Plants affected
Picea species especially Picea abies (Norway spruce or Christmas tree), Picea sitchensis (Sitka spruce) and Picea pungens (blue spruce)
Symptoms
Old foliage develops a pale mottled discolouration during
the winter and many of these needles fall off in the spring.
A black sooty mould may be noticeable on the stem joints.
New growth produced in the spring is unaffected and its bright green appearance contrasts strongly with the discoloured and sparsely foliated older stems.
Cause
Green spruce aphid is dull green with dark red eyes and is up to 2mm long. The aphid differs from most other aphids by being active from autumn to spring, instead of the spring and summer. The summer is spent as non-feeding immature nymphs. This pest is particularly damaging in mild winters that enable it to breed more rapidly.
Do not confuse aphids with other insects known as barklice or psocids. They feed on algae and fungal spores and may be numerous on trees affected by sooty mould. These are aphid-sized, winged or wingless, and brownish-white in colour. They run rapidly over foliage, unlike the slow-moving aphids.
Control
Natural enemies such as ladybirds, lacewings, hoverfly larvae and parasitic wasps may help to limit infestations during the summer.
Small trees can be sprayed with imidacloprid (Bayer Provado Ultimate Bug Killer Concentrate), acetamiprid (Scotts Bug Clear Ultra Concentrate or bifenthrin (Scotts Bug Clear Gun, Bayer Sprayday Greenfly Killer Plus, or Doff All-In-One Garden Pest Killer) in late August or September, and again during the winter if the foliage is becoming discoloured and aphids are present.
Little can be done to protect tall trees, other than spraying the lower branches.
Damaged trees can be helped to recover by keeping them watered during dry spells and feeding with a general fertiliser in the spring. It will take several years for a badly damaged tree to regain an attractive appearance.