Control is unnecessary on established box hedges or topiary plants that are going to be clipped.
If damage is serious to plants that need to grow larger can be sprayed with deltamethrin (e.g. Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Fruit & Vegetable Bug Killer) lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), acetamiprid (e.g. Bug Clear Ultra) or thiacloprid (e.g. Bayer Provado Ultimate Bug Killer). Use when newly-hatched nymphs first appear on the new growth in spring. There is no point in spraying later in the spring once the feeding damage has occurred..
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)