Synthetic chemicals will probably continue for some time as the major weapon against most pests because of their general reliability and rapid action, and their ability to maintain the high quality of agricultural products that is demanded by urban consumers today. Although new chemicals offer a short-term solution, this approach to pest control alone will rarely provide a viable, long-term strategy. Moreover, a few years of commercial exploitation may not justify the investment required to develop a new pesticide today, except where there are reasonable prospects that a pesticide's mode of action may be beyond <gjso. the capability of the pest for genetic adaptation.

Despite the continual threat of resistance, we may still be able to exploit our expanding knowledge of the genetic and biochemical makeup of pests by <g .| designing pesticides that can circumvent existing resistance mechanisms, at ^ <D least long enough to provide chemical manufacturers a reasonable rate of financial return on the investment needed to develop a new pesticide. Realistically, though, it is difficult to be optimistic on this point in practical situations where a synthetic pesticide is applied repeatedly to the same crop or environment to control a well-adapted pest. History promises no

J $ encouragement, at least for most pests, for the discovery of a ''silver bullet.'' On "g is the other hand, it is indeed encouraging that there are examples of pesticides, both selective and nonselective (e.g., the polyene fungistat pimaricin, the widely used herbicide 2,4-D, and the insecticides azinphosmethyl and carbofuran), that have been used for years in certain situations without setting off rapid, extensive resistance. The phenoxy herbicides (e.g., 2,4-D) and the broad-spectrum fungicides (captan, dithiocarbamates, and fixed coppers) have been used successfully for decades without serious resistance problems. Still, the wisest course for future research appears to be the integration of a diversity of approaches to pest control—chemical, biological, and cultural (or ecological)— 2° g because an integrated application of multiple methods will produce minimum selection pressure for development of resistance to pesticides. Evolution of Hgio resistance to minimally selective or multitarget synthetic chemicals might be delayed indefinitely if the selection pressure were kept within "reasonable" limits. The pressure might be reduced with crop rotations and careful o management, but may be virtually impossible in agricultural areas typified by repeated monocultures.

The development of resistance is encouraged by pesticides that act upon single biochemical targets. Unfortunately, the modes of action of many E systemic plant fungicides, and most modern synthetic insecticides and herbi o tn http.//www.naGEftEcf(alBgOCHEMïCAL) AND PHYSIOLOGICAL MECHANISMS OF RESISTANCE 50 TO PESTICIDES

cides, are biochemically site-specific. Many of these fungicides and insecticides have produced a rapid, major buildup of resistance genes in pest populations after just a few seasons of use. Undoubtedly, the potential for resistance development to such compounds will continue to be a limiting factor in the widespread use of these compounds, although compounds differ in the degree of risk for rapid development of resistance. In addition, some compounds lend rô EÏS themselves to relatively effective resistance management strategy. Others do ra

ro £ '¡5 not. The genetic and biological reasons that some compounds rapidly select for

"sis resistance, whereas others do not, are presently obscure in nearly all cases.

® Further research in this area will greatly facilitate the development of o ^ w efficacious strategies to manage resistance.

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