The leaves of native elms can look a bit bedraggled at this time of the year owing to the rise of two types of aphid galls: the pouch-like elm sack galls and the descriptively named elm cockscomb galls. Fortunately, neither of these galls produce significant injury to the overall health of their elm tree host. Unfortunately, these odd-looking plant structures can spoil the aesthetics of their deep green elm leaf platforms.
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.
However, the galls may also be found on American elm (Ulmus americana). While I have yet to find a rampant population on American elms that rival those commonly found on red elm, I've noticed over the past few years that these galls are becoming increasingly common on American elms planted in Ohio landscapes.
Elms are the primary host for the elm sack gall aphid. However, grasses (Poaceae) serve as a secondary host with the aphids feeding on the roots. This gives rise to the alternate common name of "elm-grass root aphid."
The winged aphids that emerge from elm galls in the summer fly to various grasses where they colonize the roots. In the fall, winged aphids emerge from grass to make a return migration to elm where they produce immatures that feed on the bark, and mature to apterous males and females.
Elm cockscomb galls are produced by another aphid, Colopha ulmicola that also alternates between elms and grasses. Although I find these galls most often on red elm, I've also occasionally found them on American elms.
Early cockscomb galls are tubular-shaped, almost worm-like, and light green. As they mature, the galls will take-on the appearance of their descriptive common name: they look like bright red chicken cockscombs rising up from elm leaves. It's a strange sight.
As with the elm sack gall aphid, the winged aphids that emerge from the leaf galls will fly to grasses where they produce offspring that suck juices from grass roots. Winged aphids that develop on the grass roots fly back to elms in the fall where their offspring spend the winter in bark crevices.