1 Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) within the canopy of 20–25-year-old Sitka spruce are described based on data from two low-altitude sites (310–420 m above sea-level), two mid-altitude sites (500–550 m a.s.l) and one high-altitude site (610 m a.s.l).
2 Aphids were counted throughout the canopy on shoots representative of all needle age-classes present at each whorl of branches. Counts were made during the middle week of each month from September to July for 4 years (1999–2003), and mean E. abietinum densities at each canopy position were calculated separately for each month and for the low-, mid- and high-altitude sites.
3 During September to November, the highest densities of E. abietinum occurred on 3–4-year-old needles on branches low in the canopy. Over the winter and spring, the centre of the aphid’s distribution shifted outward and upward, so that by June of the next year the highest aphid densities occurred on current and 1-year-old needles on branches near the top of the tree.
4 The aphid distribution was re-set each year during July, at the time when the nutrient quality of the host was in decline and E. abietinum populations were decreasing. Aphid densities decreased less on 3–4-year-old needles than on current and 1-year-old needles, suggesting that older needles were a superior food resource at this time of year and in the autumn. However, other factors, such as higher temperatures in the upper canopy during the summer or differential mortality caused by natural enemies, could also have contributed to the change in distribution.
5 The outward and upward shift in the aphid distribution over the winter period provided no evidence that aphids at positions lower and deeper in the canopy were better insulated from freezing temperatures and had higher over-winter survival rates. Mean air temperatures at the top and bottom of the canopy during the winter were also found to differ by only 0.1–0.2 °C.
6 The percentage of the total aphids per tree that occurred on current or 1-year-old needles varied widely between seasons and between sites. Consequently, sampling programmes designed to estimate total population numbers of E. abietinum have little option but to sample needles throughout the canopy, and at regular intervals during the period when the aphid is abundant.