Deep planting of Norway spruce seedlings: effects on pine weevil feeding damage and growth

Abstract

As the use of mounding as a soil preparation method and mechanized planting become more common, the use of deep planting has increased. In deep planting, a greater portion of the stem is buried below the soil surface. However, it is feared that this increases the risk of insect damage, especially damage from the pine weevil Hylobius abietis (L.) (Coleoptera: Curculionidae). The effects of planting depth on the feeding preference of adult pine weevils in 1.5-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were investigated using choice experiments. Seedlings were planted at two depths in a pot to compare (i) normal planting depth (target depth 3 cm) and (ii) deep planting (8 cm). In deep-planted seedlings, the number of feeding scars on lower stem parts above the soil surface, as well as severe feeding, was effectively reduced. After planting, the aboveground portion of the stem in deep-planted seedlings was clearly shorter and thinner, but the stems grew more both in height and diameter during the 5-week growing period than did normal-planted seedlings. At the end of the experiment, the deep-planted seedlings were still shorter, but there was no difference in diameter. In conclusion, deep planting protected 1.5-year-old seedlings from pine weevil feeding and improved seedling growth.