The Sirex noctilio (woodwasp)–Amylostereum areolatum (fungus) complex has caused substantial losses to
pine industries in its introduced range. The nematode Deladenus siricidicola that parasitizes S. noctilio and
feeds on A. areolatum is widely used as a biological control agent for S. noctilio, but not with consistent
success. This variable success could be due to factors that influence the feeding and reproductive ability
of the nematode on A. areolatum.We test two main hypotheses that emerge from this prediction. First, we
compared the survival of D. siricidicola on the South African field strain and the Australian laboratory
strain of A. areolatum, to examine a possible incompatibility between nematode and fungal strain. Second,
we examined the competitive interactions of these two A. areolatum strains with two common sapstain
fungi, Diplodia pinea and Ophiostoma ips, that occur in S. noctilio infested trees in South Africa. The effect of
water potential on the outcome of these fungal interactions was also considered. The data showed that
D. siricidicola survives at comparable levels on the two A. areolatum strains. Water potential of the media
significantly influenced growth of the fungi and their ability to capture host resource in competitive
interactions. D. pinea competed increasingly better against A. areolatum with decreasing water potential.
The results suggest that competitive interactions between A. areolatum and sapstain fungi could negatively
influence the success of D. siricidicola, especially under conditions of lowered water potential.