What insect's nymphs create frothy 'cuckoo spit' on plants?

The insect whose nymphs create frothy “cuckoo spit” on plants is the froghopper. The foam is a protective shelter made by immature froghoppers, also widely called spittlebugs.

A nymph feeds on plant sap and mixes the excess liquid with air, producing a mass of bubbles around its body. This moist covering helps prevent dehydration and can deter predators such as ants and parasitic wasps. Despite its appearance, cuckoo spit is not produced by cuckoos or by the plant itself.

Froghoppers are true bugs in the order Hemiptera and are relatives of cicadas. The common meadow froghopper, Philaenus spumarius, is a familiar source of the foam in Europe. After several nymphal stages, the insect emerges as a winged adult capable of powerful jumps.

A common mix-up is with leafhoppers, which are also sap-feeding hemipterans but do not typically produce the characteristic foam shelters. “Cuckoo spit” is therefore best associated with froghopper or spittlebug nymphs.