Symbiont fidelity is an important mechanism in the evolution and stability of mutualisms. Strict
fidelity has been assumed for the obligate mutualism between Sirex woodwasps and their mutualistic
Amylostereum fungi. This assumption has been challenged in North America where a European
woodwasp, Sirex noctilio, and its fungal symbiont Amylostereum areolatum, have recently been
introduced. We investigate the specificity of the mutualism between Sirex and Amylostereum species
in Canada, where S. noctilio co-infests Pinus with native S. nigricornis and its mutualist A. chailletii.
Using phylogenetic and culture methods, we show that extensive, reciprocal exchange of fungal
species and strains is occurring, with 75.3% of S. nigricornis carrying A. areolatum and 3.5% of S.
noctilio carrying A. chailletii. These findings show that the apparent specificity of the mutualism
between Sirex spp. and their associated Amylostereum spp. is not the result of specific biological
mechanisms that maintain symbiont fidelity. Rather, partner switching may be common when shifting
geographic distributions driven by ecological or anthropogenic forces bring host and mutualist pairs
into sympatry. Such novel associations have potentially profound consequences for fitness and
virulence. Symbiont sharing, if it occurs commonly, may represent an important but overlooked
mechanism of community change linked to biological invasions.