Combating Insecticide Resistance

Insecticide resistance management (IRM) aims to intervene in the evolutionary process and either overcome resistance or prevent its appearance in the first place. There are several practical, economic, and political constraints on the choice of possible IRM tactics and the precision with which they can be applied:

• The properties of any resistance genes present are often unknown, and knowledge of pest ecology may still be rudimentary.

• It is often necessary to contend with a whole pest complex rather then just a single pest species.

• There may be a very limited number of insecticides available for use in management strategies.

• For highly mobile pests, at least, countermeasures may need to be standardized and synchronized over large areas, sometimes whole countries.

• Resistance is a dynamic phenomenon; that is, any mechanisms already known to exist may change over time.

• To promote compliance with management strategies, the tactics adopted should be as unambiguous, rational, and simple as possible.

A strategy first implemented on Australian cotton in 1983 against H. armigera illustrated many features of large-scale attempts at resistance management. Introduced in response to unexpected, but still localized, outbreaks of pyrethroid resistance in H. armigera, the strategy was based primarily on the concept of insecticide rotation. The threat of pyrethroid resistance was countered by restricting these chemicals to a maximum of three sprays within a prescribed time period coincident with peak bollworm damage. To diversify the selection pressures being applied, farmers were required to use alternative insecticide classes at other stages of the cropping season.

Initially, this strategy had the desired effect of preventing a systematic increase in the frequency of pyrethroid-resistant phenotypes. Additional recommendations, including the targeting of insecticides against newly hatched larvae (the most vulnerable life stage) and the plowing in of cotton stubble to destroy resistant pupae overwintering in the soil, undoubtedly contributed to this success. Unfortunately, the restrictions placed on pyrethroid use were insufficient to combat resistance in the long-term, and it has been necessary to revise the strategy to place greater emphasis on the strategic use of nonpyrethroids against this pest.

Another strategy incorporating a wide range of chemical and nonchemical countermeasures was introduced on Israeli cotton in 1987. The primary objective was conservation of the effectiveness of insecticides against B. tabaci. Under recommendations coordinated by the Israeli Cotton Board, important new whitefly insecticides are restricted to a single application per season within an alternation strategy optimized to contend with the entire cotton pest complex and to exploit biological control agents to the greatest extent possible. One major achievement of this strategy has been a dramatic reduction in the number of insecticide applications against the whole range of cotton pests, but especially against B. tabaci. Sprays against whiteflies now average fewer than two per growing season compared with over 14 per season in 1986. Most importantly, the strategy has generated an ideal environment for releasing additional new insecticides onto cotton and for managing them effectively from the outset.

An integral part of delaying or preventing the evolution of resistance is the preservation of the innate "susceptibility" of a pest species. This is arguably as valuable a genetic resource as those of the rice, wheat or apple "gene banks" that are so carefully tended in institutes around the globe. The most effective way to conserve susceptibility, based both on evolutionary models and on empirical evidence, is to ensure the presence of pesticide-free "refugia" in which susceptible genotypes may survive and reproduce. The inclusion of refugia as essential components of IRM strategies is a recent phenomenon, signaling that pest management is no longer simply about eradication, but is now at least partially focused on conservation.

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