Resistance to Pyrethroids in the Field

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The main cause of development of resistance in insects in the field is a persistent and high selection pressure as a consequence of repeated applications of a single class of insecticide (or another with the same mode of action). Therefore, despite effective control in the initial stages, a small number of survivors with innate resistance then rapidly multiply until control fails. In this regard, pyrethroids are no different from other insecticide classes. It is noteworthy that it was the development of resistance to pyrethroids that first prompted companies to take resistance seriously and to take joint action. Pyre-throids suffered an inherent disadvantage at the outset in that kdr also confers resistance to DDT, and prior use of DDT had already selected kdr alleles to significant levels in the same pests. Presently over 80 species have developed resistance to pyrethroids (Whalon et al., 2003).

Until the late 1970s, the major agrochemical companies had seen the development of resistance to established classes as commercially beneficial and motivation for the introduction of new classes. However, the increasing cost of discovery, together with the realization that pyrethroids could be rendered ineffective in the field within a much shorter period of time than other classes of insecticides, forced them to take collective action. In 1979 they set up the Pyrethroid Efficacy Group (PEG), which in 1984 become a sub-group of a larger international organization called the Insecticide Resistance Action Committee (IRAC). This group communicates its actions through a website (http://www.

irac-online.org) and also sponsors the biannual Resistant Pest Management newsletter. Before the establishment of PEG, the investigation of resistance mechanisms and their consequences had remained the domain of academic research, and the development of resistance to pyrethroids was largely overlooked and even denied by some companies. The primary aim of PEG was to prolong the effectiveness of pyrethroids in the field. It encouraged and assisted in the investigation of all aspects of resistance to pyrethroids and particularly the development and implementation of insecticide-resistance management (IRM) strategies.

Persistent selection with a single class of insecticide will invariably lead to resistance, even if synergists are used. Therefore the overall aim is to minimize use of any single insecticide class so as to limit selection pressure and thereby conserve susceptibility in pest insects. This requires an in-depth knowledge of factors ranging from resistance mechanisms to genetic and ecological attributes of both pest and beneficial insects. Any strategy has also to integrate the judicious use of different insecticide types (namely those with different modes of action and synergy where possible) with other pestmanagement options (e.g., agronomic practices), together with regular monitoring of both the levels of resistance and the nature of the resistance mechanisms. Finally, the key requirement for the success of any strategy is the cooperation of the growers. By way of example, two IRM strategies are considered below which encompass these factors.

The first successful and the most publicized IRM strategy came from Australia. Synthetic pyrethroids were introduced in Australia in 1977 when there was already widespread resistance to virtually all established classes. However, within six years of introduction, resistance to these pyrethroids had also developed in commercially important insect species (Forrester et al., 1993). A resistance-management strategy for H. armigera was implemented in the 1983/4 season. A different approach was used for each class of insecticide, depending on the severity of the resistance risk and predicted selection pressure. It integrated the use of chemical and nonchemical control methods, especially biological and cultural. For chemical control, unrelated chemistries were used with a strong emphasis on pyre-throid/ovicide mixtures. In essence, a three-stage strategy was implemented. In Stage I (September to February), only endosulfan (an organochlorine) was permitted. Stage II (January to February) allowed a maximum of three pyrethroid sprays within a 42-day window (later reduced to 38 days), enough to control only one of the five H. armigera generations present within a single growing season. In Stages I and II, ovicidal compounds (e.g., metho-myl) could also be used. Organophosphates and carbamates were used in Stage III and also if required for additional sprays during Stages I and II.

