Entomopathogenic fungi and chemical insecticides have synergistic action against adult large pine weevil, Hylobius abietis

• The large pine weevil ( Hylobius abietis ) is the most serious insect pest of forestry in Europe. • We show synergy among a range of entomopathogenic fungi and pesticides. • Field trials support the conclusions of laboratory assays. • This provides an effective future strategy to manage this important pest. The large pine weevil, Hylobius abietis , is a serious pest of reforestation across much of northern Europe. Larvae develop in the stumps and roots of recently felled conifers, and adults feed on the bark of young trees used to replant the site. Entomopathogenic fungi and chemical insecticides, alone and in combination, were tested against adult pine weevil in laboratory and field experiments. In a broad screening experiment, weevils were dipped in conidial suspension of Beauveria bassiana (Bb Naturalis and Bb SC4), Beauveria caledonica (Bc SG61) and Metarhizium anisopliae (Ma F52 and Ma SG6) and/or provided with twigs treated with chemical insecticides (cypermethrin, acetamiprid and thiacloprid). When assessed after 14 or 17 days, there was higher weevil mortality in combined treatments than expected based on individual treatments, with synergistic effects demonstrated in most cases. In a second laboratory experiment, Bb Naturalis, Bc SG61 and Ma F52 applied in each of two ways, either by dipping weevils or contaminating the substrate, gave synergistic effects in combination with cypermethrin, and mainly additive effects with acetamiprid. Weevil cadavers in all fungus-insecticide treatments tested developed mycosis, which would facilitate recycling of fungi in the field. In a field trial, a commercial formulation of Ma F52 (Met52) and cypermethrin were tested alone and in combination for protection of young Sitka spruce trees. Weevil feeding damage on trees treated with Met52 and half the recommended rate of cypermethrin did not differ significantly from that of trees that received full rate cypermethrin, but damage to trees that received half rate cypermethrin was higher. This study demonstrates the potential of entomopathogenic fungi to reduce the amount of chemical insecticide used to protect trees against pine weevil damage as part of an integrated pest management approach to pine weevil.