What is box sucker?
Box sucker is a sap-sucking insect, sometimes known as a psyllid, that feeds at the shoot tips of box plants in spring.
Box can suffer from a number of other problems.
Control is unnecessary on established box hedges or topiary plants that are going to be clipped.
Young plants that need to grow larger can be sprayed with deltamethrin (Bayer Sprayday Greenfly Killer) 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)