Each species of conifer aphid has a restricted host range and specific symptoms. Some of the conifer aphids frequently encountered in gardens are listed below.
Note: When searching for aphids, do not confuse them with harmless insects known as psocids or barklice. These aphid-sized insects, may be winged or wingless, are generally brownish-white in colour. They feed on algae and fungal spores and may be abundant on trees affected by sooty mould. They run rapidly over the foliage and stems, unlike slow-moving aphids. Adelgids are usually either associated with galling or are covered in a white waxy substance.
Green spruce aphid (Elatobium abietinum): A small dark green aphid (1.5-1.8mm long) with red eyes. It infests spruces, Picea species, especially P. abies (Norway spruce or Christmas tree), P. sitchensis (Sitka spruce) and P. pungens (blue spruce). It is unusual amongst aphids in that it is active during from autumn to spring, rather than spring and summer. It can be found on spruce throughout the year but the summer months are spent as non-feeding nymphs. These start to mature during August and infestations can gradually build over the following six months. The green spruce aphid is favoured by mild winters which may result in damaging population levels occurring by late winter-early spring. The old foliage develops a pale mottled discoloration and many of these needles fall from the tree during the spring. This species produces honeydew and the resulting sooty mould is often noticed on stem joints. New growth in the spring is unaffected and its bright green appearance contrasts strongly with the discoloured and sparsely foliated older stems.
Juniper aphid (Cinara fresai): This species is of North American origin but is now widespread on Juniperus species in gardens in southern England. It is a large aphid (2.2-4.2mm long) which varies in colour from pinkish-grey to brownish-grey. The dorsal surface of the body is marked by a darker inverted V towards the head end. It can form dense colonies on the younger shoots and is found from May to October. This species produces honeydew and the resulting sooty mould is often noticed on stems and foliage. Heavy infestations can result in dieback of shoots or even the death of plants. The cultivar 'Skyrocket' is particularly susceptible.
Cypress aphid (Cinara cupressivora): This aphid attacks Cupressus species especially C. macrocarpa, Thuja occidentalis, Chamaecyparis lawsoniana and x Cuprocyparis leylandii. It is 1.8-1.9mm long and yellowish-brown although it appears greyish due to a dense coating of fine hairs. There is a dark stripe running down either side of the upper body, particularly at the head end. The aphids are active from May to November but are most troublesome during the early summer. Small colonies tend to develop at the bases of the green shoots, which often become blackened by sooty moulds growing on honeydew excreted by the aphids. Affected shoots usually become yellowish-brown and die back in midsummer. The lower parts of hedges are often more severely affected than the upper branches.
Cedar aphid (Cedrobium lapportei): A small aphid (1.5-2mm) long and greyish-brown with a paler mid-dorsal stripe running from the head end. It infests cedars including Cedrus atlantica, C. deodora and C. libani. Dense infestations can occur at the bases of the leaves during May and June, this can result in the affected leaves being shed. This aphid excretes large quantities of honeydew and sooty mould frequently develops, both on the tree and on the ground underneath.
Large cedar aphid (Cinara confinis): The heaviest infestations occur in June-July but this aphid is active on Cedrus and Abies species from March to October. A very large aphid at up to 7.8mm long they are greenish black or dark brown with a double row of dark spots on the upper surface of the abdomen. The foliage can become sticky with honeydew and blackened by the growth of sooty moulds. Infestations can occur in late winter on Abies brought indoors for use as Christmas trees.
Black spruce bark aphid (Cinara piceae): The adult aphids are very large (5-6mm in length) and are sometimes mistaken for small spiders or beetles. The fully grown aphids are shiny black with reddish-brown legs, while the nymphs have duller greyish-black bodies. They suck sap from the bark of a wide range of mature spruce trees (Picea species) and may form dense colonies several feet across on the trunk. The aphid is active from April until the autumn with the heaviest infestations occurring in late May and June. The trunk and branches can become heavily coated with honeydew and sooty mould. Outbreaks of this aphid are often very local and heavy infestations are spectacular but infrequent.
Grey pine needle aphid (Schizolachnus pineti): Dense colonies of this 1.2-2.5mm long aphid can occur on the undersides of the mature leaves of Scots pine and other Pinus species. The aphids are coated with a greyish-white wax and they are present from May until the autumn. Heavy attacks may cause yellowing of the foliage.