Froghopper

The froth serves a number of purposes. It hides the nymph from the view of predators and parasites, it insulates against heat and cold, thus providing thermal control and also moisture control; without the froth the insect would quickly dry up. The nymphs pierce plants and suck sap causing very little damage, much of the filtered fluids go into the production of the froth, which has an acrid taste, deterring predators.
From wikipedia