A Spit Bug...

This is the 'nymph stage' of the Froghopper insect. We have these on some of our Balsam trees behind the camp, quite neat actually. ( I have mentioned this before in '09, and Elaine of Artic View, helped me in the
identification of it) :)

Never seen the actual bug, I am only assuming, that the bug caught in the foam, is possibly food for this nymph.

"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 bug would quickly dry up. The nymphs pierce plants and suck sap causing damage, and much of the excess filtered fluids go into the production of the froth, which has an acrid taste, deterring predators. "

from Wikipedia.