Insecticides and Herbicides

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INSECTICIDES AND HERBICIDES

INSECTICIDES AND HERBICIDES. Farmers first discovered cultural and physical pest controls such as crop rotations, nutrient management, intercropping, and residue destruction. They also learned to use dust prepared from plants containing naturally occurring insecticides, such as nicotine and pyrethrum. Applied entomologists studied the life history of insects, seeking the keys to control strategies. U.S. Department of Agriculture entomologists imported a predaceous ladybug, known as the Vedalia beetle, from Australia in 1888 to control cottony-cushion scale on California citrus trees. The spectacular success touched off a quest for biological control agents.

The use of chemical arsenical compounds as insecticides dates from at least 1681.Paris green and London purple, both arsenical insecticides, became the main stomach poisons of chewing insects in the late nineteenth and early twentieth centuries. Other uses of insecticides in the late nineteenth century included carbon bisulfide to disinfect stored grain and control root lice and hydrocyanic acid gas to fumigate California citrus trees. In 1880, lime sulfite was employed against the San Jose scale, and in 1882 the grape phylloxera was controlled with naphthalene. Lead arsenate, first used against the gypsy moth in New England forests in 1892, was used against the cotton boll weevil until the development of calcium arsenate was recommended in 1916.In 1906, the U.S. Department of Agriculture began using arsenical dips against the Texas fever-carrying cattle tick.

During World War II, many new chlorinated hydrocarbon insecticides were produced, DDT being the most effective and widely used. These organic, contact insecticides (usually attacking the nervous system) proved more effective than the internal arsenicals, which often scorched plants during application and built up toxic residues in the soil. But with the publication of Rachel Carson's Silent Spring in 1962, people became increasingly aware of the danger such insecticides posed to other animal life. The Environmental Protection Agency banned most uses of DDT in December 1972.Scientists concentrated more on developing the organophosphorous and carbamate insecticides, which generally circumvented the problem of transmittable residues but were more toxic to man and animals during application. The Department of Agriculture also developed systemic insecticides to protect plants and animals. In 1958, ronnel became the first systemic insecticide ingested by cattle to kill the cattle grub.

The so-called second generation of insecticides encountered two other problems: development of insect resistance and killing of nontargeted insects. In some cases, pests quickly developed resistance to organophosphates, increasing the impact of both major and previously minor pests. Furthermore, when these broad-spectrum pesticides killed parasites and predators, minor pests were released from natural controls and exploded into major pest status.

In the 1960s, entomologists started preaching integrated pest management (IPM), which utilized mutually supporting systems of cultural, biological, chemical, and other controls. Pheromones, sexual attractants, and sterile-males techniques were added to the strategies. Genes from strains of the naturally occurring bacterium Bacillus thuringiensis were introduced into several crops to ward off insect damage. Insecticides continued to be a part of IPM, but more recently developed insecticides (pyrethroids, insect growth regulators, neonicotinoids) were generally targeted to a narrower range of insects and were applied in lower doses.

American and European scientists discovered several chemicals that acted as herbicides in certain situations, including copper salts, carbolic acid, caustic soda, arsenical compounds, and kerosene. But expense, the toxicity of arsenical compounds to plants, the flammability of oils, and the lack of selectivity retarded the use of chemical weed killers. Farmers generally relied on mowing, cultivation, hoeing, fire, and crop rotation to combat weeds.

Some agriculturists recognized that it would be desirable to eliminate frequent plowing and cultivation, which bared the soil to wind and water erosion and caused the loss of organic matter and soil compaction. Just before World War II, researchers in the United States and Canada experimented with "trashy fallow." The idea was to pull blades under the soil surface to cut off the moisture-sapping roots, leaving wheat stubble and other crop residues on the surface. Plant growth regulators presented the possibility of planting through the crop residues. The herbicide 2,4D, released for use in 1946, selected broad-leaf plants and was used to control weeds in grasses, including wheat, oats, and corn. By the late 1950s, herbicides such as paraquat and diquat, which killed all herbaceous plants, had been introduced. These herbicides had the added advantage of being deactivated when they touched the ground, leaving the soil ready to accept the next crop. During the 1950s and 1970s, researchers tested combinations of equipment, chemicals, and growing methods. The Farm Bill of 1985 gave no-till and other forms of reduced tillage, known collectively as conservation till-age, a boost. The law required that farmers who received assistance from the Department of Agriculture, including price support payments, had to reduce erosion on highly erodible land to an acceptable level. Conservation tillage utilizing herbicides was the most cost-effective way for many farmers to meet the requirements. Farmers utilized conservation tillage methods on 73 million acres in 1990, 98 million acres in 1995, and 108 million acres in 2000.