Hosts and their associated microbes are being increasingly introduced around the world, which can lead to novel host/microbe associations via new sympatries. Woodwasps (Hymenoptera: Siricidae) are able to utilize wood for its nutrients due to obligate mutualistic associations with white rot fungi in the genus Amylostereum and when invasive woodwasps are introduced to new areas, their symbionts accompany them. There is increasing evidence that woodwasp-fungus associations previously believed to be highly specific are actually flexible. We show that in North America, both Urocerus albicornis and Urocerus cressoni, which develop in trees in the Pinaceae, usually use Amylostereum chailletii but sometimes carry an Amylostereum areolatum strain putatively introduced to North America by the invasive woodwasp Sirex noctilio. Symbiont spillover from invasive to native hosts is a source of new host/introduced symbiont associations that could result in changes in microbes and host fitness with the potential to impact communities.

Parasitic stages of the North American native dimorphic nematode Deladenus proximus develop within larvae of the native North American siricid Sirex nigricornis and sterilize many of the eggs in adult females. Sirex nigricornis were reared from felled trees or trapped in Arkansas and Louisiana in 2017–2019 and parasitism by D. proximus was evaluated. Deladenus proximus parasitized 0.16–0.43 of S. nigricornis individuals, among sites and years. Parasitized males were smaller than healthy males, and more males were parasitized than females emerging from the same trees. Rate of parasitism increased with increasing S. nigricornis density per tree, and, as per tree density increased, so did the proportion that was male. Egg sterilization (woodwasp eggs killed and inhabited by nematodes) was partial, with an average proportion of 0.66–0.88 woodwasp eggs sterilized by D. proximus per female S. nigricornis, differing significantly by site. As the numbers of eggs in a female increased, the proportion of eggs sterilized declined. Comparing data from this study with others, a trend is reported for variable levels of egg sterilization in Sirex females, with 0 or 100% sterilization often occurring in novel host/parasite associations and partial (averages between 50% and 100%) sterilization occurring when host/parasite associations have co‐evolved.

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Parasitic nematodes and hymenopteran parasitoids have been introduced and used extensively to control invasive Eurasian Sirex noctilio woodwasps in pine plantations in the Southern Hemisphere where no members of this community are native. Sirex noctilio has more recently invaded North America where Sirex-associated communities are native. Sirex noctilio and its parasitic nematode, Deladenus siricidicola, plus six native hymenopteran woodwasp parasitoids in New York and Pennsylvania, were sampled from 204 pines in 2011–2019. Sirex noctilio had become the most common woodwasp in this region and the native parasitoids associated with the native woodwasps had expanded their host ranges to use this invader. We investigated the distributions of these species among occupied trees and the interactions between S. noctilio and natural enemies as well as among the natural enemies. Sirex noctilio were strongly aggregated, with a few of the occupied trees hosting hundreds of woodwasps. Nematode parasitism was positively associated with S. noctilio density, and negatively associated with the density of rhyssine parasitoids. Parasitism by the parasitoid Ibalia leucospoides was positively associated with host (S. noctilio) density, while parasitism by the rhyssine parasitoids was negatively associated with density of S. noctilio. Thus, most S. noctilio come from a few attacked trees in a forest, and S. noctilio from those high-density trees experienced high parasitism by both the invasive nematode and the most abundant native parasitoid, I. l. ensiger. There is little evidence for direct competition between the nematodes and parasitoids. The negative association occurring between rhyssine parasitoids and I. l. ensiger suggests rhyssines may suffer from competition with I. l. ensiger which parasitize the host at an earlier life stage. In addition to direct competition with the native woodwasp Sirex nigricornis for suitable larval habitat within weakened trees, the large S. noctilio population increases the parasitoid and nematode populations, which may increase parasitism of S. nigricornis.

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