The tree should be fine and nothing needs to be done; the later in the growing season a tree loses foliage (whether to insect feeding, disease, or storm damage), the less impact it has on long-term tree health, because it was photosynthesizing and storing energy all season prior to that point. Unless an unrelated issue has caused root stress or trunk damage, we expect the tree will leaf-out normally in spring. If it does produce a few new leaves between now and autumn, that's fine, but generally trees and shrubs stop new growth by midsummer and focus instead on both maturing buds for next year's foliage (which will sit dormant on the twigs between now and spring) and hardening-off tissues for overwintering.

The caterpillar pictured is a White-marked Tussock Moth larva, and they can feed on a variety of host plant species. However, they are rarely abundant enough to defoliate a plant, so we suspect a different caterpillar (likely Orange-striped Oakworm, which is also active this time of year and which feeds in large groups) was responsible. Both species are native insects, and are eaten by many predators, including wasps and other insects as well as birds, and they can also fall victim to insect-specific infections like certain viruses and fungi.

Caterpillars often wander off of their host plant when they are done feeding and ready to pupate (form a cocoon in this case). Therefore, applying a pesticide (even if it were warranted, which it isn't in this situation) wouldn't provide much benefit since there is not enough foliage left to treat and the insects are no longer present.

Miri