Making a correct tree identification is the critical first step in correctly diagnosing a tree problem. Of course, sometimes the diagnosis identifies the tree. Insect galls are often so host-specific, they can give you six-legs up on tree identification.
Elm Sack Galls (= Pouch Galls) are produced by a non-native European aphid, Tetraneura ulmi, that was introduced into the U.S. in the 1890s. If you find an elm in an Ohio woodland that's festooned by these odd looking galls, it's highly likely the gall-adorned tree is a red elm (= slippery elm) (Ulmus rubra); it's almost a sure-fire identification.
Elm Cockscomb Galls are also helpful, but a little less trustworthy. They are produced by another aphid, Colopha ulmicola. Although I find these galls most often on red elm, I've also occasionally found them on American elms (U. americana). On the other hand, these galls never occur on Japanese Zelkova (Zelkova serrata), hornbeams (Carpinus spp.), birches (Betula spp.), or other trees that I used to confuse with elms before I discovered the gall tree identification method.
On a side note, both the sack galls and cockscomb galls release winged aphids that fly to grass plants where they produce offspring that suck juices from grass roots. Indeed, an alternate common name for elm sack gall aphids is elm-grass root aphids. Their offspring fly back to elms in the fall to spend the winter in bark crevices.
Ever struggle with identifying baldcypress (Taxodium distichum) versus dawn redwood (Metasequoia glyptostroboides)? Struggle no more! The Cypress Twig Gall Midge Fly, Taxodiomyia cupressiananassa, will only produce their spongy white galls on Baldcypress.
You will never again need to remember such trifling trivia like the leaves and needles on dawn redwood are opposite while bald cypress leaves and needles are alternate. On the other hand, this information could be helpful in case a baldcypress suffers reduced ornamental value because it lacks midge galls.
Be honest. How often have you used Hackberry Nipple Galls produced by the gnat-like psyllid, Pachypsylla celtidismamma, to make a slam-dunk identification of common hackberry (Celtis occidentalis)? Hackberry Disc Galls (= Button Galls) produced by another psyllid, P. celtidisumbilicus are an equally dependable tree ID aid. In fact, there is a whole group of not-so-silly psyllids, known as the "celtidismamma complex," whose gall-making handiwork is invaluable for identifying hackberry.
Gall-making mites can also serve as helpful tree ID partners. Is it a black walnut (Juglans nigra), or a dreaded tree of heaven (Ailanthus altissima)? No need to risk stinky fingers if the petioles are embellished by fuzzy Walnut Petiole Galls harboring the eriophyid mite, Aceria caulis. The same is true for Eriophyes brachytarsus which produce Black Walnut Hairy Leaflet Galls (a.k.a. Walnut Pouch Galls).
Is it a black cherry (Prunus serotina)? No need to count the number of dark red-brown bud scales (6) or decide if it has alternate simple leaves if the upper leaf surfaces are adorned with Cherry Spindle Galls produced by the "cherry-ophyid mite," Phytoptus cerasicrumena.
So, if you're out hiking in the woods this weekend and have trouble identifying a tree; look to the galls.