Damage caused by the Green Spruce Aphid to Norway and Sitka spruce needles

Three-month-old needles of Sitka spruce were less susceptible to Elatobium abietinutn than 15-month-old needles. Symptoms appeared after longer aphid feeding times but only a proportion of damaged needles fell. After short feeding periods symptoms appeared in more Norway spruce needles than in Sitka spruce, whereas longer feeding periods resulted in more needles producing symptoms in the Sitka spruce. The symptoms took 4—6 days longer to appear in Norway spruce, and needle fall followed a longer feeding period than on Sitka spruce. Following 72 h feeding, needle fall occurred more quickly on Sitka spruce than on Norway spruce. The time taken for needle fall to occur was inversely related to the feeding time in Sitka spruce but such a response was not evident in Norway spruce. The results are discussed in relation to the differences exhibited in the probing behaviour of the aphid on the two spruces.