Potential of Beauveria bassiana application via a carrier to control the large pine weevil

Abstract The large pine weevil (Hylobius abietis) is an important pest of freshly planted coniferous trees. Feeding damage to conifer seedlings can be prevented by protecting the seedlings (mechanically or chemically), or by reducing H. abietis abundance. The entomopathogenic fungus Beauveria bassiana has promise for H. abietis control. To test the effects of the B. bassiana-colonized carriers (25 mm in diameter and colonized by strain AMEP20) on H. abietis adults that move into forest clearances, we performed several experiments. These included an experiment in Petri dishes under laboratory conditions, an experiment in cages with Norway spruce seedlings under semi-natural conditions, and a field experiment. Infection by B. bassiana and mortality of H. abietis was confirmed in all three experiments. The total mortality rate in all Petri dishes with B. bassiana carriers after 14 days was 74% (80% with fresh carriers; 68% with carriers that had been treated with UV light; 82% for carriers that had been frozen; 45% for adults that were exposed to carriers for only 24 h; and 95% for carriers that had been stored at 4°C for 2 months). The mortality of H. abietis adults was affected by the absence/presence of carriers in the cages with spruce plants. The number of dead adults after 4 weeks of treatment in forest clearing were higher in plots with carriers than in the control plot. Mortality was highest for adults trapped in the plot with carriers on trap barks (58 %), followed by the plot with carriers and an attractant (48 %), and followed by the plots with carriers without an attractant (35 %). The results suggest that carriers colonized by the entomopathogenic fungus B. bassiana could be suitable for reducing H. abietis abundance in forests. The results indicated that H. abietis adults may be infected by B. bassiana after only a short exposure to carriers. The B. bassiana-colonized carrier is ready-to-use, i.e., it could be immediately produced and applied.