Frothy blobs of liquid appear on stems of plants in early summer.
The immature, nymph stage of froghoppers which feed on the plants and produce this froth for protection. Once they mature, the froghoppers stop producing this froth.
Very little, unless they feed on very tender shoot tips, which may cause some distortion.
Since these froghoppers rarely cause much damage, it is not worth spraying them. They are best picked off as and when they occur.