Resistance In Nonpest Species

The ability of insect predators and parasitoids to develop pesticide resistance would be of enormous benefit to pest management strategies that are chemically dependent. Although pyrethroid and organophosphate resistance has been documented in predatory mites (e.g., Typhlodromus pyri in orchards and Amblyseius womersleyi in tea fields) and hymenopterous parasitoids (e.g., Aphytis holoxanthus in orchards and Anisopteromalus calandrae in grain stores), reports of insecticide-resistant beneficial species from the field are far rarer than they are for pest species. Reporting bias aside, the most likely reasons for this are the difficulty in host location when both natural enemy and host are under selection pressure and, in comparison with herbivorous species, the possibility that the enzyme systems of predators and parasites are less well adapted to detoxify xenobiotics.

Resistance may, therefore, be more likely to develop if the hosts or prey are themselves resistant, thereby making their location easier. For example, a parasitic wasp (A. calandrae) of a stored-grain beetle (Sitophilus oryzae) is resistant to insecticides, and it is thought that this adaptation has been encouraged because the host organism is sheltered from insecticides by the grain kernels it inhabits.

Many attempts have been made to select resistance in beneficial species in the laboratory, but limitations on the size of the populations (and hence their genetic variability) that can be maintained under these conditions means that resistance tends to arise through the development of polygenic traits. Once released into natural populations, these are more likely to fragment and dissipate than rarer, but generally more robust, single mutations.

In general, when resistance does occur in nonpest species, its mechanisms are similar to those exhibited by pest species. Organophosphate resistance in strains of A. calandrae has been linked to the presence of carboxylesterase-like enzymes similar to those conferring organophosphate resistance to the M. persicae. The expression level of the carboxylesterase-like enzyme in this wasp is approximately 30-fold higher in the resistant strain relative to that in the susceptible strain, and the mechanism seems to have its basis in a single nucleotide replacement. Organophosphate resistance in strains of the warehouse pirate bug (Xylocoris flavipes) has also been linked to the presence of a carboxylesterase. Resistance to this chemical group in the lacewing, Chrysopa scelestes, has been attributed to increased AChE activity.

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