Geographic variation in resistance of the invasive Drosophila suzukii to parasitism by the biological control agent, Ganaspis brasiliensis

Host-parasitoid interactions are often tied in coevolutionary arms races where parasitoids continuously have to evolve increased virulence as hosts evolve increased resistance. Invasion theories predict that when a host is introduced to a novel region without its coevolved natural enemies, they will evolve lower defenses. Resistance may also differ geographically and temporally due to abiotic and biotic factors. We investigated spatial, temporal and host plant related differences in resistance of the invasive D. suzukii in seven geographically distinct populations in Michigan and of one population from Oregon against a coevolved parasitoid, Ganaspis brasiliensis. Encapsulation rates (resistance) of G. brasiliensis eggs by D. suzukii reached 39% in August and 48% in September regionally. These relatively high levels of resistance in North America contrast with expectations and may be due to the low levels of competition D. suzukii experiences in the invaded range. Encapsulation rates of D. suzukii differed regionally and temporally but not between fruit types. The northernmost site with the lowest encapsulation rate had the highest rate of parasitism suggesting that parasitoids may be able to detect the defensive capacities of their hosts and adjust attack rates accordingly. The lowest encapsulation rates at the northernmost and thus overall coldest site indicate a negative effect of temperature on resistance. However, temporal differences in resistance from August to September were not consistent among sites. These results indicate that there can be regional and temporal variation in the outcome of host-parasitoid interactions between D. suzukii and G. brasiliensis. This may influence the efficacy and biocontrol potential of G. brasiliensis that has been recently approved for field releases against D. suzukii in North America.