METAPOPULATION DYNAMICS AFFECT RESISTANCE DEVELOPMENT IN THE PREDATORY MITE, METASEIULUS-OCCIDENTALIS (ACARI, PHYTOSEIIDAE)

A stochastic metapopulation model was used to examine implementation of a pesticide-resistant strain of the predatory phytoseiid mite, Metaseiulus occidentalis (Nesbitt). A resistant strain could be established at frequencies >50% per tree within 1 yr in simulations, which corresponded well with field trials. Comparisons between simulations with and without metapopulation dynamics and local extinctions suggest that metapopulation dynamics affect establishment of resistant strains of biological control agents primarily by increasing local homozygosity within patches. Metapopulation dynamics therefore accelerate resistance development most when the resistance mechanism is recessive, because dominant alleles respond to selection regardless of the level of homozygosity. The predator population went extinct more often when the simulated prey species, based on the mite Tetranychus urticae Koch, was initially susceptible, as expected from the food limitation hypothesis. These extinctions differed from those predicted by the food limitation hypothesis, however, because they occurred at overwintering and not immediately following pesticide applications. This difference may be important when testing the food limitation hypothesis in field experiments. The survival of the predator population after pesticide treatments is linked to the high reproductive capacity and rapid recovery of the prey population as well as heterogeneity in patch dynamics introduced by the metapopulation model. Frequent loss of rare resistance alleles during overwintering suggest that reducing the size of the population during this genetic bottleneck through management tactics could retard the ability of a species (either natural enemy or pest) to adapt to pesticide treatments. Overwintering may represent a genetic weak point in many life cycles which could be exploited by resistance management strategies.