Cypress aphid

Symptoms
A black powdery coating of sooty mould often develops on the stems and foliage. Yellowing shoots in summer; by late summer many of these will be brown and dead. On clipped hedges the dieback can be quite pronounced, with the lower parts often more severely affected than the top.

Spot them
Greyish brown aphids are active from May to November, reaching their peak in early to midsummer. Although the aphids are up to 4mm long, they are difficult to see.