Parasitism of Sirex noctilio by non-sterilizing Deladenus siricidicola in northeastern North America

• S. noctilio with non-sterilizing (NS) D. siricidicola produced fewer eggs.
• Fecundity was also mediated by the effect of NS D. siricidicola on host size.
• S. noctilio with NS D. siricidicola emerged earlier than unparasitized woodwasps.
• There was an inverse relationship between NS D. siricidicola and parasitoids.
• NS D. siricidicola was present in all sites and reached 57.7% parasitism.

The parasitic nematode Deladenus siricidicola Bedding (Tylenchida: Neotylenchidae) has been extensively used for biological control of the wood wasp Sirex noctilio F. (Hymenoptera: Siricidae), an invasive pest of pine trees (Pinus spp.) in the Southern Hemisphere. The virulent strain of D. siricidicola used for biological control sterilizes S. noctilio females, although non-sterilizing strains of this nematode also occur. A non-sterilizing (NS) strain is established in the most recent invasion of S. noctilio, in northeastern North America. This study analyzed the effects of the NS strain of D. siricidicola on invasive S. noctilio collected from New York State and Pennsylvania. Nematode parasitism had a direct negative relationship on the number of eggs produced by the pro-ovigenic adult female S. noctilio. Nematode presence also negatively influenced the number of eggs indirectly, due to smaller size in nematode-parasitized females. Nematode-parasitized females produced an average of 29 fewer eggs than healthy females. On average, male and female nematode-parasitized S. noctilio emerged earlier from trees than healthy S. noctilio, and the size of female S. noctilio was not related to emergence date. S. noctilio with NS nematodes were found in 44.0% of trees and 26.9% of all individuals diagnosed, reaching 27.9 ± 26.0% parasitism (mean ± S.D.) when averaged across sites. There was greater parasitism of female S. noctilio than males. We also examined parasitism by hymenopteran parasitoids in association with parasitism by NS D. siricidicola. Parasitism by nematodes averaged 31.9 ± 35.4% per tree, while parasitism by hymenopteran parasitoids averaged 41.8 ± 19.6%. In terms of management, NS D. siricidicola may be less effective in providing biological control compared with sterilizing D. siricidicola or parasitic hymenopterans; the latter two natural enemies prevent all or most reproduction of S. noctilio, while the NS strain is primarily associated with a reduction in S. noctilio size and fecundity.

Direct and indirect effects of parasitism by non-sterilizing Deladenus siricidicola nematodes on egg production in Sirex noctilio. Left photo: Mycophagous D. siricidicola with eggs Right photo: S. noctilio female drilling with ovipositor (Photos: E. Erin Morris).Figure optionsDownload as PowerPoint slide