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Population Dynamics and the Rate of Evolution of Pesticide Resistance
Pages 170-193

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From page 170...
... We review the effects of insect migration to andfrom untreated regions and of density-dependent aspects of the population dynamics of the target species. Combining population dynamics with gene flow considerations, we review ways in which the evolution of resistance may be speeded or slowed; in particular, we contrast the rate of evolution of resistance in pest species with that in their natural enemies.
From page 171...
... The standard example of microevolution in the current generation of introductory biology texts is industrial melanism in the peppered moth. This tired tale could well be replaced by any one of a number of field or laboratory studies of the evolution of pesticide resistance that would show in detail how selective forces, genetic variability, gene flow (migration)
From page 172...
... The equation relating the gene frequencies of R in successive generations is then the standard expression (Crow and Kimura, 1970~: ~_ WRRpt + WRSP'q' WRRp2 + 2WRSp,q, + Wssqt In the early stages of pesticide application, the resistant allele will usually be very rare, so that p, << 1 and q, ~ 1. The initial ratio ply, will, indeed, usually be significantly smaller than the ratio wRs/wRR or wss/wRs, so that to a good approximation equation 1 reduces to ~, ~.
From page 173...
... We will return to the systematic trends exhibited in Table 2 and crudely summarized in Table 3, after the discussions of migration, density dependence, and other miscellaneous factors. The approximate expression for TR in equation 4 mixes factors that are intrinsic to the genetic system underlying the resistance phenomenon (such as Tg, pO, and the degree of dominance of R)
From page 174...
... 174 Cal ._ C)
From page 177...
... , it is possible in principle that a pesticide may have cycles of useful life: the gene frequency of R first increases under the selection pressure exerted by use of the pesticide; eventually R attains a frequency sufficiently high to produce a noticeable degree of resistance, and shortly thereafter the pesticide is discontinued as ineffective; in the absence of the pesticide, usually wss > WRR, and selection will now cause the frequency of R to decrease. Applying equation 4, mutatis mutandis, to this back-selection process, we note that the time elapsed before the population is again effectively susceptible to the pesticide will depend on (1)
From page 178...
... One illuminating study contrasts two examples of industrial melanism: BiStOn betUIaria iS relatively vagile and thus is predominantly in the melanic form over most of England's industrial midlands; individuals of GOnOJOntiS bid~entata move significantly less in each generation, leading to weaker gene flow and a patchy pattern of local adaptation with melanic forms predominating near cities and wild types predominating in the intervening countryside (Bishop and Cook, 19751. This academic literature is directly relevant to the problem of the evolution of pesticide resistance in the presence of migration.
From page 179...
... In the treated region, the final steady state will be one of resistance or continued susceptibility, depending on the strength of migration relative to selection, as measured by m. There is a fairly sharp boundary between these two regions (indicated by the hatched line in Figure 11; the boundary depends weakly on the magnitude of PR, with slightly higher gene
From page 180...
... In some situations it could pay to introduce susceptible adult males following treatment, which could enhance the gene frequency of S in the next generation without producing any additional pest larvae. These analytic and numerical insights have been corroborated by laboratory experiments on Musca domestica exposed to dieldrin at various dosage levels and with various levels of influx of susceptibles (Taylor et al., 1983~.
From page 181...
... As indicated in Figure 2, if a population with undercompensating density dependence (b < 1) is driven to low values in one generation (by pesticide application, for example)
From page 182...
... With undercompensating density dependence (b ~ 1) , the population densities of the next generation of pests on average will be lower in treated regions than in untreated ones.
From page 183...
... Because undercompensating density dependence makes migration relatively more important, TR(m; b < 1) is always greater than TR(m; b = 1)
From page 184...
... POPULATION DYNAMICS OF PESTS AND THEIR NATURAL ENEMIES The propensity for pest species to evolve resistance more quickly than their natural enemies do has often been remarked (Tabashnik, this volume; Roush, this volume)
From page 185...
... We think it is useful to buttress these concrete studies with the very general observation that pesticide resistance is likely to appear faster among pests than among their natural enemies, by virtue of the interplay between population dynamics and migration; in this sense, the phenomenon illustrates the general arguments made in the previous section. Other work in this area includes the numerical studies by Gutierrez and collaborators on management of the alfalfa weevil, taking account of pest
From page 186...
... MISCELLANEOUS TOPICS This section comprises brief notes on a variety of factors that complicate the analyses presented above. Life History Details Throughout we have considered pests with deliberately oversimplified life cycles, in which pesticide application and density dependence acted only on one stage.
From page 187...
... concludes that both high dosage rates and the use of drug mixtures may tend to retard the evolution of resistance. Drug administration to humans and other animals often does permit close control in a closed population, such that these strategies have a chance to work (rather than be washed out by gene flow; see Life History Details, above)
From page 188...
... ; the second column places some organisms along this scale in a very approximate way; and the third column comments on some rough correlations between the time scale and life histories or treatment efficiencies. application; the cost of insect damage to the crop may then be estimated as Aw.
From page 189...
... ; the basic features of Figure 6 are not qualitatively dependent on these parameter values. Both dosage levels and total costs are shown as a function of the parameter combination bTo, which is essentially the ratio between the intrinsic time scale associated with the evolution of resistance and the doubling time of invested money (at interest rate b: for more precise definitions, see the text)
From page 190...
... To advance this enterprise we need a better understanding of the detailed genetic mechanisms underlying resistance and more information about the population biology of pests and natural enemies in the laboratory and in the field. Insofar as the dynamical behavior of pest populations influences the rate of evolution of resistance, we must be wary of extrapolating the laboratory studies into field situations; it would be nice to see more control programs being designed with a view to acquiring a basic understanding at the same time as they serve practical ends.
From page 191...
... 1981. The descriptive properties of some models for density dependence.
From page 192...
... 1975. Gene frequency clines in the presence of selection opposed by gene flow.
From page 193...
... 1985. Evolution of pesticide resistance in apple pests and their natural enemies.


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