1. Invasive species with global distributions encounter unique environmental and biotic variables that can greatly affect the magnitude of their impact. The European woodwasp, Sirex noctilio Fabricius, is a prime example that has invaded climatically and ecologically distinct ecosystems across the Southern Hemisphere and, more recently, North America. 2. Northeastern North America presents a unique set of conditions, including pine host species, native parasitoids, a diverse assemblage of native co‐colonising insects, and fungal associates of these co‐colonisers. In North America, S. noctilio attacks both a naturalised ancestral host (Pinus sylvestris L.) from Europe and a naïve native host (P. resinosa). A large assemblage of insects and their associated bluestaining ophiostomatoid fungi colonise these pines. Competition between S. noctilio and this group is a hypothesised mechanism of biotic resistance in the invaded region of North America, possibly via superior resource capture abilities or alteration of host tissue by bluestain fungi. 3. Investigating these ecological interactions is challenging because they manifest deep in the xylem tissue. To overcome this, 30 experimentally stressed trees were systematically dissected with an electric log splitter to investigate the effects of bluestain fungi and tree factors on S. noctilio development and parasitism by native hymenopterans. 4. Body size and colonisation density were affected by pine species, with S. noctilio being 25% larger and densities three‐fold greater in P. sylvestris than P. resinosa. Survivorship was slightly negatively correlated with the proportional volume of bluestain infection. Interestingly, rhyssine (Rhyssa and Megarhyssa spp.) parasitism responded positively to greater S. noctilio density, but there was no density relationship with Ibalia leucospoides ensiger parasitism. Pine host species appears to play a strong role in S. noctilio development, which is important considering uninvaded regions of North America have a diversity of pine species that likely vary in their susceptibility to this woodborer.

Body size is a key biological trait frequently used to assess fitness. Variation in body size stems from genetic and environmental factors and can have strong affects on reproduction. Here, we quantify narrow-sense heritability of size, fecundity, and sexual size dimorphism in four bark beetle species across two genera: Ips pini (Say), Ips lecontei Swaine, Dendroctonus frontalis Zimmermann, and Dendroctonus brevicomis LeConte. For each species, we conducted rearing experiments pairing parents of large or small size classes and measuring body length of all progeny. There was significant narrow-sense heritability of body size in Ips, but not Dendroctonus. Male parent size in I. pini, I. lecontei, and D. frontalis was positively correlated with progeny production and female parent size was positively correlated with gallery length in I. pini, D. frontalis, and D. brevicomis. All species exhibited sexual size dimorphism (SSD), with males being larger than females (male-biased SSD) in Ips and females larger than males (female-biased SSD) in Dendroctonus. Although mean differences in body length between sexes was small, ≈l–2% for Ips and 2–4% for Dendroctonus, the pioneer sex (first colonizer) was statistically larger than the nonpioneer sex in all species. In addition, the pioneer sex displayed greater phenotypic variation in body length than the nonpioneer sex, and Dendroctonus exhibited greater phenotypic variation in body length than Ips. Differing selection pressures within species and even between sexes likely affect body size and heritability patterns exhibited by bark beetles.