Dense colonies of small (<2.5mm) pinkish grey aphids develop on the underside of the foliage in spring and early summer. Leaves at the shoot tips can become curled and yellowish and in some cases shoots show stunted growth with distorted leaves that start to turn brown during the summer. The crop and future health of the tree  however, can be unaffected. These aphids often support large numbers of predators.

The leaves can also become sticky with honeydew, on which black sooty moulds may develop. During summer populations on pear die out and the aphids migrate to bedstraws (Gallium species).

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. Once leaf distortion has occurred there is little that can be done to control this insect, this is also true on tall trees as treatment is 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, 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
• There is evidence that earwigs on fruit trees can reduce aphid numbers and they do not cause damage to the trees or fruits. Providing shelters such as flower pots loosely stuffed with hay in trees can help increase 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.
This aphid does not normally affect cropping. 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. There is no point in spraying pear trees in late spring or summer, when the aphid damage has already occurred.

• 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)

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 home gardeners (pdf document)

Pear-bedstraw aphid overwinters on pear trees as eggs that are laid in autumn in bark crevices and around the buds on the shoots. These eggs hatch in spring as the leaves begin to emerge from the buds. While sucking sap, the aphids secrete chemicals into the foliage and fruitlets, which cause the distorted growth.

Several generations of wingless aphids develop between bud burst and early summer. During June-July, winged forms of the aphid develop that migrate away to wild plants known as bedstraws, Gallium species, where they spend the rest of the summer. Populations on pear die out during the summer but there is a return migration from bedstraws in autumn when overwintering eggs are produced.