Susceptibility of different developmental stages of large pine weevil Hylobius abietis (Coleoptera: Curculionidae) to entomopathogenic fungi and effect of fungal infection to adult weevils by formulation and application methods

The large pine weevil, Hylobius abietis, is a major pest in European conifer forests causing millions of Euros of damage annually. Larvae develop in the stumps of recently felled trees; the emerging adults feed on the bark of seedlings and may kill them. This study investigated the susceptibility of different developmental stages of H. abietis to commercial and commercially viable isolates of entomopathogenic fungi, Metarhizium and Beauveria. All the developmental stages of H. abietis can be killed by Metarhizium robertsii, Metarhizium brunneum, and Beauveria bassiana. The most virulent isolate of M. robertsii ARSEF4556 caused 100% mortality of pupae, larvae and adults on day 4, 6 and 12, respectively. This strain was further tested against adult weevils in different concentrations (10(5)-10(8) conidia cm(-2) or ml(-1)) using two types of fungal formulation: 'dry' conidia and 'wet' conidia (suspended in 0.03% aq. Tween 80) applied on different substrates (tissue paper, peat and Sitka spruce seedlings). 'Dry' conidia were more effective than 'wet' conidia on tissue paper and on spruce or 'dry' conidia premixed in peat. The LC(50) value for 'dry' conidia of isolate ARSEF4556 was three folds lower than 'wet' conidia on tissue paper. This study showed that 'dry' conidia are more effective than 'wet' conidia, causing 100% adult mortality within 12 days. Possible strategies for fungal applications are discussed in light of the high susceptibility of larvae and pupae to fungal pathogen.