Cypress aphid is a blackish brown sap-sucking insect that feeds on the stems of some hedging conifers (Cupressus species especially C. macrocarpa, Thuja occidentalis, Chamaecyparis lawsoniana and x Cuprocyparis leylandii) in early summer. This often causes a gradual drying up and browning of the foliage.

Aphids are sap-sucking true bugs. They range in size from 1 to 7mm (¼in or less) long. Some aphids are known as greenfly or blackfly, but there are species that are yellow, pink, white or mottled. There are more than 500 aphid species in Britain. Some feed on only one or two plant species, but others can be found on a wide range of plant hosts. Many have lifecycles that involve more than one host plant. Almost any plant can be a host to aphids, including ornamentals, vegetables, fruits, greenhouse plants and houseplants. More information on aphids. 

There are a number of aphids found on other conifers and aphid like adelgids can also affect these plants.

• Yellowing and shoots in summer; by late summer many of these will be brown and dead. Brown patches on conifers can be caused by a number of other reasons, including disease, but are often caused by growing conditions or routine operations such as trimming. Read more about brown patches on conifers here
• On clipped hedges the dieback can be quite pronounced, with the lower parts often more severely affected than the top
• A black powdery coating of sooty mould may develop on the stems and foliage, this is often the only symptom which will confirm aphids as the cause
• Damaged hedges can recover but it is likely to be a slow process

Following care and pruning guidelines for susceptible conifers during the growing season will improve resistant to cypress aphid damage. 
Aphids form the basis of many food chains in the garden and it is not unusual to have some of these animals in a healthy balanced garden ecosystem.  Check susceptible conifers frequently from spring onwards so action can be taken before a damaging population has developed. On established trees aphids can usually be considered part of the biodiversity they support, natural enemies will normally reduce numbers during summer. On conifers however this is not always the case and small populations of some species can cause browning. When choosing management options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section and avoiding pesticides. Within pesticides the shorter persistence products (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. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

Non-pesticide control

• 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 (on very small trees)
• 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
• Once aphid damage has occurred renovating the hedge can be attempted however the host conifers are often intolerant of hard pruning. Information on hedge selection can be found here

Pesticide 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. Where spraying is feasible below are some suitable products for the home gardener;

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear Fruit & Veg, Vitax Rose Guard) 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, Growing Success Bug Stop, Rose Clear 3 in 1 Action 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) 
• 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.

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Pesticides for gardeners (pdf document)

The greyish brown aphids that are active from May to November, reaching peak numbers in early to mid-summer. This is one of the possible causes of browning in susceptible host conifers and can be particularly prevalent on Lawson's cypress (Chamaecyparis lawsoniana) and Leyland cypress (× Cuprocyparis leylandii) hedges.

• They suck sap from the stems and even quite light infestations can have a significant effect
• Although the aphids are 2-4mm long (about 1/8in), they are difficult to see as their colour matches that of the bark 
• A useful confirmation of aphid damage is the presence of a black sooty mould on the stems and/or foliage, as there are other causes of dieback on conifers
• Other causes of dieback include poor establishment, weather damage such as drought, scorching caused by wind or extreme temperatures, excessive clipping of a hedge, and root diseases such as honey fungus and Phytophthora root rot