Evidence that the availability of suitable pine limits non-native Sirex noctilio in Ontario.
Natural controls may not be effective in limiting non-native species that invade a new environment. Alternatively, factors present in the new environment can prevent an invader from becoming a serious pest. We investigated whether existing natural enemies or the availability of susceptible and suitable host plants limited an established, non-native species Sirex noctilio F. (Siricidae: Hymenoptera). Using several pine forests throughout Ontario, we examined the relationships between parasitism and S. noctilio activity, as well as S. noctilio activity and the availability of susceptible and suitable pine. We also evaluated parasitism at two points in time (5-7 years apart). We found correlative evidence indicating that the availability of suitable hosts limited the S. noctilio population in Ontario. Sirex noctilio killed more pine when more pine was available, especially suppressed (highly suitable) pine. More co-dominant (healthy) pines were attacked but not killed by S. noctilio at sites with more S. noctilio activity, indicating that higher densities of S. noctilio were needed to overwhelm healthy pines. The role of parasitoids was less clear. Parasitism did not vary consistently with the amount of S. noctilio activity at sites. The two groups of parasitoids, Rhyssa and Ibalia, likely responded differently to presence of S. noctilio.