Why vegetation increases pine weevil damage: Bridge or shelter?

Abstract

Conifer seedlings suffer less feeding damage from adult pine weevils ( Hylobius abietis (L.)) when planted in pure mineral soil than in undisturbed humus. We investigated how vegetation surrounding planted seedlings affects damage by pine weevil and the causes of such effects. In three experiments conducted on newly clear-felled conifer forest sites in southern Sweden the impact of vegetation was studied by using several artificial vegetation cover systems constructed around Norway spruce seedlings planted in pure mineral soil or humus. Most treatments included arranging wavy hair-grass ( Deschampsia flexuosa (L.)) in various positions and densities around the seedlings. The presence of vegetation significantly increased pine weevil feeding for seedlings planted in mineral soil. The findings clearly demonstrated that the weevils did not use the vegetation as a “bridge” to reach the seedling, nor did differences in microclimate close to the seedlings appear to explain the differences in feeding damage. A likely explanation for the vegetation effect is that pine weevils perceive the vegetation as a shelter providing protection, from predators or temperature extremes, and therefore feed predominantly on sheltered seedlings. In practical forestry, the aim should therefore be to avoid vegetation growing close to the seedlings, e.g. by creating sufficiently large spots of mineral soil to plant in, and by planting immediately after cutting, when vegetation is not abundant.