I am relatively relaxed about the appearance of ‘cuckoo spit’ on the plants in the garden. This froth is created by the froghopper from the plant’s sap, to protect the nymph during growth and retain moisure. It is relatively harmless unless it occurs on the growth tip of the plant, where sucking the sap can distort the growth.
I found this young froghopper hiding in a ball of foam on the tip of one of my young asters, and to preserve the precious growing tip I rubbed the froth off with my fingers to reveal the insect which hopped off on to the roof of the woodstore and away. You can also blast the foam off with a hose if it bothers you.