Evaluation of Beddingia siricidicola as a biological control agent of Sirex noctilio in North America (2009)

Sirex noctilio F. was identified in 2005 from bark beetle trap collections in Oswego County, NY, the previous autumn. This exotic invader does not seem to be under natural control, is already distributed over a wide area in the United States and Canada, and poses a serious threat to pine forests and plantations in North America if it is not controlled. Its most effective natural enemy, the entomopathogenic nematode (Beddingia (Deladenus) siricidicola (Bedding)), has been used very successfully as a biological control agent in management programs in the Southern Hemisphere. Sirex nematode’s remarkable biology facilitates its use as a management tool. It can occur in two forms: a mycetophagous form that feeds on the Sirex symbiotic fungus, Amylostereum areolatum (Fries) Boidin, as it builds populations inside a tree and an entomopathogenic form that attacks S. noctilio larvae and ultimately sterilizes the woodwasp adults. We report on our recent activities to use the nematode in a developing biological control program, including consideration of some ecological challenges and issues in adapting the system for use in North America. Australian scientists have investigated the biology of Sirex nematode during the past 30 years and developed technology for delivering it as an effective biological control agent. The Australian biocontrol program schedules activities through three seasons. In the spring, trap trees are created. Plots of 10 suppressed pine trees at the edge of a plantation are treated with an herbicide. The trees die slowly and attract S. noctilio females to attack them in their stressed state. In the summer, the nematodes are mass reared by Ecogrow Ltd., the licensed commercial nematode producer in Australia. Nematodes are delivered in lots of one million, which are sufficient to treat 10 trees. In the fall, the trap trees are felled and inoculated with nematodes. A special hammer is used to punch holes into the xylem; nematodes are then injected into the holes mixed in a polyacrylamide gel and they swim into the cut tracheid fibers of a tree. Given the environmental differences between Australia and North America, transferring the biological control technology poses some ecological challenges. Two broad areas for discussion are (1) the ecological factors that affect nematode establishment and spread in North America and (2) the possible effects of nematode releases on non-target native species, especially other siricid woodwasps. Within-tree competition is one important ecological factor. Pinus species are exotic in Australia, and native pine feeding organisms are non-existent. By contrast, pine trees in North America contain numerous indigenous species, including borer competitors for S. noctilio, fungus competitors for A.areolatum, and nematode competitors for B. siricidicola. Climate is another factor affecting nematode growth and reproduction. Overall, the area into which the nematode will be introduced—currently New York and Pennsylvania—is much colder and wetter than Australia. Multiple strains of A. areolatum from different areas of origin and with varying growth rates also pose a challenge. The North American fungus strain isolated from S. noctilio in New York State grows at approximately one-third the rate of the strain used for mass rearing in Australia. This difference in growth of the fungus resource also affects the growth rate of nematodes and results in lower yields, by a factor of about four, in our mass rearing process. Another important factor in nematode ecology is tree species. North America has many native and exotic pine species with individual physiological characteristics that may affect nematode growth, development, and reproduction differently.