cuckoo spit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A frothy mass of liquid secreted on plant stems as a protective covering by a nymph of a froghopper.

from The Century Dictionary.

• noun A froth or spume secreted by sundry homopterous insects, as the common frog-hopper, Aphrophora or Ptyclus spumarius. Also called froth-spit.
• noun An insect which secretes a froth or spume, as a frog-hopper: called in full cuckoo-spit froghopper.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A frothy mass of white liquid that is secreted onto plants by froghopper larvae and serves as a protective covering.