Photo credit: http://www.forestryimages.org
The Orange-striped oakworm can be abundant on Red oak species from June through September. Small trees are often defoliated by mid summer. Total defoliation is never a good situation for any tree, and the negative long-term affects on tree energy reserves are unfairly discounted by many diagnosticians.
Defoliation in the middle of the growing season results in the tree expending energy reserves, and can result in a net reduction of produced energy by the tree.
Caterpillars are easily identified as large adults, but in younger stages they can go undetected by the casual observer since the small caterpillars are green and easily blend into the foliage.
From http://www.ugaurbanag.com/content/orange-striped-oak-worm:
The full grown caterpillars are 1 1/2 to 2 inches long, black in color with several narrow, yellow-orange longitudinal lines. Behind the head are a pair of stiff, blunt spines, about the thickness of the body. The remaining segments of the body have pairs of smaller spines. The orange-striped oakworm caterpillar generally appears in Georgia in August, September and sometimes as late as October. They defoliate sections of various oak species and sometimes completely defoliate smaller trees. This insect is extremely troublesome when present in oaks over patios, driveways, sidewalks, etc. because of the large amount of excrement from the insect's body.