Overview of natural enemies of insect pests

Agents of biological control (natural enemies) of insects include predators, parasitic insects, nematodes, and pathogens.

Orius (pirate bug) preying on an aphid Photo credit: S. Yoo

Predators may be insects or other insectivorous animals, each of which consumes many insect prey during its lifetime. Predators are often large, active, and/or conspicuous in their behavior, and they are therefore more readily recognized than are parasites and pathogens.

Parasites (also called parasitoids) of insects are other insects that lay their eggs in or on the host insect. When the parasite egg hatches, the young parasite larva feeds on the host (the pest) and kills it. Usually that one host is sufficient to feed the immature parasite until it becomes an adult. Many parasites are specific to the type of host insect they can attack, and they are not harmful to humans.

Although insect parasites are very common, they are not well known because of their small size. One of the smallest, Trichogramma, is only about the size of the period at the end of this sentence; it parasitizes the eggs of other insects. There are many types of aphid parasites; these, too, are very small, being no larger than its aphid host.

Aphelinus albipodus, a parasitic wasp, laying eggs in an aphid Photo credit: Bob Ellingson

Insects, like other animals, are subject to attack by disease organisms. Insect pathogens include viruses, bacteria, fungi, and other microorganisms. Disease epidemics among insects are not commonly encountered in nature except when insect populations are large or when environmental conditions favor the growth of the disease organism. Nevertheless, insect pathogens are important in the constant suppression of pest populations.

An aphid covered with mycelium of the fungal aphid pathogen, Paecilomyces lilacinus. Photo credit: Bill Stoneman

Certain insect pathogens have been successfully manipulated to achieve biological control of specific pests. For example, different strains of the bacterium Bacillus thuringiensis, commonly known as Bt, are marketed to control many insects, including various caterpillars, mosquito larvae, and Colorado potato beetles. Many insect pathogens attack only one species or a limited group of insects and therefore are unlikely to harm non-target species such as beneficial insects, humans, livestock, wildlife, or plants.

Insect-parasitic nematodes are small worms that attack and kill insects that live in moist habitats, especially water and damp soil. These nematodes are not harmful to other animals or plants. A few types are mass-produced and sold for pest control.