Effect of entomopathogenic nematode (Steinernema carpocapsae Weiser) application technique on the efficacy and distribution of infection of the large pine weevil (Hylobius abietis L.) in stumps of Sitka spruce (Picea sitchensis Carr.) created at different

The large pine weevil (Hylobius abietis) is a major pest of young conifer and broadleaf transplants on restock sites in the UK. The objectives of this study were two-fold. The first was to test the effectiveness of two methods of application of the entomopathogenic nematode Steinernema carpocapsae in infecting the various life stages of the weevil in Sitka spruce (Picea sitchensis) stumps. The second objective was to investigate the importance of distribution and development of each of the weevil life stages in order to improve our understanding of factors influencing infection by S. carpocapsae. The method of nematode application on 24 July 1997, as well as the distribution and life stage of H. abietis within the stump and root system of Sitka spruce, were all shown to affect nematode efficacy as a field-based biological control agent. In contrast, the time of felling of the trees from July to September 1995 to create these stumps did not result in different rates of nematode infection of H. abietis in the stumps 2 years post-felling, but did influence development of the pest. H. abietis larvae were more widely distributed throughout the stump and root system than pupae, callow adults and pre-emergent adults that tended to be in the upper regions of the soil and close to the stump bole. Callow adults were highly susceptible to nematode infection, but larvae and older adults were not. Pupae had similar infection to larvae possibly due to late nematode application, as laboratory studies showed similar infection rates to callow adults. The results showed that S. carpocapsae tended to remain near the point of application, with the incidence of H. abietis infection declining with increasing distance from the stump and with increasing soil depth. The distribution of the most susceptible stages was, therefore, optimal for maximum nematode infection. At the time of the trial, susceptible H. abietis life stages comprised only 14–20% of the H. abietis population and the low level of control achieved (5.2–18.3%) reflects the differential susceptibility of these stages. The short duration of the callow adult and pupal stages, combined with the variation in development rates, indicated a fairly narrow application window in which to maximise infection rates. Timing of nematode treatment to proceed or better coincide with the susceptible stages of the lifecycle, multiple applications and/or increased dose rates of nematodes would probably have increased the overall level of control. Of the two application techniques used the subsurface drench was more efficient than the surface spray application, the former targeting a greater proportion of the susceptible life stages of H. abietis.