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.

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Functional traits are useful for comparing the resource use of invasive and native species, with goals of identifying resource overlap to predict competitive interactions. The invasion of northeastern North America by the woodwasp Sirex noctilio has resulted in competition with the native congeneric Sirex nigricornis for suppressed and weakened pines. We compared sizes of adults, venom glands, fecundity, tree species use, voltinism and abundance of the invasive woodwasp S. noctilio with the native S. nigricornis in northeastern North American pines. Rearing adults from attacked pines showed that these species used the same tree species but S. noctilio were far more abundant, especially with increasing time since establishment. Adults of the invasive S. noctilio were larger than S. nigricornis, female S. noctilio had larger glands carrying phytotoxic venom in relation to body size, average-sized S. noctilio females carried more eggs, and S. noctilio developed faster than S. nigricornis. Sirex noctilio was the dominant woodwasp infesting suppressed pines in our study areas. We hypothesize that the future abundance of S. nigricornis could depend in part on the availability of wood for oviposition by this native that is not available or acceptable to the earlier-emerging S. noctilio.

1 Establishing host-parasitoid relationships is challenging for cryptic insects feeding deep in the xylem tissue of trees. Sirex noctilio is a xylophagous feeder and recent invader of North America that co-occurs in pines (Pinus) with a native siricid (Sirex nigricornis) and a suite of shared hymenopteran parasitoids. This insect has a long history as a destructive pest in Southern Hemisphere pine plantations. 2 To establish explicit trophic linkages between native hymenopteran parasitoids and two species of woodwasp (Sirex) in North America, a polymerase chain reaction assay was developed to amplify mitochondrial DNA that was restriction digested with the aim of identifying species. The assay was used to identify species of Sirex from cadavers and gut contents from different parasitoid stages with the aim of establishing host-parasitoid relationships. 3 The host woodwasp species was identified successfully from parasitoid gut contents collected from larvae, pupae and adults. Across all parasitoid life stages, woodwasp host was identified in 52.9% of ibaliid parasitoids and 78.5% of rhyssine parasitoids. Host was successfully identified from 88.1% of associated siricid cadavers. 4 These results demonstrate the utility of molecular techniques to verify host-parasitoid associations that previously had only been inferred. Development of molecular tools is a rapid and crucial tool for establishing trophic linkages and is particularly useful in situations involving non-native species with cryptic lifestyles.