• Plum leaf-curling aphid is green and can reach 2mm in length. It causes tight leaf distortion on the foliage of all types of plum during April to late May. Leaves that develop later are not affected, although the earlier damage often remains visible throughout the summer
• Mealy plum aphid Is dusky green and usually covered in a white waxy material, it can teach 2.6mm in length. Thsi aphid is active in mid- to late summer, when dense colonies of pale greenish white aphids cluster on the underside of leaves and shoot tips. It does not cause leaf distortion but it excretes honeydew that can make foliage and fruit sticky and allows the growth of sooty mould

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. On tall trees aphids can be considered part of the biodiversity they support, natural enemies will normally reduce numbers by late summer, in addition artificial treatments are only likely to be successful if the entire plant can be reached. 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, the plum aphids often do not affect cropping or future health of the tree. These aphids can 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
• Research has indicated that earwigs on fruit trees can reduce aphid numbers and in fruit trees they do not cause damage. Providing shelters such as flower pots loosely stuffed with hay in trees can help increase earwig numbers

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. Treatment after leaf curling has occurred will also have limited effects. 

• On fruit trees and shrubs. Overwintering aphid eggs can be destroyed by using a plant oil winter wash (organic e.g. Growing Success Winter Tree Wash). This can be used when the buds are fully dormant in November-early February on a dry frost-free day. Plant oil winter washes are less likely to be detrimental to natural enemies and can mean that spring sprays are unnecessary 
• 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

Download

Pesticides for gardeners (pdf document)

Both mealy plum and plum leaf-curling aphids overwinter as eggs that are laid in autumn in bark crevices and around the buds. Eggs of plum leaf-curling aphid hatch at bud burst but those of mealy plum aphid hatch later in April.

Aphids feed by sucking sap from the foliage. Leaf-curling plum aphid secretes chemicals into the foliage that cause the young leaves to develop in a crumpled and distorted fashion. In this species, winged forms of the aphid develop during May and the winged adults then fly away to various herbaceous plant where they spend the summer. The population on plums dies out in late May-early June and after that time the tree produces normal foliage.

Mealy plum aphid populations develop during mid to late summer. Winged mealy plum aphids develop during the summer and these migrate to various grasses and reeds.

Both species produce a winged generation in autumn that flies back to plums and other host plants to lay the overwintering eggs.