, Braconidae, Chalcididae, and Scelionidae--are particularly 
significant in biological control. The tachinid flies of the order Diptera are 
important parasites of caterpillars. 
Other orders with prominent insectivorous species are Coleoptera (the 
beetles), Hemiptera (the true bugs), and Lepidoptera (the moths and butter- 
flies). 
Certain species of predaceous mites are instrumental in checking 
injurious populations of other mites. For example, the typhlodromid mites 
suppress the two-spotted spider mite and other tetranychid mites. 
Certain larger animals also are important in controlling insect pests 
although their value is difficult to measure. Birds are traditionally consid- 
ered heavy consumers of insects under certain situations. Skunks, snakes, 
1Many microbes--bacteria, viruses, nematodes, protozoa, and fungi--also are true parasites. However, they 
are discussed only briefly in this report because the subject has been covered in Special Report 22-74, **The 
Use of Diseases to Kill Plant Insect Pests--A Research Progress Report.”* 
4 
