Disentangling the effects of the invasive pest, Dryocosmus kuriphilus, and the introduction of the biocontrol agent, Torymus sinensis, on native parasitoids in an isolated insular chestnut-growing area

Dryocosmus kuriphilus Yasumatsu (Hymenoptera Cynipidae) was first recorded in Sardinia (Italy) in 2007. Two years later a biological control program started with the introduction of its non-native parasitoid, Torymus sinensis Kamijo (Hymenoptera Torymidae). Over a nine-year period (2009–2017), data on the relative abundance of D. kuriphilus, its native parasitoids and T. sinensis were collected. Native parasitoids reacted quickly to the large availability of hosts. However, a dramatic change in the abundance of each native species was recorded from year to year. In the first few years, the native parasitoid community and pest abundance did not fluctuate much. The turning point was 2014, when, at the same time, the native parasitoid populations increased and the T. sinensis populations also began to increase. In the following year, both the D. kuriphilus and native parasitoid populations decreased considerably, whereas the T. sinensis populations reached over 50% parasitism. The univoltine D. kuriphilus acted as an asynchronous resource for native bivoltine parasitoids. An asynchronous host-parasitoid interaction produces wide fluctuations in their populations. In this case, D. kuriphilus strongly altered the local community, which was subjected to a chaotic change in composition and species abundance. When T. sinensis became established, it monopolized the pest, making this resource unavailable to native parasitoids that returned to attack almost exclusively the native oak cynipids. In fact, the successful control of D. kuriphilus by the introduction of T. sinensis rebalanced the situation that had been altered by the arrival of the invasive pest.