Efficacy of Kamona strain Deladenus siricidicola nematodes for biological control of Sirex noctilio in North America and hybridisation with invasive conspecifics

Abstract

Sirexnoctilio is an invasive woodwasp that, along with its symbiotic fungus, has killed pine trees (Pinus spp.) in North America and in numerous countries in the Southern Hemisphere. We tested a biological control agent in North America that has successfully controlled S.noctilio in Oceania, South Africa, and South America. Deladenussiricidicola nematodes feed on the symbiotic white rot fungus Amylostereumareolatum and can switch to being parasitic on S.noctilio. When parasitic, the Kamona nematode strain can sterilise the eggs of S.noctilio females. However, in North America, a different strain of D.siricidicola (NA), presumably introduced along with the woodwasp, parasitises but does not sterilise S.noctilio. We tested the sterilising Kamona biological control strain of D.siricidicola against S.noctilio in North America. Interactions between the biological control strain and the NA strain could include competitive exclusion, co-infection within hosts or hybridisation. We reared D.siricidicola Kamona on an A.areolatum strain native to North America (IGS-BE) and another strain (IGS-BDF) used commercially to mass-produce the nematode in Australia. We inoculated Kamona reared on either strain of A.areolatum into logs infested with S.noctilio larvae and compared parasitism rates against control logs. Individual nematodes were isolated from S.noctilio hemocoels and from sterilised eggs and were genotyped with eight microsatellite loci. A high rate of parasitisation of S.noctilio by D.siricidicolaNA was found for all treatments and we found evidence of both co-infection and hybridisation. Surprisingly, sterilisation rates were not related to the rates of parasitisation by D.siricidicola Kamona.