Pin oak problems
I noticed a couple weeks ago that some of the leaves on my young pin oak tree were turning brown. The tree was just planted last fall and is approximately 9 feet tall, with not much growth occurring this year, as far as I can tell. When I looked at the cluster of brown leaves more closely this afternoon, I found the leaves were being eaten by dozens of tiny caterpillars. The very top of the tree had been eaten, but no caterpillars were on that area. The ones I found were a little lower, but still near the top. I used a strong stream of water from a garden hose to blast the caterpillars off. I left the damaged leaves and twigs in place, hoping new leaves would grow next spring. I was also hoping that the caterpillars would not crawl back to the tree after being blasted off. What type of caterpillars are they (see attached picture), and do they pose a real threat to the health of the tree? It has been a tough year for all of my new trees, with the high heat and sporadic rain, so I was wondering if the stressed condition makes them more attractive to this pest, or if they would attack it regardless of its health. A second issue I found was a single green caterpillar that had made a home for itself by gluing two leaves together and living in between them (see attached picture). What type of caterpillar is this, and is it related at all to the others I found, or is it a separate issue? Is this caterpillar harmful to the tree? It is too late for that individual because I already disposed of it, but it would be nice to know for future encounters. Thanks for the help!
Washington County Maryland
These are a native caterpillar known as the yellownecked caterpillar. (The way they hold up their heads and back ends is very distinctive.) They are super gregarious when young and great, important, food for birds (especially baby birds), so spraying them off and letting the birds dispatch them is a good approach. You can also manually pull them off and toss in a wooded area.
On an established tree, they are not a problem. On a young one, it is good to minimize the leaf consumption as you did--however--this late in the growing season, all trees have already had plenty of time to carry on photosynthesis and build up reserves for winter. So, losing a few leaves is not a big deal.
Also, trees grow more leaves than they actually need, so losing some, again, not a big deal except for a young tree that doesn't have many leaves total. In that case, again, removing the caterpillars is the way to go.
The third photo is a leaftier insect. Does not do significant damage and no steps called for.
The most important thing for your new tree is to water during droughty spells. This is critical for 2 year, as it gets established. Supplement rainfall when there is less than about 1" a week from spring through fall. Fall droughts can be killers, and we often don't notice them. Don't let your plants go into winter in dry soil.
Thank you very much for the advice!