This is the time of year that folks get concerned about defoliation of their elm trees. Usually the culprit is a small yellow beetle called, appropriately enough, the Elm Leaf Beetle. It eats elm leaves.
The adults cause small shot holes in the leaves, but the larvae will turn a leaf into skeletons. That's what we're seeing now in late July and early August. The larvae have big appetites!
This is the second generation of the pest this year. The first happened in June. Soon the larvae will crawl down the tree to pupate and repeat the cycle once again. Aberdeen and points south will probably have 3 generations this year. Bishop and Big Pine may also have 3 if the weather stays warm and we have a long autumn, but usually have 2 generations since the growing season is shorter.
There isn't much point in spraying the tree canopy now or using systemic insecticides this late in the season. If you notice the larvae crawling down the tree, it is probably worthwhile to do a band spray on the trunk with an approved insecticide. This is about when that happens on average in Independence and Lone Pine. Big Pine and Bishop are a week or two behind. Since the heat started early, we may be sooner than average this year. The only way to know is to inspect your trees often. Temperature drives development of this pest.
If you notice an accumulation of pupae at the base of the tree, vacuum or sweep them up right away.
Largely this is a pest of humans more than elms if your trees are otherwise healthy. They aren't doing the tree any good, but most of the problem is the mess and lack of foliage in landscapes. Keep that in mind if you decide to treat. There are dozens of elms in Owens Valley that get these every year and never get treated, yet are still alive and well.
You can find a lot more information about these insects on the Elm Leaf Beetle page at the UC IPM program website. It includes information that can help you to decide whether to attempt control of them.