Regeneration Methods to Reduce Pine Weevil Damage to Conifer Seedlings

Petersson, M. 2004. Regeneration methods to reduce pine weevil damage to conifer seedlings. ISSN: 1401-6230, ISBN: 91 576 6714 4 Damage caused by the adult pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) can be a major problem when regenerating with conifer seedlings in large parts of Europe. Weevils feeding on the stem bark of newly planted seedlings often cause high mortality in the first three to five years after planting following clear-cutting. The aims of the work underlying this thesis were to obtain more knowledge about the effects of selected regeneration methods (scarification, shelterwoods, and feeding barriers) that can reduce pine weevil damage to enable more effective counter-measures to be designed. Field experiments were performed in south central Sweden to study pine weevil damage amongst planted Norway spruce (Picea abies (L.) H. Karst.) seedlings. The reduction of pine weevil damage by scarification, shelterwood and feeding barriers can be combined to obtain an additive effect. When all three methods were used simultaneously, mortality due to pine weevil damage was reduced to less than 10%. Two main types of feeding barriers were studied: coatings applied directly to the bark of the seedlings, and shields preventing the pine weevil from reaching the seedlings. It was concluded that the most efficient type of feeding barrier, reduced mortality caused by pine weevil about equally well as insecticide treatment, whereas other types were less effective. Soil scarification reduces feeding by pine weevil, but different soil features associated with type and cultivation strongly influences the results. In our experiments, mortality was highest in undisturbed humus and lowest in pure mineral soil. Pine weevil damage was reduced somewhat when the humus was cultivated, and feeding levels were lower than on pure humus when humus and mineral soil were mixed. The results indicate that pine weevils are more willing to stop and feed when suitable places for hiding are available close to the seedling. When grass vegetation surrounded a mineral patch pine weevil feeding increased significantly, but the pine weevils did not use vegetation as a "bridge" to reach the seedling. The most probable explanation for the increase in feeding is that pine weevils perceived the vegetation as a shelter.