The woodwasp Sirex noctilio Fabricius, native to Eurasia and North Africa, regularly co‐occurs with a North American origin Pissodes sp. on Pinus trees in South Africa. The nature of this co‐occurrence and potential impacts on either of the species is unknown. Using structured sampling, we investigated the pattern and degree of co‐occurrence of S. noctilio and Pissodes sp. in six sites in P. patula plantations in South Africa aiming to better understand the potential for interactions and population‐level feedbacks. We compared density, adult body size and within‐tree distribution of both insect species in trees where they co‐occurred or occurred singly. Sirex noctilio and Pissodes sp. co‐occurred on 68.0 ± 3.8% of infested trees (range 55–80%). Both insect species were more abundant in trees where they occurred alone relative to co‐inhabited trees. The within‐tree distribution of S. noctilio did not differ in the presence of Pissodes sp.; however, in the presence of S. noctilio, Pissodes sp. were more frequent in the bottom than the middle tree section where S. noctilio was at higher density and this could be an indication of resource partitioning. Body size of S. noctilio and Pissodes sp. was not influenced by co‐occurrence. The results obtained in the present study suggest a broad overlap for both insects in preference for stressed trees in similar states of decline, as well as moderate antagonism between the species leading to lower emergence of both species in shared trees. The influence of S. noctilio on Pissodes sp. population densities may still be a net positive at a landscape scale because Pissodes sp. prefers declining or recently dead trees that are readily created by S. noctilio via attacks on healthy trees.
