Control Of Insect Pests

Under natural conditions, insect numbers are controlled by various means, principally by climatic strictures and by predators, parasites, parasitoids, and disease (Aguilar 1989, Strong 1984). Insects living in unwanted proximity to humanity and competing with people for food and fiber require artificial control (Martin and Wood cock 1983). Practical (applied or economic) entomologists have devised a great many strategies to deal with pest species. Today, eradication is not the goal, as in the past, so much as reducing damage to acceptable tolerance levels, an approach referred to as "pest management" (Metcalf and Luckman 1982). Methods of pest management are successful to different degrees, depending on local conditions, damage levels, and availability of funds. In recent years, they have been combined in appropriate ways to capitalize on the best aspects of each, in the technique of "integrated control" (Apple and Smith 1976, van Huis 1981). This is usually the most logical and productive approach, rather than relying entirely on just one method, primarily chemicals, to achieve quick and cheap results. The use of chemicals alone has not been totally successful because of the ability of insects to develop resistance to most poisons (Georghiou and Saito 1983) and the deleterious side effects that accompany pesticide use (environmental pollution and destruction of nontarget organisms) (Green 1976). In fact, heavy and exclusive use of insecticides may actually lead to a decrease in crop production. A classic case of this in Latin America took place in the Cañete Valley in Peru (Barducci 1971). In the 1920s, agricultural emphasis here shifted from sugarcane to cotton. In the following two decades, cotton pest control was accomplished unevenly with chemicals and some ecological methods, and yields varied. In the 1950s, however, treatments with organic insecticides increased greatly and became pervasive. Yields dropped dramatically, and pests increased in kinds and intensity of damage. Finally, integrated methods were introduced, and after a few years, the crisis abated.

A key aspect in successful integrated control is vigilance and monitoring, using various kinds of trapping and sampling techniques to assess pest and damage levels before control measures are instigated

(e.g., Silveira Neto 1972). Integrated control takes advantage of the following diverse methods of keeping checks on injurious insects.

1. Chemical control. Insecticides (pesticides) kill insects by their chemical action, usually by interfering with some essential metabolic function, such as transmission of electrical impulses across nerve synapses or blocking nutritive pathways (Corbett et al. 1984, Wilkinson 1976). Poisonous compounds reach their target tissues by ingestion (stomach poisons), by passage through the integument and sense organs (contact poisons), or by entering the tracheal system (fumigants).

Insecticides come in almost infinite variety and chemical structure (Martin and Worthing 1976, Wiswesser 1976), and their mode of action, application, safety, and effectiveness comprise the complex subject of insect toxicology (Matsumura 1975). The major categories of insecticides based on chemistry (Buchel 1983) are the inorganics, for example, arsenic compounds, cyanide gas, and the botanicals ("first-generation insecticides"), the naturally occurring types of which (rotenone, pyrethrum, nicotine) have been in use for centuries in many parts of the world. Then there are the synthetic organics ("second generation"), products of modern chemistry, including such well-known chlorinated hydrocarbons as DDT, benzene hexachloride, and Chlordane. To these have been added recently the organophosphates (Malathion, Parathion) and the carbamates, both classes noted for their great potency.

Insecticides are formulated or applied in various ways, as sprays, aerosols, or gases (fumigants), in pellets or granules, oils, injectates, dusts, and so on.

Other chemicals are useful in combat ing insects by actions other than killing them. Such are repellents that prevent the pest from doing damage in the first place (Davis 1985). These are most often used against biting flies and evolved from the use of natural substances for centuries by many peoples to spare them the ravages of mosquitoes, punkies, and blackflies. Indians and rural people everywhere build smoky fires to repel insects. The Peruvian Amazon Indians rub their skin with the fruit of the Siparuna (Monimiaceae) shrub, which produces a citronella-like odor that is intended to ward off mosquitoes. Very effective artificially made repellents are dimethyl phthalate and DEET (N,N-diethyl-3-methylbensamide).

Another form of chemical control is the release of laboratory-produced phe-romones to confuse the mating behavior of crop pests and suppress their numbers by interfering with reproduction (Jacobson 1965). This seems to have the best potential with lepidop-terous pests (Roelofs and Cardé 1977). A substance mimicking the courtship pheromone of the pink bollworm ("gos-syplure") has been fairly successful in this way in cotton fields. Sex-attractant pheromones also are used in traps to catch Mediterranean ("medlure") and Mexican Fruit Flies so that their presence can be detected and monitored in infested areas. Hormone analogues, mostly growth regulators, are now known which disrupt normal growth and kill. Such sophisticated chemical attacks, engineered to spurn only the offensive species and operating on fundamental life processes so that resistance is unlikely, constitute the modern front of pest control ("third generation insecticides" or semiochemicals).

