Why Are My Pear Tree Leaves Covered in Sticky Honeydew?

If your pear tree's leaves, shoots and sometimes fruit are coated in a thick, sticky substance, possibly turning black with sooty mould, and you can see small flat brownish insects on the undersides of leaves and in shoot crevices, you are looking at a pear psylla infestation. Pear psylla is one of the most damaging pear pests in warm, dry summers and can significantly reduce tree vigour and fruit quality if populations build up unchecked.

Identifying pear psylla

Pear psylla (Cacopsylla pyri and related species) are small, jumping insects related to aphids. Adults are 2–3 mm long, brownish or greenish, with clear wings held roof-like over the body. Nymphs are flat, pale green or yellowish discs that feed in groups on leaves and in bud axils, each surrounded by droplets of honeydew. The jumping habit of adults — they spring away when disturbed — distinguishes them from aphids. Populations build quickly in warm, dry conditions and crash in wet summers as rain washes nymphs off the tree and promotes fungal diseases that attack psylla.

Sooty mould consequences

The honeydew coating on leaves is colonised almost immediately by airborne sooty mould spores. Heavily moulded leaves cannot photosynthesise effectively, reducing the tree's energy production throughout summer. Fruit coated in honeydew at harvest becomes sticky and may be difficult to store. Washing fruit gently in water removes most surface contamination. The mould recedes as soon as the psylla population is controlled and rain clears the residue.

Encouraging natural enemies

Several predatory insects feed on pear psylla, including anthocorid bugs (flower bugs), earwigs and lacewing larvae. Populations of these beneficials are suppressed by broad-spectrum insecticide use. Earwigs are particularly effective psylla predators; encouraging them by placing damp rolled corrugated cardboard or inverted flower pots filled with straw around the tree provides overwintering habitat. Avoiding unnecessary spraying is the foundation of psylla management.

Targeted management of severe infestations

A winter wash with plant-oil based products applied while the tree is fully dormant (December to February) reduces the overwintering egg and adult population. In summer, insecticidal soap sprays targeted at nymph stages — reaching leaf undersides — help on small trees. Time any spray for early morning or evening to minimise contact with bees and other pollinators.