Efficacy of entomopathogenic fungi against large pine weevil, Hylobius abietis, and their additive effects when combined with entomopathogenic nematodes

The large pine weevil Hylobius abietis is an important pest of reforestation in northern Europe. In field trials, we assessed the efficacy of entomopathogenic fungi (EPF) alone and in combination with entomopathogenic nematodes (EPN) against immature stages. We used commercial strains of Metarhizium brunneum and Beauveria bassiana and a strain of Beauveria caledonica isolated from the pest's habitat. The EPN used were Steinernema carpocapsae and Heterorhabditis downesi. Efficacy was assessed by weevil infection in stumps and by numbers of adult weevils emerging in traps above the stumps. EPF infected up to 23% of pine weevils, at depths up to 25 cm below ground. Three EPF application methods were compared, but application method did not affect infection rates. When applied at half doses, EPF and EPN had additive effects: M. brunneum and S. carpocapsae caused a reduction in emergence of 58% relative to control, M. brunneum and H. downesi 93%, B. caledonica and H. downesi 86%. EPN alone offered good suppression of H. abietis populations and suppression by the mixture did not surpass suppression afforded by EPN alone. Our trials show B. caledonica is a promising species, rivalling the success of the other two species. Applied EPF, especially M. brunneum, can persist for at least 2 years in the forest setting. Additionally, different fungi can have differential action against weevils above versus below ground; therefore, EPF combinations may be beneficial. Based on our trials, further development of strains and application methods are required before EPF can be recommended for suppression of pine weevil.