Defoliation and growth relationships for mid-rotation Sitka spruce attacked by the green spruce aphid, Elatobium abietinum (Walker) (Homoptera: Aphididae)

Green spruce aphid (Elatobium abietinum) feeds primarily on the 1-year-old and older needle leaves of spruce (Picea spp.) and is a major defoliator of commercial Sitka spruce plantations in the UK and other maritime regions of north–west Europe. The impact of E. abietinum on mid-rotation (23–28-year-old) Sitka spruce in Radnor Forest, in mid-Wales, was determined by comparing aphid population densities, defoliation and growth between plots of trees that were either treated with insecticide or were left untreated. The experimental treatments were maintained for 5 years and over this period (2002–2006) peak E. abietinum densities in the untreated plots varied from 5 to 36 aphids per 100 needles. These densities, which were representative of low to moderate rates of infestation, were associated with low rates of defoliation (0–8%), but they had a significant (P ⩽ 0.01) impact on mean radial increment (RI) and mean volume increment (VI). In 2005, the year with the highest aphid populations, peak densities in the untreated plots averaged 14 aphids per 100 needles and this rate of infestation reduced RI by 17%, VI by 10% and the dry weight of current-year needles by 10%. On average, across all years, infestation by E. abietinum reduced mean annual VI by 6%. Comparisons with previous studies on the impact of the aphid on 4-year-old, 7-year-old and 9-year-old Sitka spruce ( Straw et al., 2005) indicate that defoliation by E. abietinum, on a per capita basis, decreases as trees grow older, but that the impact on VI increases. The different growth response of young and mid-rotation Sitka spruce to infestation is related to differences in canopy structure, particularly in the ratio of current-year needles to older needles, and the greater demands on photosynthetic production in older trees that arise from the need to support an increasing quantity of non-photosynthetic structural tissues in branches, stem and roots.

► Quantitative data on the impact of Elatobium abietinum on Sitka spruce.
► Detailed relationships between aphid population densities and needle loss.
► Trends in per capita defoliation rates and impact in relation to tree age.
► Significant long-term effects on tree performance from low rates of infestation.