Aphids are sap-sucking insects which multiply very quickly and can cause tremendous damage to fruit trees. There are numerous forms, but most are green, brown, or pink, and tend to congregate on young shoot tips and the undersides of leaves at the ends of branches.
Female winged forms can arrive at any time in the spring or summer. They quickly give birth to large numbers of live young which rapidly overwhelm the tree.
Infestations occur most commonly in spring, and in the case of new young trees they can quickly stunt new shoots, thereby preventing the tree achieving the growth and form expected.
A second wave of infestation often occurs in late summer, the mature aphids having spent the intervening period on other soft plants.
No fruit tree varieties are resistant to aphids. However they most commonly attack apple trees and plum trees. In the case of plum trees it is sometimes the sweeter-tasting varieties that are more likely to be attacked, but this is by no means certain.
Results of attack
Aphids usually congregate on the growing tips of young shoots. They weaken the tree, and also distort or stunt the growth of the shoot. This damage is permanent and it is usually best to cut back to a healthy section lower down the shoot the following winter / spring and start again.
Aphids have many natural predators, particularly ladybirds and ladybird larvae. However by the time these arrive the damage has invariably been done.
A number of chemical sprays are also available - but again these tend to work after the event.
Therefore vigilance is the best policy. Keep a close watch on your trees, especially the undersides of the growing tips of shoots. If you see individual winged aphids, squash them. Another sign of aphids is a patrol of ants climbing the tree or active at the top of the tree - they farm the aphids and prevent other insects (such as natural predators) getting near.
Some customers have been successful with lemon solutions, made by boiling lemon peel, spread on or near the tree, since the citrus odour seems to deter aphids - more details from Fennel and Fern.