Plants affected
Several popular hedging conifers, including Cupressus macrocarpa, Lawson's cypress (Chamaecyparis lawsoniana), Thuja and Leyland cypress (x Cupressocyparis leylandii).
Symptoms
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 more severely affected than the top. A black powdery coating of sooty mould often develops on the stems and foliage.
Cause
The damage is caused by greyish brown aphids (Cinara cupressi) which are active from May to November, reaching their peak in early to mid-summer. They suck sap from the stems and even quite light infestations can have a significant effect. Although the aphids are fairly large, 4mm long, they are difficult to see. A useful confirmation of aphid damage is the presence of the black sooty mould, as there are other causes of dieback on conifers. These include drought, scorching caused by wind or extreme temperatures, and root diseases such as honey fungus and phytophthora.
Control
Spray in early summer at the first sign of an attack to prevent damage occurring. Suitable products contain imidacloprid (Bayer Provado Ultimate Bug Killer Concentrate) or bifenthrin (Scotts Bug Clear, Doff All-In-One Garden Pest Killer or Bayer Sprayday Greenfly Killer Plus). It is often difficult to spray large hedges thoroughly and in years when the aphid is abundant some damage will occur. Damaged hedges can recover but it is likely to be a slow process.
Andrew Halstead