Cuckoo spit is seen as a frothy mass on plants.
It is a sign that a tiny froghopper insect is hiding within, and feeding on the sap of your plants, as it sucks it produces bubbles to hide itself from predators.
Apart from looking unsightly, feeding nymphs do little harm other than causing some distortion to the plant shoots.
Control them by picking them off by hand rather than using an insecticide spray.
The nymphs soon grow into adults and if you try and catch them with your fingers you will find that they will jump away, hence the insect's common name froghopper.