Resistance The Bugs Fight Back

1908 1940 1950 1960 1970 1980 1990 Year

FIGURE 8-6 Chronological increase in the number of insect and mite species resistant to at least one type of insecticide (total), and species resistant to each of the five principal classes of insecticides. Not discussed in this chapter were polychlorinated cyclodiene (cyctodienes), pyreth-rums (pyrethroid), and carbamates. From G. P. Georghiou, in Managing Riointance to Agrochem-icals, ACS symposium series 421, p. 20. American Chemical Society, Washington, DC, 1990.

biochemical pathways to deal with synthetic pesticides developed over the past 250 million years as a result of warfare between plants and insects. Most plants don't just sit there allowing themselves to be chewed up by worms and beetles. For example, the members of the chrysanthemum family produce pyrethrums, a mixture of compounds toxic to many insects. (The pyrethrums extracted from the dried flowers of the chrysanthemum family are used as insecticides.)

The insect strains that survived these wars were the ones that evolved a biochemical apparatus to deal with the natural pesticides, and they use the same apparatus to deal with the synthetic agents. Another example is that birch trees generate a varying array of chemicals throughout the growing season to overcome the genetic ability of insect predators to adapt to the birch tree "insecticides.'' Resistance to DDT and the cyclodiene insecticides is controlled by genes that initiate the synthesis of proteins that bind these compounds. The protein-insecticide complex then induces the synthesis of detoxifying enzymes. In the case of DDT, the products are glutathione transferases, which catalyze the loss of HCl from DDT, and cytochrome P-450s, which deactivate by addition of a hydroxyl group to the DDT, are produced.

Application of the same or similar pesticides to a field for several years may result in the development of resistant strains of insects. Since the insects that

DDT -o-O-P —■— Carbamate -A- Pyrethroid

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1908 1940 1950 1960 1970 1980 1990 Year

TABLE 8-5

An Abbreviated Chronology of Colorado Potato Beetle Resistance to Insecticides in Long Island, New York

TABLE 8-5

An Abbreviated Chronology of Colorado Potato Beetle Resistance to Insecticides in Long Island, New York

Insecticide

Year introduced

Year first failure detected

Arsenicals

1880

1940s

DDT

1945

1952

Dieldrin

1954

1957

Endrin

1957

1958

Carbaryl

1959

1963

Azinphosmethyl

1959

1964

Monocrotophos

1973

1973

Phosmet

1973

1973

Phorate

1973

1974

Disulfoton

1973

1974

Carbofuran

1974

1976

Oxamyl

1978

1978

Fenvaleratea

1979

1981

Permethrina

1979

1981

Fenvalerate + p.b.a

1982

1983

Rotenone + p.b.a

1984

?

aM. Semel, New York State Agriculture Experimental Station, Riverhead, NY, personal communication, 1984: p.b. = piperonyl butoxide.

Source: G. P. Geovghiou, in Pesticide Resistance: Strategies and Tactics for Management Natl. Acad. Science, Washington, DC, 1985, Table 6, pg. 33.

aM. Semel, New York State Agriculture Experimental Station, Riverhead, NY, personal communication, 1984: p.b. = piperonyl butoxide.

Source: G. P. Geovghiou, in Pesticide Resistance: Strategies and Tactics for Management Natl. Acad. Science, Washington, DC, 1985, Table 6, pg. 33.

are not resistant to the pesticide are killed off, the farmer is helping to select and build up a population of resistant insects that are not killed by the pesticide. Since most pesticides are not specific for an insect pest, they will usually also kill off insects that are natural predators of the harmful insects. Consequently, when a resistant insect strain develops, it causes even more damage because the number of natural predators has been reduced by the pesticide.

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