Why Are My Apple Tree Leaves Distorted and Sticky in Spring?

Apple growing tips in spring that produce tightly curled, puckered or inward-rolled leaves — often sticky with honeydew and with black sooty mould developing on the stickiness — are infested with rosy apple aphid (Dysaphis plantaginea). This spring-active aphid is one of the most damaging of all apple pests because it not only stunts and distorts the growing tips but also affects the nearby developing fruitlets. Apples that form on a shoot where rosy apple aphid has been actively feeding can be severely malformed, small and lopsided at harvest — even if the aphids have long since departed.

Identification

Rosy apple aphid colonies are found on the undersides of tightly curled young leaves and on the stems of developing fruitlet clusters. The aphids themselves are small (1–2 mm), pale greyish-pink to rosy-brown. The leaf curling caused by rosy apple aphid is tighter and more persistent than that caused by green aphid species — even after the aphids leave, leaves that have been infested may remain curled for weeks. Honeydew and sooty mould on the upper leaf surfaces indicate the presence of aphids underneath.

Timing and the damage window

Rosy apple aphid eggs hatch as apple buds open in early spring. The crawling nymphs infest the first available growing tissue before natural predator populations have built up. The critical damage window is from bud burst through to fruitlet set — about four to six weeks. After this, winged forms migrate to plantain weeds and the apple infestation declines naturally. The fruit-distortion damage, however, is already done by the time the aphids leave.

Control measures

Apply a winter plant oil emulsion spray in January to kill overwintering eggs on the bark — this gives the most significant population reduction. At the green cluster stage in early spring (before flowers open), apply insecticidal soap, pyrethrum or thiacloprid to reduce the initial crawling nymphs. Avoid broad-spectrum insecticides at flowering, which kill pollinating bees. Encourage aphid predators: ladybirds, hoverfly larvae and lacewings feed actively on aphid colonies.