Did a cryptogenic nematode strain control a sirex wood wasp outbreak?

Sirex noctilio (sirex) is a significant pest of Pinus plantations in the Southern Hemisphere. Biological control and stand management to reduce tree stress (e.g. thinning) are effective in managing sirex in commercial plantations, but the relative contribution of each of these in controlling a sirex outbreak is untested. In this study we attempt to disentangle the relative roles of ‘top-down’ (natural enemies) and ‘bottom-up’ (susceptible host tree availability) forces in regulating a sirex outbreak. We monitored a sirex outbreak in an unthinned Pinus radiata plantation in New South Wales to determine whether biological control alone could effectively reduce the sirex population. The plantation was 11 years old in 2015 when sirex attack was first detected and had very high stocking (3000–6500 s.p.h.) due to ineffective pine wildling control. Inoculation of naturally struck trees with the Kamona strain of the nematode Deladenus siricidicola began in 2016, with a total of 16 million nematodes inoculated up to 2019. By 2020, the outbreak had subsided with only a few sirex-attacked trees detected. Ground plots revealed that 40% of trees were struck by sirex and that suppressed trees and wildings were by far the most susceptible. The number of susceptible trees had significantly reduced by 2020, likely contributing to sirex control via reduced host availability, but there were still a substantial number of susceptible trees in the stand due to the large number of wildings. The wasp Ibalia leucospoides parasitised approximately 30% of sirex larvae, also contributing to control. Nematode parasitism increased steadily from