Hylobius Abietis – Host Utilisation and Resistance

Abstract

The pine weevil, Hylobius abietis, breeds predominantly in the bark of roots of felled conifers and occasionally in moribund trees or in logs in contact with the soil. In unmanaged forests, these resources are unpredictable in time and space and usually scarce, except after relatively rare events such as extensive windblow. H. abietis is therefore, expected to occur at low density in its natural habitat. In managed forests, where clearfelling at the end of a rotation is the silvicultural norm, high weevil populations develop on the abundant root-stumps left in the ground. Hylobius abietis is therefore a ‘silvicultural’ pest whose population size is determined to a large extent by the availability of breeding sites (Eidmann 1979). The size of weevil populations is one of the main factors determining the extent of mortality of young conifers planted after clearfelling in central and northern Europe. But emphasis on the quantitative aspects of root-stump availability as a determinant of population size tends to obscure possible qualitative variation, both between and within host species, in their ‘suitability’ for larval development. The ‘quality’ of the resource is also likely to change over time, depending on such site-related factors as rainfall, temperature and rate of fungal decay. Weevil population dynamics and abundance could therefore be influenced by qualitative as well as quantitative variation in their breeding resource. The young conifers used in replanting clearfell sites are vulnerable to attack by adult weevils that emerge from the root-stumps. Weevils are large relative to the transplants and because they feed on the bark of the main stem, relatively small amounts of damage can be lethal. Mortality of unprotected young conifers on reafforestation sites is normally high (Langstrom & Day, chapter 19), giving the impression that most plants are very susceptible to weevil attack. However, because weevil population densities and therefore rates of attack on transplants are so high,