Ever seen a plant where it looks like someone spit on it?  Those frothy masses have small spittlebug nymphs inside. The insect hides inside the bubbled up plant sap. Damage is minor but can include curled or distorted growth. They have only one generation per year and will transform into adults and no longer cause the spittle masses or damage by late June. You can hand destroy the nymphs. If you decide to spray, make sure you use enough pressure to get the material into the spittle mass.