Seeing a long trail of ants trudging up the trunk of your fruit tree and congregating on a particular branch can cause you to run for the ant killer, but the ants themselves don't damage fruit trees. It is the tiny aphids they are tending that cause the leaves to curl up and turn brown, so you should manage both pests to prevent damage to your fruit trees. You are most likely to notice an infestation in early summer, when new shoot growth is at its height.
Aphids are 2 mm (1/12 inch) in size, and have soft, pear-shaped bodies. They may be green, brown, black, yellow or red. Aphids are not host-specific, so the same species may attack your cherry, plum and apple trees. They suck liquids from the underside of the leaves, usually the young, tender shoot tips. Other indicators of aphids, besides the ants, are curling leaves, stunted shoot growth, and a sticky substance on the leaves.
Ants and aphids have a symbiotic relationship. Aphids digest leaf juices and excrete this sweet, sticky substance called honeydew, which then attracts the ants who eat it. Ants "milk" the aphids to have them excrete the honeydew, and they "tend" the aphids by protecting them from natural predators, including ladybirds and wasps.
Feeding damage to mature trees is usually minor and self-limiting, but young trees may be overwhelmed if the infestation is extensive, because defoliation can occur. Once the leaves are curled, they do not recover after the pests are controlled, so identify the problem early. Aphids can carry diseases from one tree to another. The honeydew itself may cause black sooty mould to develop, which reduces photosynthesis in the leaves and can reduce fruit quality.
Prevention begins in late winter with the use of dormant oil sprays, which smother the overwintering aphid eggs. You should follow the product directions exactly for the desired results. Using dormant oil spray when the tree is not dormant can cause extensive damage to the tree. Use a lower nitrogen or slow-release nitrogen fertiliser to slow shoot growth that attracts aphids in the spring.
For best results, manage both the aphid and ant populations. For aphids on small young trees or localised infestations, wipe them off by hand or prune off the shoot tips where they are congregating. On larger trees, wash off aphids with water sprayed from a hose. Spraying the affected trees with insecticidal soap is the least-toxic way to kill aphids, but its effect is temporary and may need to be repeated every few days.
To keep ants off trees, band the trunks with 7.5 cm (3 inch) wide tree paper and paint it with a sticky ant trap product. Don't paint this directly on to the bark, as the substance will damage the tree. Check the paper every week or so and replace it when it becomes so covered with ants and debris. You can also use ant bait stations near the bases of trees to kill the entire ant colony.