Spruce aphid population dynamics in relation to canopy character: scope for cultural regulation.

The dynamic relationships between annual population densities of the spruce aphid in plantations of Sitka spruce are reviewed. The aphid is anholocyclic in western Europe and unlike many aphids on trees, it can be strongly affected by winter weather. In general, mild winters may be followed by higher aphid populations which are capable of causing widespread defoliation, but overcompensating density dependence between years is also indicated and among those processes responsible are behavioural responses of aphids to increased crowding in spring. Alate aphids were found to accumulate on foliage bearing low initial aphid populations, but it was also significant that they preferentially accumulated on tree canopies which were more exposed (apparent) than those under normal plantation conditions (cryptic). From initially similar population densities of aphids, larger aphid populations were shown to develop on apparent trees than on cryptic trees of the same provenance. The redistribution of alate aphids can therefore be seen as a significant (although not the only) process responsible for generating differences in aphid numbers between trees. The process is density dependent but different canopy types could support different equilibrium densities of aphids. The consequences of differences in canopy exposure in plantations (edges or spacing between trees) may be an increased risk of aphid damage, and this should be considered in silvicultural planning, particularly as better interactive models for the growth of Sitka spruce plantations become available and the economic effects of the aphid become better understood.