Biological Control is the art and science of using biological agents to prevent insect outbreaks. There are two basic approaches which we will call natural enemy manipulation and classical biological control. In this segment we look at some examples of natural enemy manipulation involving the preservation and/or augmentation of native natural enemies to control native pests. In fact, consideration of predators and parasitoids should be part of every silvicultural prescription.

EXAMPLES

Ants. In some parts of Europe ant nests are dug up and moved to areas where there are problems from pests (slide).

Clerid beetles (slide) are important predators of bark beetles. Clerid populations can be manipulated by releasing laboratory reared adults into the forest; protecting them during spray operations by leaving the stumps of infested trees (where clerid larvae accumulate) unsprayed; and populations can be concentrated by the use of bark beetle pheromones, to which clerids are attracted.

Ladybird beetles overwinter in dense aggregations of adults (slide) where they are easy to collect and distribute to nurseries or plantations in which aphids or scales are causing problems.

Birds can play important roles in regulating insect pest populations at sparse densities. Many insectivorous birds nest in holes in dead trees, and their numbers and effectiveness can be increased by leaving snags (slide) or by providing nest boxes.

Insectivorous mammals (slide) can also be important in regulating insect pest populations at sparse densities. They may be particularly important in pine plantations where sawfly cocoons are a favorite food. The numbers and effectiveness of small mammals can be increased by creating nesting sites like brush piles and by providing food when it is scarce; e.g., in years of poor seed production small mammals may have a difficult time surviving the winter.

Herbivorous insects could possibly be used to control forest vegetation. For example, manzanita brush-fields often dominate a site after logging or forest fire in California and severely inhibit the growth and succession of conifers (slide). In order to decrease reliance on herbicides, we have been trying to control manzanita by setting off outbreaks of native herbivorous insects (slide), so far without success. It seems to be more difficult to set off outbreaks than one would think, which attests to the power of natural controls on insect populations.

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