Plants affected
Hydrangea, Viburnum species, Prunus species and some other ornamental woody plants.
Symptoms
In early summer the stems and foliage of hydrangeas are covered with brown scale insects about 4-5mm long and their white oval egg masses. The remnants of the egg masses can persist on the plant throughout the summer.
Cause
The white objects are the waxy egg masses of the hydrangea scale, an introduced pest in Britain that originates from Japan. It was first found in the London area in the late 1980s and is now well established in southern England.
Young nymphs hatch from eggs in midsummer and suck sap from the undersides of leaves. In late summer the nymphs move to the stems where they overwinter before maturing in late spring. Heavily infested plants lack vigour and look unsightly.
Control
There are no effective non-chemical controls, other than destroying infested plants and replanting in the autumn.
Spray against the newly hatched nymphs in mid-July with imidacloprid +sunflower oil (Provado Ultimate Bug Killer concentrate), acetamiprid (Scotts Bug Clear Ultra concentrate) or thiacloprid (Provado Ultimate Bug Killer Ready To Use). To reduce the risk of spray damage to the leaves avoid spraying hydrangeas in hot sunny weather or if the soil is dry.