The first thing to do is determine is whether or not there is a tent, or web. That, along with time of year, will allow you to figure out what you're dealing with. Caterpillars are cyclic, which means there will be a bad year here and there, then natural predators chime in to help get the population back under control.
Gypsy moth caterpillars have 5 pairs of blue dots, followed by 6 pairs of red dots, down the back. They are hairy and do not form tents. In outbreak years when the cycle is heavy, these caterpillars can do extensive damage in one season.
Tents formed at the ends of branches in late summer or early fall are fall webworm. This caterpillar stays within the tent through all instar stages, feeds within the tent, and only leaves to overwinter and pupate in the soil.
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Forest tent caterpillars have 2 blue lateral stripes down the side of the body and white 'footprint' like markings down the center of the back. They do not form tents, contrary to the name's implication, and are commonly found on foliage or buildings through late spring and summer.
Found in the spring in large numbers, eastern tent caterpillars form a web at a branch union, and expand the tent as they grow. There is a solid white line down the back, outlined by yellow lines. The caterpillars will congregate in the nest during the day, leaving at night to feed on foliage in the canopy.
There are a few species of caterpillar that you may see on your trees. Proper identification is the first step towards effective pest control and luckily, it is fairly easy to tell the caterpillars apart.
Treatment costs are highly variable depending on the situation, and may not be possible/merited. Below are photos to help you identify what is in your yard, and from there we can help you figure out if a treatment should be applied and advise approximate costs.