I love treehoppers, and it was a delightful treat to find this brilliantly-colored white one with bright red eyes, orange and black stripes, and a prominent horn. The oak treehopper (Platycotis vittata) is generally about 8-9 mm long, not including the horn, which doesn’t seem to be related to the insect’s gender, and indeed may or may not even be present.
Oak treehoppers are common all over North America, and actually come in a bunch of color varieties across their range, four of which have been formally named (the one above is technically Platycotis vittatta var. vittata (Fabricius)). They seem to prefer “coastal” areas with moderate climate.
As one might guess, they do live gregariously on oak trees, using their piercing mouthparts to suck liquids from the tree, but they generally don’t get numerous enough to do lasting harm to the tree.