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. 

Currant blister aphid is one of a number of aphids that feed on currants. Is pale yellow and up to 2.54mm in length. It can be found on the lower leaf surface in early summer and causes leaf distortion. 

• Leaves at the shoot tips appear puckered or blistered
• The distorted foliage takes on a reddish or yellowish green discolouration
• Pale yellow aphids can be found underneath the blistered areas in late spring-early summer
• This aphid is a food source for many aphid predators and colnies can be wipped out 

Currants affected by blister aphid still make growth and produce a good crop, so treatment is not necessary.

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 currants frequently from spring onwards so action can be taken before a damaging population has developed. On established bushes aphids can usually be considered part of the biodiversity they support, natural enemies will normally reduce numbers during summer. 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. Some damage from these aphids can be tolerated with no effects on quality or quantity of yield. These aphids 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

• There is no point in spraying after the foliage has become distorted
• 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) is available for use on ornamentals and selected edibles

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.
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)
 

• Currant blister aphids are pale yellow and live on the underside of the leaves where they feed by sucking sap
• They also secrete chemicals into the foliage and this causes puckering, distortion and discolouration
• Other species of aphid also occur on currants but they do not produce this characteristic blistering symptom
• Wingless forms of the blister aphid are active on currants from bud burst until July
• In midsummer, winged aphids develop and migrate to the wild flower known as hedge woundwort (Stachys sylvatica)
• Populations on currants come to an end at that time, although damaged foliage remains visible until leaf fall
• The aphids return to currants in the autumn when overwintering eggs are laid