2-3 Reading 2: Pest Control Methods

Biological and Integrated Control of Pests

Biological Control

#1. Biological control refers to the use of natural predators or parasites to control insects and related pests. Unlike the use of insecticides, which is aimed at controlling and destroying the pest temporarily, biological control is aimed at a continuing reduction of the pest population.

#2. There are two basic approaches to biological control. In one approach, the selected predator or parasite is introduced into the region where a particular pest is abundant. An example of this approach was the introduction of the vedalia beetle into California in the late 1880s to help control the cottony-cushion scale, an insect that had almost entirely destroyed the citrus groves there. In the second basic approach to biological control, the environment of the infested region is adjusted to encourage the growth of natural predators or parasites already living in the area. An example of this approach is the practice of planting wild blackberries near grape vineyards because an insect that parasitizes the eggs of the grape leafhopper, a serious pest of grapes, breeds throughout the winter on the eggs of a type of leafhopper that feeds on wild blackberries. Thus, planting of wild blackberries near the vineyard encourages the growth of the insects that parasitize the eggs of the grape leafhopper.

#3. The most spectacular results of biological control are seen in cases where insect predators have been introduced from the same region. The first time this method was used was in 1873, when a predatory mite native to the United States was brought to France to help control the grape phylloxera, a type of aphid that had been accidentally introduced into France from the United States in 1860. A similar situation occurred in 1883. In that year a small wasp was introduced into the United States from England to control the imported cabbage worm, which had been accidentally brought from England years earlier.

#4. Among the organisms most often used in biological control are insects, spiders, birds, and mammals. However, microscopic organisms, such as bacteria, have also been effective in controlling insect pests. Chief among these microorganisms is the bacterium Bacillus thuringiensis, which causes a fatal disease of the Japanese beetle and European chafer. The larvae of these insect pests normally feed on the roots of grasses, so that the application of the bacterial spores to the soil effectively eliminates the pests from the area. The same bacterium is also used to control a wide variety of agricultural pests, especially the butterflies and moths whose larvae attack cabbage and corn plants. Although viruses have been used experimentally to control certain insect pests, it is more difficult to create the conditions necessary for viruses to be effective.

Problems with biological control of pests

#5. One of the key conditions of biological control is that there must be an adequate supply of pests on which the predators or parasites can feed. This is one of the major drawbacks of biological control, because the pest population may reach tremendous size and cause substantial damage before it becomes large enough to be controlled by predators or parasites.

#6. A second drawback to biological control is that the species introduced to control a pest may itself become a serious pest. In the early 1850s the English, or house, sparrow was introduced into New York City to control the cankerworms that were destroying the shade trees. For several years the birds were generally limited to the larger cities along the eastern coast, but by 1900 they were widespread throughout the country. House sparrows are now considered to be a serious agricultural pest because they feed on grains and other cultivated crops.

#7. Integrated control is the combination of biological, chemical, and other techniques in order to obtain the advantages of each of these methods while minimizing their inherent weaknesses. In an integrated control program the pests in the area are divided into four groups: key pests, occasional pests, potential pests, and migrant pests. The basic aim of the program is to allow as many non-destructive and even mildly destructive insects as possible to remain in the area while reducing the population of destructive key pests by manipulating the environment. For example, in controlling the spotted alfalfa aphid, it has been found that low dosages of insecticides reduce the aphid population temporarily but do not affect the aphid predators and parasites, which can control the aphids after the use of the insecticide is discontinued.