Influence of stem diameter on the survival and growth of containerized Norway spruce seedlings attacked by pine weevils (Hylobius spp.)

Pine weevils (Hylobius spp.) feeding on stem bark of young conifer seedling s pose a serious threat to forest regeneration-planting programmes in Nordi c countries. This study was designed to determine the threshold diameter fo r planted, untreated containerized seedlings, above: which pine weevils cau se little or no damage. The effects of sublethal weevil damage on seedling growth were also assessed. In total, 5320 containerized spruce seedlings we re planted on scarified and unscarified plots on three sites in southern Sw eden. Seedlings in six size classes, which differed with regard to age (1.5 -3.5 yrs) and cultivation density (28-446 seedlings m(-2)) were grown using the Combicell system. None of the seedlings was treated with insecticides, except for those in the smallest class. where both untreated and treated s eedlings were used. Inspections were made periodically during the first 3 y rs and after both 5 and 7 yrs. A statistically significant relationship was found between seedling losses due to pine weevil attack and seedling stem- base diameter at the time of planting out, on both scarified and unscarifie d plots. For seedlings with a stem-base diameter of around 10 mm, mortality due to pine weevil attack on scarified plots was low enough to be consider ed negligible. This threshold diameter was several millimetres greater for seedlings planted on unscarified plots, An analysis of the relationship bet ween the extent of weevil damage and seedling growth rate showed that among surviving seedlings, those that grew fast tended to show low levels of dam age. On unscarified plots, the mortality rate amongst seedlings treated onc e with a permethrin insecticide was only one-third that of untreated seedli ngs. On scarified plots, the corresponding difference was somewhat smaller. Repeated insecticide treatment resulted in a pronounced reduction in seedl ing mortality on the unscarified plots, whereas the effect was weaker on sc arified plots.