2. Physical control. These methods often involve the use of special equipment, machinery, and electrical or radiation-producing devices. They are generally costly in time and labor and rarely give general control. Some, however, are simple, such as the use of screens or barriers. A very direct approach is hand picking and destruction by gathering and disposing of the pest. This was done against larval Lepidoptera, such as sphinx caterpillars and earworms, by ancient Americans but is not practiced on a commercial scale today. However, not very long ago, bounties were offered for scorpions in Durango, Mexico. It is reported that from April 1785 to October 1787, prizes were paid on 506,644 scorpions in that city (Baerg 1929: 422).

Pests may be caught in machines where they are killed by exposure to fumigants or excessively high or low temperatures. Most recently, electromagnetic radiation has been applied in different forms for control. Light draws many insects to their death in traps set at the edges of fields or around habitations. Ionizing radiation is a very effective device in the war against the screwworm and various fruit flies. It is used to sterilize males in great numbers so that they may be released to flood local populations and eliminate effective reproduction ("sterile male technique"). Ultrasonic waves have been tried for insect control but are totally ineffective (Ballard and Gold 1983, Lewis et al. 1982). Other physical forces, such as lasers and radiowaves, are now undergoing trials as potential control agents.

3. Biological control. An attractive approach because it concentrates on the target organism and causes minimal harm to the environment is biological control (Cock 1985, Hoy and Hertzog 1985). Classically, this has depended on the introduction of the pest's natural enemies (Caltagirone 1957, Sweetman 1958), which include predators, parasites, parasitoids, and disease microor ganisms (Maramorosch and Sherman 1985). Many of the first three are other insects but may include insectivorous vertebrates such as birds, fish, toads, or lizards. This method is usually also self-sustaining and can maintain pest populations at low levels indefinitely as long as some hosts are left to sustain predator-parasite populations.

There are many examples of biological control in Latin America. The area serves both as the recipient of control agents and as the source of them. In the former category are five parasitic wasp species introduced into Cuba and Mexico from the Middle East which finally controlled the citrus blackfly (Aleurocan-thus woglumi) in the first half of this century after many attempts (De Bach 1974: 139fi, 167f.). An example of the latter case is an internal wasp parasitoid from Brazil (Tetracnemus peregrinus) which helped stop the mealybug, Pseudococcus adonidum, in parts of the United States early in this century. The Amazon fly (Metagonistylum minense, Tachinidae) has been transported from its native South America to several areas of the Caribbean where it now helps to keep the sugarcane borers (Diatraea) in check (De Bach 1974: 143 f.).

Other forms of biological control are being devised in endlessly ingenious ways by contemporary researchers in practical entomology. Bacillus thuringi-ensis is a bacterium lethal to caterpillars and other larval forms. Commercial preparations of the protein crystalline inclusions in the spores may be disseminated and act like a specific insecticide (Thuricide). Genetic control (Kirsch-baum 1985, Pal and Whitten 1974) takes advantage of lethal or repressive genes that entomologists artificially introduce into wild populations from laboratory-reared individuals carrying these genes.

4. Cultural control. This method of control uses ordinary farm or management prac tices to reduce damage from pests as much as possible. It is the cheapest of all control measures but must be planned far in advance of the season of potential damage. It is also necessary to understand in detail the life history and habits of the insect pests involved.

The most common application of cultural control is crop rotation and timed tilling or soil cultivation. The objective is to remove the insect's food source and modify its environment at critical times of its life cycle. By varying the time of planting and harvesting, infestations may be avoided or much reduced. The use of resistant crop or animal varieties also keeps pest problems to a minimum (Maxwell and Jennings 1980). Some strains or varieties of cultivars are more or less resistant to insect attack and can be selected for propagation in pest-prone areas.

5. Legal control. The law can be applied against insect infestations and considerably ameliorate major problems. A powerful weapon against both medical and agricultural pests is quarantine. Spread of the offending species outside of the primary area is prevented, and control efforts can be concentrated on eradication. Legal measures are also important in making sure that pests are kept out of a country or region. Historically, the most serious injurious insects have been imported from other places. Free of their natural enemies in their home territories, their populations explode in the new lands. Laws are necessary to enforce safe use of dangerous insecticides and movement of materials that might spread problem species. They also establish agencies to control and study injurious insects and related arthropods, such as agricultural schools and experiment stations, pest control commissions and boards, institutes, and abatement districts.


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