Complex ecological relationships among an assemblage of indigenous hymenopteran parasitoids, the exotic European woodwasp (Sirex noctilio; Hymenoptera: Siricidae), and a native congener

We collected naturally infested Pinus resinosa Aiton (Pinaceae) and P. sylvestris Linnaeus to investigate phenological patterns and quantify parasitism by a suite of native hymenopteran parasitoids on two woodwasps (Hymenoptera: Siricidae): the invading non-native European woodwasp, Sirex noctilio Fabricius, and a co-colonising native, S. nigricornis Fabricius. We sampled a total of 76 trees from two field sites in 2010 and seven sites in 2011. In raw abundance, S. noctilio outnumbered S. nigricornis by 2:1 in 2010 and by 7.5:1 in 2011. We collected the egg/early instar parasitoid, Ibalia leucospoides ensiger Norton (Hymenoptera: Ibaliidae); four species of Rhyssinae (Hymenoptera: Ichneumonidae) late larval parasitoids; and Pseudorhyssa nigricornis (Ratzeburg) (Hymenoptera: Ichneumonidae), a cleptoparasitoid of rhyssines. Variation in siricid and parasitoid species assemblage and abundance was explained primarily by site, with tree-level factors playing a secondary role. Parasitism was significantly lower in P. sylvestris (13.6%±4.1 SE), a naturalised pine from Europe, than in the native P. resinosa (28.5%±5.0). Total parasitism was 27.6%±5.0 in 2010 and 20.9%±4.7 in 2011. This study represents the most robust analysis of the diverse woodwasp and parasitoid assemblage infesting pines in North America.