Insects reproduce rapidly. They are capable of building up tremendous populations in a few years or even a few months. Fortunately, natural enemies of insects, such as pathogens, parasites and predators, usually prevent populations of insects from reaching outbreak proportions. The regulatory role of natural enemies is referred to as biological or natural control. Biological control may also include the release of predators or parasites, and the application of insect pathogens, such as microbial insecticides.
An understanding of the natural enemies that regulate insect populations is critical to the success of landscapers and nursery managers because destruction of natural enemies will most certainly result in outbreaks of insects that feed on ornamental landscape plants. Each insect species has a complex of viruses, bacteria, fungi and protozoans capable of infecting individuals and producing disease. Some of the more spectacular diseases are the viral infections of caterpillars that leave thousands of flaccid, black caterpillars hanging dead from tree branches, and the fungal diseases of grasshoppers and flies that cause the insects to climb up the plants, where they die clinging to the very top. Some pesticides have adverse effects on these pathogens of insects. In particular, many fungicides suppress important fungal pathogens of insects.
Parasites of insects live in or on the bodies of their hosts, where they feed during at least part of their life cycle. Many insect parasites are referred to as parasitoids because they consume a large portion of their host, eventually killing it. Most insect parasites are flies, bees and wasps. There are at least six important families of fly parasites and 10 families of important wasplike parasites. In just one family of parasitic wasps, the ichneumons, more than 3,100 species have been identified in North America. Most ichneumon species are specialized to parasitize only one or a few host insect species. Insect parasites are very susceptible to insecticides, and some may also be killed by other pesticides, such as fungicides and herbicides.
The final groups of natural enemies are the predators. Predators usually feed on insects smaller than themselves, ingesting one or more for a single meal. In general, they are very active insects that seek out other insects to feed on. Some of the more important families of predators are ground beetles, tiger beetles, ladybird beetles, social wasps, lacewings, some stinkbugs, assassin bugs, nabid bugs, robber flies, syrphid flies and some midge flies. A tremendous number of species of predaceous insects exist. For example, more than 2,500 species of ground beetles live in our area. Like parasites, predaceous insects are susceptible to insecticides. In some cases, they are far more sensitive to insecticides than the plant feeding insects they prey on.
Homeowners and landscapers should avoid unnecessary destruction of natural enemies. Avoid using insecticides unless you are certain they will give you the necessary reduction of insect pest numbers. Applying insecticides to resistant life stages of insects, such as the eggs of many insects, adults of armored scales or gall insects inside plant tissue, may kill off natural enemies and so result in increased numbers of plant feeding insects. Also, insecticide drift may destroy natural enemies on other plants in the area. In general, the use of pesticides destroys natural enemies and interferes with natural control. Use cultural and biological controls whenever possible to prevent the need for insecticides.