Geographic Variation in Resistance of the Invasive <i>Drosophila suzukii</i> to Parasitism by the Biological Control Agent, <i>Ganaspis brasiliensis</i>

Host-parasitoid interactions are tied in coevolutionary arms races where parasitoids continuously have to evolve increased virulence as hosts evolve increased resistance. Over time, geographic structure in virulence and resistance can arise because of spatial and temporal differences in parasitoid communities, in the strength of reciprocal selection pressures, in genetic variation in local populations, and as trade-offs are balanced between defense and fitness traits. It is crucial to understand the resistance structure of pest populations to successfully implement biological control programs against invasive insect hosts. We investigated spatial and temporal variations in the resistance of the invasive <i>Drosophila suzukii</i> in seven geographically distinct populations in Michigan and of one population from Oregon against a newly approved biocontrol agent, the larval parasitoid <i>Ganaspis brasiliensis.</i> We found regional and temporal variations in the resistance (encapsulation rates of parasitoid eggs) of <i>D. suzukii</i> populations that ranged from 11% to 48%. The northernmost, and thus the coldest site, had the highest rate of parasitism and the lowest encapsulation rate. Large regional differences in the resistance of <i>D. suzukii</i> populations can render the ensuing biocontrol program more variable and less predictable, and release strategies may need to be altered at sites where flies have high resistance.