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1 Introduction RESISTANCE IS A CONSEQUENCE of basic evolutionary processes. Pop- ulations have genetic variance, and plants and herbivores have a history, respectively, of evolving chemical defenses and overcoming them. Some individuals in a pest population may be able to survive initial applications of a chemical designed to kill them, and this survival may be due to genetic differences rather than to escape from full exposure. The breeding population that survives initial applications of pesticide is made up of an ever-increasing proportion of individuals that are able to resist the compound and to pass this characteristic on to their offspring. Because pesticide users often assume that the survivors did not receive a lethal dose, they may react by increasing the pesticide dosage and frequency of application, which results in a further loss of susceptible pests and an increase in the proportion of resistant individuals. Often, the next step is to switch to a new product. With time, though, resistance to the new chemical also evolves. During the early 1950s, resistance was rare, while fully susceptible pop- ulations, of insects at least, have become rare in the 1980s. Known to occur for nearly 76 years, resistance has become most serious since the discovery and widespread use of synthetic organic compounds in the last 40 years. (See Georghiou, this volume, for a fuller treatment of the magnitude of the problem.) Resistance in plant pathogens became a problem in the mid- 1960s and has increased over the last 15 years along with use of systemic fungicides. Resistance is being detected with increasing frequency in weeds that have been intensively treated with herbicides. Pesticide resistance in rodents now occurs worldwide. 1 1
12 INTRODUCTION Resistance in insects and mites rose from 7 species resistant to DDT in 1938 to 447 species resistant to members of all the principal classes of insecticides, i.e., DDT, cyclodienes, organophosphates, carbamates, and pyrethroids, in 1984. Nearly all (97 percent) of these species are of agri- cultural or veterinary importance. Almost half of these species are able to resist compounds in more than one of these classes of insecticides, and 17 species can resist compounds in all five classes. Resistance occurs as well in at least 100 species of plant pathogens (primarily to the fungicide benomyl), 55 species of weeds (mainly to the triazine herbicides), 2 species of nema- todes, and 5 species of rodents. To appreciate the gravity of resistance to pesticides in agriculture and public health, though, it is necessary to look beyond lists of species known to exhibit resistance. For example, the rate of increase in species of arthropods newly reported as resistant to some pesticide has actually declined since 1980 because more of the new cases of resistance now occur in species already "counted" as resistant to some other compound. This is an even greater cause for alarm, however, since resistance to more than one compound usually means that the pest is harder to control. Furthermore, when pests are subjected to prolonged and intensive selection, frequency of resistance may stabilize at high levels over wide areas for example, the hops aphid in England; the green rice leafhopper in Japan, the Philippines, Taiwan, and Vietnam; cattle ticks in Australia; and anopheline mosquitoes nearly world- wide. Resistance is probably the major contemporary problem in control of vectorborne diseases, particularly malaria, in most countries. When pest organisms are resistant to one class of pesticide compounds, they may evolve resistance more rapidly to new groups of chemicals having either similar modes of action or similar metabolic pathways for detoxication. There is particular concern that the pyrethroids may have a short useful life against many pest species because of a gene identified as kdr. This gene played a key role in the genetic evolution of DDT resistance and appears to provide certain insects with protection against pyrethroids. Resistance to DDT is widespread, so this genetic predisposition to cross-resistance poses a po- tential threat to the efficacy of pyrethroids. Pesticides remain effective in many areas where selection has been less severe. On the Atlantic coast of Central America, Anopheles albimanus can still be effectively controlled by organophosphates and carbamates. In the Midwest these compounds also control the Colorado potato beetle, which is resistant to every insecticide applied to control it on Long Island. Resistance to insecticides has not yet been detected in the European corn borer, but this is an exceptional case. Nevertheless, agricultural production and public health programs can no longer rely on a steady stream of new chemicals to control resistant pest species. Resistance is spreading at an increasing rate, while development of
INTRODUCTION 13 new compounds has declined since 1970 (Georghiou, this volume). New compounds that are superior or have different modes of action are difficult to discover and are increasingly expensive to develop. Many are not pursued because of estimates that they may not return their cost of development, which is at least partly due to the potential for resistance. Pesticide costs for many agricultural and nonagricultural uses have been increasing because of resistance, which compels a switch to generally more expensive chemicals and/or more frequent applications of pesticides. Rational pest-control strategies must be designed to manage resistance, both to prolong the effectiveness of pesticides and to reduce environmental contamination by excessive use of chemicals. These strategies should be based on integrated-pest-management (IPM) techniques. It is also vital to pursue development of new chemicals that are effective through new modes of action. Better understanding of resistance will emerge from more effective methods to detect and monitor resistance, along with better coordination of interdisciplinary research on critical areas of genetics, biochemistry, and population biology. Many people in science and business anticipate gains in crop protection from applications of biotechnology and other new developments. Pests, how- ever, can be expected to evolve strains that are resistant to virtually any control agent, including pest-resistant crop varieties. This is likely to hold true whether resistant plant cultivars are developed with the new tools of biotechnology or by traditional genetic methods. While it is unrealistic to expect biotechnology to eliminate the problem of pesticide resistance, emerg- ing science does indeed offer great hope in helping reduce the impact of resistance episodes while keeping down the economic and environmental costs of pest control. For a more detailed discussion of an optimistic view of the future and data showing falling pesticide prices to farmers, see Mir- anowski and Carlson (this volume).
