Controlling the Pine-Killing Woodwasp, Sirex noctilio, with Nematodes

Abstract

The pine-killing woodwasp Sirex noctilio, a native to Eurasia/Morocco, was accidentally introduced into various Southern Hemisphere countries during the last century and has recently (2005) been detected in north-eastern North America. The parasitic nematode Beddingia siricidicola is by far the most important control agent of sirex and has been introduced into each Southern Hemisphere country soon after sirex became established. The nematode has a complex life cycle with morphologically very different forms. One form feeds on the tree-pathogenic, sirex-symbiotic fungus (Amylostereum areolatum) as this fungus grows throughout the tree, while the other form grows in and then sterilises adult female S. noctilio. The fungal-feeding form of B. siricidicola is used to mass-produce the nematode. Methods are described for liberating nematodes in pine plantations. The nematode has caused major crashes in S. noctilio populations so that sirex-infested trees can no longer be found in many plantations. A problem arose when it was discovered that long-term in vitro culture using only the fungal cycle without intervention of parasitic cycles had selected, over many years, for a nematode strain (the “defective strain”) that rarely formed the infective stage and was therefore much less effective in the field. Isolation of the “Kamona strain”, annual replenishment from liquid nitrogen storage and other procedural changes are enabling strain replacement in the field. While nematode control in most of the Southern Hemisphere has proved to be highly successful, there are problems in the KwaZulu-Natal region of South Africa where warm dry winters cause sirex-infested trees to dry out before the nematode populations can spread throughout the tree. In North America an inferior strain of nematode appears to have been accidentally introduced with sirex. The symbiotic fungus of sirex introduced to North America is a different strain of A. amylostereum to that in the Southern Hemisphere and does not permit optimal nematode development.