OUTBREAKS OF THE WINTER MOTH ON SITKA SPRUCE IN SCOTLAND ARE NOT INFLUENCED BY NUTRIENT DEFICIENCIES OF TREES, TREE BUDBURST, OR PUPAL PREDATION

Since the early 1980s, the winter moth, Operophtera brumata L. (Lepidoptera: Geometridae) has emerged as a serious pest of Sitka Spruce, Picea sitchensis Bong. plantations in southern Scotland. Outbreaks are characterised by susceptible sites within plantations which can occur immediately adjacent to resistant sites. We investigated the level of some nutrients in the trees, the date of budburst of the trees, and the numbers of some potential predators of winter moth pupae. None could satisfactorily explain outbreak patterns. Although foliage analysis demonstrated that many trees were marginal or deficient in phosphorus, nitrogen and potassium, these deficiencies were not related to the susceptibility of a site. Within sites, the numbers and weights of O. brumata were positively related to phosphorus content and negatively related to calcium content of foliage. Other evidence suggests, however, that these correlations may not represent direct effects of phosphorus and calcium on larval growth and survival. Date of budburst, which commonly determines susceptibility of deciduous hosts to O. brumata, was unrelated to density, and pupal predators were more, not less, abundant in susceptible sites. Although it is difficult to distinguish between factors that initiate outbreaks and those that maintain them, these data suggest that nutrient deficiencies of trees, budburst date, and the distribution of pupal predators of the winter moth cannot explain patterns of outbreak of the winter moth on spruce.