Biological Control

Biological control is the use of an organism to control the growth rate of a pest population through predation, parasitism, herbivory, or competition. Optimally, biological control reduces a pest population to a level that is below economic injury to the crop, but [Page 52]high enough to sustain the population of the beneficial organism. Because biological control relies on ecological processes, it is considered an essential tactic in integrated pest management programs.

Most animal populations experience fluctuations in numbers as a result of abiotic disturbances, such as drought, floods, and hurricanes, and biological factors, such as natural enemies, disease, and food quantity and quality. When resources are abundant, favorable environmental conditions exist, and natural enemies are scarce, population growth rates can be high. Conversely, when resources are limiting, environmental conditions are unfavorable, and activity from an antagonist organism increases, the animal population can decline.

This U.S. Department of Agriculture Agricultural Research Service staffer is working with an insect called Encarsia formosa, which may be beneficial in controlling the whitefly pest.

Source: U.S. Department of Agriculture Agricultural Research Service/Scott Bauer

Most plants possess physical (e.g., spines, waxy coatings, tough cell walls) or chemical (e.g., plant secondary metabolites) defenses against plant-feeding organisms. In agricultural crops, breeding has mainly focused on characteristics such as uniform growth, durability during shipping, and palatability. In the process, the plant's natural defenses against herbivores may be reduced or lost, increasing their susceptibility to damage from pests. When susceptible crops are planted in large “monocultures,” they can be more apparent to and therefore more easily located by herbivores than in natural systems, which tend to be more diverse. Hence, in agroecosystems, food resources are generally not limiting for herbivores, and their populations may increase to levels that reduce crop yield or marketability, thereby becoming a “pest.” Furthermore, intensively managed monocultures may lack the diversity and stability of resources to support a diverse natural enemy community. Thus, the idea behind biological control is to increase the population numbers of natural enemies of pests by either introducing or attracting them to fields or conserving those that may already be present.

Biocontrol agents can be herbivores, predators, parasitoids, or pathogens. Within those functional groups, biocontrol agents may be generalists (use a broad number of species) or specialists (use a limited number of species). When their numbers are sufficiently high, specialist natural enemies can be particularly effective at quickly and efficiently reducing the population of a single host species or genus. On contrast, the collective activity of generalists, which may be common in the farm landscape, can prevent a host population from reaching pest status.

Weed populations are controlled by herbivores and plant pathogens, which use above-ground (stems, leaves, flowers, and seeds) or below-ground (roots and tubers) plant parts. [Page 53]Ground beetles (Coleoptera: Carabidae) have been widely documented for their ability to consistently consume weed seeds present in the soil (the seed bank), thereby reducing the following generation of weed seedlings. The combined granivory (seed-feeding) of crickets, earthworms, slugs, rodents, and birds can also reduce the seed bank of several economically important weed species, such as jimsonweed, pigweed, and common lamb's-quarters, by as much as 90 percent.

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