Hi Kristen,

Those are most likely pine webworms, a type of tent-making caterpillar. Their damage rarely causes severe damage or mortality in mature trees such as yours. So it doesn't require management unless you take issue with the aesthetics of it.

The most effective management strategy for tent-making caterpillars is to cut out the foliage their "tents" are attached to. That would remove the bulk of caterpillars, as well as their protection. Remaining caterpillars would be vulnerable to attack from their natural enemies, of which there are many. 

Alternatively, you could spray them with the biological agent Bacillus thuringiensis. This is a bacteria that only attacks butterfly/moth larvae. If you go this route, make sure to read the instructions on the label carefully and follow them exactly. The label is the law. 

Let me know if you have further questions.