Does enemy loss cause release? A biogeographical comparison of parasitoid effects on an introduced insect

The loss of natural enemies is a key feature of species introductions and is assumed to facilitate the increased success of species in new locales (enemy release hypothesis; ERH). The ERH is rarely tested experimentally, however, and is often assumed from observations of enemy loss. We provide a rigorous test of the link between enemy loss and enemy release by conducting observational surveys and an in situ parasitoid exclusion experiment in multiple locations in the native and introduced ranges of a gall-forming insect, Neuroterus saltatorius, which was introduced poleward, within North America. Observational surveys revealed that the gall-former experienced increased demographic success and lower parasitoid attack in the introduced range. Also, a different composition of parasitoids attacked the gall-former in the introduced range. These observational results show that enemies were lost and provide support for the ERH. Experimental results, however, revealed that, while some enemy release occurred, it w...