Pellets beneath oak tree
What are these pellets which are beneath an oak tree? They seem to expand with moisture and leave stains on our deck. Several leaves on the tree looked as though something was eating them and all that was left were the veins. There are some dead caterpillars on the deck. This occurred last year as well (we can’t recall if it happened in prior years).
This is all connected.
The pellets are the excrement of the caterpillars, which are orange-striped oakworms. These caterpillars are common in the late summer. At that point, the feeding they do on the oaks doesn't harm the oaks. The oaks have had plenty of time to carry on photosynthesis and store energy for leafing out next year.
The caterpillars get diseases, just like all creatures do. In particularly wet years, they tend to get a fungal disease that kills them, and that's why you're seeing dead ones on your deck.