Resistance assays of Drosophila suzukii populations from seven locations in Michigan and one location in Oregon

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 how
invasion changes the coevolutionary dynamics of host-parasitoid
interactions to successfully implement biological control programs against
invasive insect hosts. We investigated spatial and temporal variation in
resistance of the invasive Drosophila suzukii in seven geographically
distinct populations in Michigan and of one population from Oregon against
a newly approved biocontrol agent, the larval parasitoid Ganaspis
brasiliensis. We found regional and temporal variation in resistance
(encapsulation rates of parasitoid eggs) of D. suzukii populations that
ranged from 11– 48%. 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 coldest site suggest a negative effect of temperature on resistance.
Large regional differences in resistance of D. suzukii 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 to accelerate biocontrol.