Natural Enemies in Field Crops: A Guide to Biological Control (E2721)
Centuries ago, Chinese farmers observed that ants in their citrus orchards were feeding on caterpillars, beetles and leaf-feeding bugs.
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What is Biological Control?
Centuries ago, Chinese farmers observed that ants in their citrus orchards were feeding on caterpillars, beetles and leaf-feeding bugs. The farmers discovered that collecting the papery nests of a specific type of ant from trees in the countryside and moving them into their orchards gave them better control of some pests. They also provided aerial bamboo bridges among the citrus trees to help the ants move easily from tree to tree. These efforts to increase the numbers of ants in the orchard and to heighten their efficiency as predators is the first recorded occurrence of biological control of insects. Biological control is the manipulation of populations of living beneficial organisms, called natural enemies, to reduce the numbers of pests or the amount of damage caused by pests.
Agents of Biological Control
Most of us are familiar with spiders, ladybugs and praying mantids and how they feed on pests. Many other natural enemies also control pests. There are three major groups of natural enemies: predators, parasitoids and pathogens.
Predators may be insects or other arthropods, such as spiders. Each individual consumes many insect prey during its lifetime. Predators are often large, active and conspicuous in their behavior and are, therefore, readily recognized in the field.
Parasitoids are insects that lay their eggs in or on the host insect. When the parasitoid egg hatches, the young parasitoid larva feeds on the host (sometimes an important pest) and kills it. Usually the parasitoid larva feeds on one host until it becomes an adult. Many parasitoids are very specific to the type of host insect they can attack and not harmful to humans. Though parasitoids are very common, they are small and often go unnoticed.
Pathogens are viruses, bacteria, fungi, nematodes and other microorganisms that cause insect diseases. Disease epidemics among insects occur naturally when insect populations are very large or when environmental conditions favor the growth of the disease organism. Certain insect pathogens are available commercially as microbial insecticides and have been convenient and successful in achieving biological control of pests.
Approaches to Biological Control
There are three broad approaches to biological control. Classical biological control is conducted by federal and state agencies when pests are exotic in origin. Exotic natural enemies are imported and released to bring about control. Though you will not be importing natural enemies into the field, some important natural enemies such as parasitoids of alfalfa weevil and cereal leaf beetle are here because of past classical biological control programs. Conservation biological control improves the effectiveness of natural enemies through farming practices that provide necessary resources for their survival and protect them from toxins and other adverse conditions. Augmentation biological control temporarily increases the numbers of natural enemies through periodic releases, thereby increasing the overall numbers of natural enemies and improving biological control.