Pest Control
I -INTRODUCTION
Pest Control, any of a wide range of environmental interventions that have as their objective the reduction to acceptable levels of insect pests, plant pathogens, and weed populations. Specific control techniques include chemical, physical, and biological mechanisms. Despite all the control efforts used, pests annually destroy about 35 percent of all crops worldwide. Even after food is harvested, insects, microorganisms, rodents, and birds inflict a further 10 to 20 percent loss, bringing the total destruction to about 40 or 50 percent. With so many areas of the world facing serious food shortages, researchers seek to reduce this loss by improving pest control.
II -CHEMICAL CONTROLS
The chemical agents called pesticides include herbicides (for weed control), insecticides, and fungicides. More than half the pesticides used in the United States are herbicides that control weeds. The United States Department of Agriculture (USDA) estimates indicate that 86 percent of U.S. agricultural land areas are treated with herbicides, 18 percent with insecticides, and 3 percent with fungicides. The amount of pesticide used on different crops also varies. For example, in the United States, about 67 percent of the insecticides used in agriculture are applied to two crops, cotton and corn; about 70 percent of the herbicides are applied to corn and soybeans, and most of the fungicides are applied to fruit and vegetable crops.
Most of the insecticides now applied are long-lasting synthetic compounds that affect the nervous system of insects on contact. Among the most effective are the chlorinated hydrocarbons DDT, chlordane, and toxaphene, although agricultural use of DDT has been banned in the United States since 1973. Others, the organophosphate insecticides, include malathion, parathion, and dimethoate. Among the most effective herbicides are the compounds of 2,4-D (2,4-dichlorophenoxyacetic acid), only a few kilograms of which are required per hectare to kill broad-leaved weeds while leaving grains unaffected.
Agricultural pesticides prevent a monetary loss of about $9 billion each year in the United States. For every $1 invested in pesticides, the American farmer gets about $4 in return. These benefits, however, must be weighed against the costs to society of using pesticides, as seen in the banning of ethylene dibromide in the early 1980s. These costs include human poisonings, fish kills, honeybee poisonings, and the contamination of livestock products. The environmental and social costs of pesticide use in the United States have been estimated to be at least $1 billion each year. Thus, although pesticides are valuable for agriculture, they also can cause serious harm.
Indeed, the question may be asked—what would crop losses be if insecticides were not used in the United States, and readily available nonchemical controls were substituted? The best estimate is that only another 5 percent of the nation's food would be lost. Many environmentalists and others advocate organic farming as an alternative to heavy chemical pesticide use.
III -NONCHEMICAL CONTROLS
Many pests that are attached to crop residues can be eliminated by plowing them underground. Simple paper or plastic barriers placed around fruit trees deter insects, which can also be attracted to light traps and destroyed. Weeds can be controlled by spreading grass, leaf, or black plastic mulch. Weeds also may be pulled or hoed from the soil.
Many biological controls are also effective. Such insect pests as the European corn borer, Pyrausta nubilalis, and the Japanese beetle, Popillia japonica, have been controlled by introducing their predators and parasites. Wasps that prey on fruit-boring insect larvae are now being commercially bred and released in California orchards. The many hundreds of species of viruses, bacteria, protozoa, fungi, and nematodes that parasitize pest insects and weeds are now being investigated as selective control agents.
Another area of biological control is breeding host plants to be pest resistant, making them less prone to attack by fungi and insects. The use of sex pheromones is an effective measure for luring and trapping insects.
Pheromones have been synthesized for the Mediterranean fruit fly, the melon fly, and the Oriental fruit fly. Another promising pest-control method is the release of sterilized male insects into wild pest populations, causing females to bear infertile eggs. Of these techniques, breeding host-plant resistance and using beneficial parasites and predators are the most effective. Interestingly, the combined use of biological and physical controls accounts for more pest control than chemical pesticides.
Integrated pest management (IPM) is a recently developed technology for pest control that is aimed at achieving the desired control while reducing the use of pesticides. To accomplish this, various combinations of chemical, biological, and physical controls are employed. In the past, pesticides were all too often applied routinely whether needed or not. With IPM, pest populations as well as beneficial parasite and predator populations are monitored to determine whether the pests actually present a serious problem that needs to be treated. If properly and extensively employed, IPM might reduce pesticide use by as much as 50 percent, while at the same time improving pest control. If this goal were achieved, the environmental problems would be minimized, and significant benefits would result for farmers and society as a whole.