Elm cockscomb and elm sack galls: what extension services say

This page describes what the named university pages say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat or leave it.

What the two galls look like

Ohio State University Extension (Joe Boggs, May 28, 2020) says early cockscomb galls are tubular, almost worm-like and light green, and as they mature look like bright red chicken cockscombs rising from the elm leaf. The University of Wisconsin-Milwaukee Field Station (Kate Redmond, August 20, 2025) also describes them as tubular and wormlike, starting green, turning red as summer goes on and ending up brown, and notes that they sit in plain sight on the leaf surface, with several sometimes on one leaf. Virginia Tech (Eric Day and Theresa Dellinger, June 2, 2022) says one to several may appear on the upper leaf surface of American and slippery elm, and that the name comes from the likeness to a rooster's convoluted comb.

The sack gall, which Virginia Tech also calls the slippery elm pouch gall, is an elongated pouch on the upper leaf surface made by a different aphid, Tetraneura ulmi. Virginia Tech says the galls appear in May, are filled with female aphids and their young, are often reddish, and can be found in large numbers on a leaf in heavy infestations. Ohio State says this aphid is a non-native European species introduced into the United States in the 1890s.

Which elms

Ohio State says that in an Ohio woodland a tree covered in sack galls is very likely a red elm (slippery elm, Ulmus rubra), though the galls also occur on American elm, and that it has seen them become more common on American elms planted in Ohio landscapes. It finds cockscomb galls most often on red elm, and only occasionally on American elm. The Wisconsin page says cockscomb galls turn up on American and red elm in an odd, patchy distribution.

Life cycle, with grass as the second host

The Wisconsin page gives the fullest account of the cockscomb aphid, Colopha ulmicola. A single egg is hidden in elm bark over winter, though one source it cites says between bud scales. As new elm leaves grow in spring, the newly hatched nymph feeds and stimulates the gall. All the aphids of this generation are female; the first becomes a stem mother and produces hundreds of young inside the gall, all feeding and making honeydew. In early summer this generation, which is winged, leaves through a slit on the underside of the leaf and goes to feed on grass roots. In fall a winged generation of males and females emerges and flies back to elm, mates, and the females lay the overwintering eggs. Ohio State and Virginia Tech agree that the aphid feeds on grass roots after leaving the leaf gall, and Ohio State says the sack gall aphid does the same, which gives it the other common name elm-grass root aphid. Ohio State describes that aphid's fall generation as feeding on the bark, then maturing into wingless males and females.

Whether the sources call them harmful

Ohio State says neither gall produces significant injury to the overall health of the elm, though both can spoil the look of the leaves. The Wisconsin page says that other than being unsightly or interesting, they typically cause no damage to the tree. Virginia Tech says most of the aphid, adelgid, phylloxeran, psyllid and midge galls it covers are harmless to trees and plants, and that a few are pests. None of the three pages gives treatment steps for these two galls.

The wider group is on the tree galls page, and elm pests that regulators track are on the what attacks elm page.

Elm cockscomb and elm sack galls: what extension services say

What the two galls look like

Which elms

Life cycle, with grass as the second host

Whether the sources call them harmful

Sources