Conifers can be affected by several species of aphid (greenfly, blackfly and related insects). Honeydew and sooty mould will often reveal their presence and conifer aphids can cause shoots to dieback.
Plants affected Various conifers
Main symptoms Honeydew and sooty mould, dieback
Caused by Aphids
Timing All year
What are conifer aphids?
Aphids are sap sucking true bugs often called blackfly, greenfly or plant lice. There are more than 500 species found in Britain.
Several species feed on conifers, these populations can result in the presence of sticky honeydew and sooty mould and in some cases dieback. Closely related adelgids can also cause be found on some conifers, these insects are often covered in a white wax or cause galls.
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, it is easy to 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 feeds on 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. 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 numbers 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 discolouration 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. Large populations can result in dieback of shoots or even the death of plants. The cultivar 'Skyrocket' is particularly susceptible.
Cypress aphid (Cinara cupressivora): This aphid feeds on 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. It is active from May to November but are most noticable 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 colonies 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 largest populations 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. It cann become active in winter on Abies brought indoors for use as Christmas trees.
Black spruce bark aphid (Cinara piceae): The adult aphids are large (5-6mm in length) and sometimes mistaken for spiders or beetles. The adult 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 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 populations reaching a peak in late May and June. The trunk and branches can become heavily coated with honeydew and sooty mould. This aphid is often very local and large colonies 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. Large populations may cause yellowing of the foliage.
Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. Little can be done to deal with aphids on tall trees as treatment is only likely to be successful if the entire plant can be reached.
When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
- Where possible tolerate populations of aphids they form an important part of many food chains and can be part of a healthy garden ecosystem
- Use finger and thumb to squash aphid colonies where practical
- Encourage ‘aphid predators’ in the garden, such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs. Be aware that in spring aphid populations often build up before natural enemies are active in sufficient numbers and then give good control
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
Little can be done to deal with aphids on tall trees as treatment is only likely to be successful if the entire plant is sprayed. It is difficult to penetrate dense conifer hedges with sufficient thoroughness to prevent conifer aphids causing damage. To prevent damage it may be necessary to spray the trees as soon as signs of aphids are seen. With most species this will be in late spring or early summer. For the green spruce aphid, however, it is late August or September, with a further treatment on a mild dry day in early February.
- Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
- Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These products contain some synthetic ingredients and so are not considered organic
- More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
- A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
- The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides.
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener
Pesticides for gardeners (pdf document)
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