Between Spring and Early Summer little white, fluffy looking dots may start appearing on box hedging and topiary. These are woolly aphids, a small sap sucking insect feeding on the soft stems of box.
Woolly aphids are actually black aphids who cover themselves in white, waxy secretions for protection. Infestations are rarely bad enough to cause any lasting damage to the box so rather than using chemicals, I prefer to brush the aphids off with a stiff brush.
Another way to control woolly aphids is to encourage their natural predators into the garden. Ladybirds, lacewings and hoverfly larvae are excellent at reducing the aphid population. Try using bug houses, such as this Ladybird Tower to attract the natural enemies of woolly aphids.
A pesticide can be used to control the woolly aphids. I would choose a spray based on plant oils, fatty acids or pyrethrum, such as Bug Clear. Many repeat applications maybe required to be effective.
Woolly aphids are one of the cuddlier looking box pests but it is worth trying to control their population. In mid summer the aphids will disappear as they grow wings and go off in search of new host plants. But they will be back the following Spring unless treated.