Trading spaces: Fungus and nematode switch off as predator and prey

Use of the parasitic nematode Deladenus siricidicola to control invasive pine-killing Sirex noctilio woodwasps in the Southern Hemisphere is one of the most successful examples of classical biological control. The commercially available nematode parasitizes and ultimately sterilizes adult female woodwasps. Since the discovery of established S. noctilio in North America in 2005, the nematode is being considered for control in the United States, as the southern pine industry is potentially threatened. Both nematode and woodwasp rely on the white rot fungus, Amylostereum areolatum, for continued survival, and the nematode is commercially mass produced on this fungus. Multiple Sirex species occur in the United States, and these woodwasps carry different species and strains of Amylostereum fungus. We hypothesized that fungal strain would impact reproduction of the nematode. Evidence gathered by examining two species of Deladenus nematodes on various fungal isolates showed D. siricidicola, as well as a closely related native North American nematode, D. proximus, achieve significantly higher population numbers on some strains of the fungus than on others. We then focused on one possible reason for the differences in population number by hypothesizing that a role reversal occurs wherein fungal hyphae invade and kill nematode eggs. Nematode eggs were exposed to multiple strains of A. areolatum to quantify the number of eggs lost to fungal invasion. Additional cryogenic scanning electron microscopy and fluorescence microscopy showed that fungal hyphae are able to make extensive assimilative hyphae in a nematode egg. We discuss implications of these findings in a biocontrol context.