Spread of the introduced biocontrol agent Torymus sinensis in north-eastern Italy: dispersal through active flight or assisted by wind?

In successful classical biological control, natural enemies can provide enduring pest suppression if they reproduce and disperse without continued human management. To explain the efficient control exerted on the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) by the parasitoid Torymus sinensis Kamijo (Hymenoptera: Torymidae), it is required that the latter has the capacity to track the range expansion of its host. In the present study, we hypothesized that the distribution of infested patches and the prevailing wind directions would interactively influence and accelerate the spread of the parasitoid. It emerged that the spread capability of T. sinensis is high and likely affected by a combination of short- and long-distance flights (stratified dispersal). In particular, the ability to disperse long distances represents a successful trait of the parasitoid because the natural dispersal of its host is high and frequently aided by unintentional human-mediated transportation.