Sometimes called Mirids, up to 6mm long, very active insects (hemiptera miridae) winged, membrane type legs and long antennae and proboscis. The nymphs are similar but without wings. Their colours are yellow, green and brown depending on species. Adults & nymphs pierce plant tissue with their fine stylets, inject saliva and suck sap. Their saliva kills plant tissue; some species also feed on mites, aphids and small caterpillars so they have some beneficial effect. 

The main species are:- Common Green Capsid (Lygocoris pabulinus) Potato Capsid (Colocoris novegicus) Aplle Capsid (Plesiocoris rugicollis) Tarnished Plant Bug (Lygus rugicollis).
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They are often confused by gardeners with Frog Hopper (Philaenus spumarius) which are known as cuckoo spit. However Frog hopper damage is different. They pierce the young stems and often cause cocked heads or buds. 

Controls
Treatment for both is best by spraying with a systemic insecticide such as Imidacloprid.

above - Capsid leaf damage
right - bloom mis-shapen by capsid damage at bud stage (cultivar 'Lorna Wood')