The use of a ''softer'' insecticide (endosulfan) in the early season was deliberate to minimize disruption of beneficial parasitoids and predators and to avoid a potential upsurge of secondary pests such as mites, aphids, and whiteflies, which were not controlled by the pyrethroids available at the time. Examples of nonchemical countermeasures incorporated in the strategy to reduce selection pressure included the use of early-maturing crops, avoiding early-growing crops (e.g., maize) in adjacent fields, which may act as early-season nurseries for resistant H. armigera, and utilization of host plants in refugia to maintain a large pool of susceptible individuals, which would continually dilute the resistant population in the crop. Resistance was monitored on a weekly basis using a discriminating dose of fenvalerate with and without the synergist PBO. Later monitoring showed a rise in metabolic resistance attributed to MFOs and this resulted in the inclusion of PBO in the last of the maximum of three pyrethroid sprays. This strategy was successful for many years but the underlying trend of upward increase in the proportion of resistant insects continued and finally led to a complete reorganization of the strategy in the mid 1990s with a shift away from reliance on pyrethroids.

Initial resistance to pyrethroids was thought to be due to the presence of the knockdown site-insensiv-ity resistance (kdr) mechanism probably as a direct result of cross-resistance to DDT. Over the years of the IRM strategy, metabolic resistance mechanisms appear to have taken over. However, there is still controversy over whether it is primarily due to elevated levels of esterases or mixed-function oxidases (MFOs). The main reason for the uncertainty concerns the role of synergists used in the studies. For example, PBO is now thought to inhibit both these types of enzymes. Furthermore, it has been suggested that such inhibitors may themselves have become resisted in the field over time (McCaffery, 1998), thus obscuring the mechanism of resistance.

The second IRM strategy involved the whitefly, B. tabaci, a representative sucking pest on a wide range of crops (Denholm etal., 1998). In 1995, over-reliance on a limited range of pyrethroids in Arizona had led to a classic treadmill scenario, with farmers responding to rising levels of resistance by increasing the number of sprays (as many as 8-12 applications per season). In this pest, the haplodiploid breeding system encourages rapid selection and fixation of resistance genes. Males are produced uniparentally from unfertilized, haploid eggs, and females are produced biparentally from fertilized diploid eggs. In addition, for this (and other) highly polyphagous species, the interaction between pest ecology and resistance is complex and generally not well understood, making formulation of IRM strategies even more difficult. The strategy of Dennehy and Williams (1997) implemented in 1996 had several features in common with that for H. armigera. It too relies on the continuous availability of a pool of susceptible whiteflies in refugia (e.g., Brassica crops) throughout the year and alteration of agronomic practices (e.g., timing of planting) in crops to minimize whitefly numbers whilst still maintaining the level of natural enemies. The first of the three-stage IRM strategy involved use of a single spray each of pyriproxyfen and buprofezin (then newly available insect-growth regulators) with a 14- or a 21-day gap. The second stage allowed nonpyrethroid conventional insecticides and the third used pyrethroids synergized with acephate as late as possible in the season. A threshold of infestation was defined to initiate insecticide applications. Mixtures were limited to no more than two compounds and any one active ingredient was restricted to no more than two applications in one season. This strategy has been extremely successful in reducing the number of sprays required and regaining susceptibility both to synergized pyrethroids and key nonpyrethroid insecticides.

As for H. armigera, the need for monitoring, establishing threshold levels to trigger spray applications, and cooperation of the growers was key to the strategy. Long-term success of any IRM strategy depends on many factors because, even when an IRM strategy has been successful (McCaffery, 1998), effectiveness of synergized pyrethroids can be lost after just two applications in areas with a history of resistance to pyrethroids.

In conclusion, the key requirement for the development and sustainability of an IRM strategy is diagnosis of the resistance mechanism(s) in field populations. This is especially important when assessing the relative importance of individual mechanisms when several are present, which influences changes to the strategy with time. As alluded to earlier, there is still much uncertainty in the diagnosis of mechanisms. The use of established inhibitors or just in vitro bioassay data has been shown to be unreliable in this regard. For example in M. persicae, elevated esterase was considered for many years to be the main resistance mechanism.

It was only recently shown (Devonshire et al., 1998) that this mechanism made only a minor contribution to the observed resistance to pyrethroids, with a kdr mechanism being the main contributor. This case further emphasized the need for not only accurately identifying the resistance mechanisms present but also their relative contribution. One way forward would be to design selective inhibitors on the basis of in vivo and in vitro structure-activity relationship studies from susceptible and resistant insects.

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