When we speak of “pesticide resistance” we are referring to genetic changes that take place in a population (pests) over time as a result of the natural selection of individuals whose genetic make up confers some measure of immunity to the pesticide. Over time, the genetic pool of the population includes a larger and larger percentage of individuals with resistance to a particular pesticide or class of pesticide.
Pesticide resistance spreads gradually. Initially resistance appears as isolated incidences on patches of land. As insects disperse over larger areas of heavy application, resistance grows (Georghiou, 1986).
Although resistance was rare in the 1950s, by the 1980s and 1990s many insects had become increasingly unresponsive to chemical treatments (NRC, 1986). In 1938, seven species of insects and mites were resistant to DDT; by 1984, 447 species were resistant to all the principal insecticide classes, including DDT, cyclodienes, organophosphates, carbamates, and pyrethroids (NRC, 1986). In the 1980s, the rate of increased cases of resistance declined, largely because most of the newly reported cases occurred in species already documented as resistant to a pesticide (NRC, 1986). Insects and other pests that are resistant to more than one pesticide are even more difficult to control (NRC, 1986).
Since the first case of resistance was discovered in the house fly Culex molestus in Italy over 25 years ago, 98 species of economically important insect species subsequently have developed resistance (Metcalf, 1973). These include fourteen species of human body louse as well as the human bedbug, the oriental rat flea, four other fleas, eighteen species of culicine mosquitoes, and the vectors of filariasis, western equine encephalitis, yellow fever, typhus, and the plague (Metcalf, 1973).
Continual application of pesticides on populations allows for large groups of resistant individuals to thrive in sprayed areas. Examples of pests that have become resistant in large areas are the hops aphid in England; the green rice leafhopper in Japan, the Philippines, Taiwan, and Vietnam; cattle ticks in Australia, and anopheline mosquitoes worldwide (NRC, 1986). In addition, pesticide applicators often increase initial pesticide doses in order to ensure efficacy; as a result, reactive pests of the pest populations are over-treated while resistant individuals continue to thrive (NRC, 1986).
Applicators may switch to new chemicals to combat resistance; however, resistance to new chemicals will eventually develop (NRC, 1986). As resistance increases for each newly applied agent, higher and more frequent intervals of application enhance insect resistance, even in non-targeted species. In the Pacific coastal zone of Central America and southern Mexico during 1979, 30 liters of mixtures of active ingredients were applied in 30 treatments on cotton crops over a six-month period. The magnitude of these applications resulted in increased selection of resistance among mosquitoes, which had not been targeted in the agricultural spraying (Georghiou, 1986). Often, such application processes or schedules expedite or exacerbate normal pest resistance responses.
Resistance is one of the greatest problems opposing the control of vector-borne diseases around the world, particularly in developing countries. Late in the 1940s, the World Health Organization (WHO) initiated programs to eradicate malaria around the world with the use of DDT. Eventually the targeted anopheline mosquitoes, which are vectors of malaria, grew resistant (Georghiou, 1986). Fifty-one of these species are resistant; 47 resistant to dieldrin, 24 to DDT, 10 to organophosphates, and 4 to carbamates (Georghiou, 1986). By 1984, DDT resistance in Anopheles culcifacies was found over much of India (Georghiou, 1986). [figure 8, pg. 28] The number of Anopheles albimanus in Guatemala that are responsive to DDT has declined from nearly 100 percent in 1959 to approximately 5 percent in 1980. In areas of El Salvador, the susceptible gene in the same species of anopheles was reduced by 52 percent by 1972 (Georghiou, 1986).
The development of previously uncultured land in tropical areas has intensified the problem of insect resistance, especially in anopheline mosquitoes. Advances in technology and improvements in agricultural science have allowed for widespread pesticide application in tropical areas to produce high-yielding crops. Humans living in tropical areas had rarely farmed these areas, due to the prevalence of illness and death from malaria (NRC, 1986). Malaria eradication efforts enabled farmers to reach previously uncultivated areas. Yet, the new vegetation covering these areas provides a receptive habitat for mosquitoes, and drainage and irrigation ditches generate suitable breeding sites for the mosquito larvae (NRC, 1986). Thus, the cycle continues as farmers apply toxicants to ever-increasing resistant organisms.
Pest resistance problems are difficult to resolve because the development of pesticide formulas with new modes of action is extremely expensive and complex (NRC, 1986). In the past, the direct costs of pest control soared for farmers who continually applied increasing amounts of pesticides or changed to new, more expensive substances. The costs of spraying homes for malaria control in various malaria control programs in the 1970s increased 5.3 times when malathion replaced DDT and between 15 and 20 times when propoxur, fenitrothion, and deltamethrin replaced DDT (Table 3, p. 35) during the 1970s.
The rising costs of developing new pesticides limited development of the industry in the 1980s. For instance, in 1956 it cost approximately $1.2 million to develop an agricultural chemical; by 1981 the cost had risen to at least $20 million. Because pest resistance negatively affects the performance of these chemicals, developers faced diminished return on their investments and significantly curtailed production (NRC, 1986).
