Ash flower gall: what extension services say

This page describes what the named extension publications say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat, prune or leave it. Pesticide advice in a source is mentioned only in general terms, and the guide on who may apply pesticides covers licensing.

What it looks like

North Carolina State Extension's ash flower gall mite publication (Abbey Stewart and Kelly Oten) calls the galls green, lumpy plant growths, yellow-green at first, that turn dark brown in late summer as they dry, and gives their diameter as 1/2 to 1 inch. The University of Minnesota Extension diagnostic page describes them as broccoli-like and green in summer, dark brown or black in winter, and as distorted clumps of male flowers. It says they first form in April or May and stay on the tree through summer and winter. Nebraska Extension in Lancaster County (Sarah Browning, May 14, 2024) describes clusters of black structures attached to the tips of branches.

What causes it

All three sources name an eriophyid mite feeding in the male flower clusters. North Carolina State names the ash flower gall mite, Aceria fraxiniflora, a native North American species, and says it attacks only male trees because ash trees are dioecious, with male and female flowers on separate trees. It says the mite is microscopic, under 0.5 mm, and cigar-shaped, and is usually identified from the galls it makes rather than on sight.

The same source gives the cycle: females spend the winter in bark crevices under bark scales, come out in spring to feed on male flowers so that the flower tissue swells into galls, and lay eggs in the developing galls. The young feed inside the gall tissue, there are several generations through spring and summer, and after mating in fall the females move to crevices under bud scales for the winter. The University of Minnesota's insect and mite galls publication (Wawrzynski, Hahn and Ascerno) says the mites begin feeding on male ash in spring before the flower buds fully expand, and that their tiny size lets them enter the flower structure before it looks open.

How long the galls stay

Source	What it says
University of Minnesota, insect and mite galls publication	Galls are green at first, turn brown and become more obvious in August and September, and can remain on the tree for up to two years.
North Carolina State Extension	Many galls stay on the tree long after the mites have left and the leaves have fallen, and in some cases are visible until the following spring.
Nebraska Extension	The black clusters persist until the year after they develop, then fall from the tree.

Whether the sources call it harmful

North Carolina State says the galls are unattractive but do not usually affect an ash tree's long-term health, do not stop a male tree from producing pollen, and in rare situations severe infestations may cause minor tree stress or the weight of the gall tissue may strain branches. It calls the mite an aesthetic pest of all ash species. Nebraska says the galls are unsightly but do not affect tree health. The University of Minnesota's general publication says galls seldom threaten plant health and control is generally not suggested.

What the sources say about management

North Carolina State says management is not typically necessary or recommended because this is predominantly a cosmetic issue. It lists pruning off galls before the mites become active, replacing highly susceptible trees with female ash or another species, and chemical treatment timed when blossoms first begin to form, and adds that once a gall starts to grow it is too late because the mites are protected inside. The University of Minnesota says control is difficult because the mites get into the flower before it opens, that the timing falls in mid to late April before most people think about pest management, and that this often results in poor control. North Carolina State also notes that ash trees are exposed to the emerald ash borer, which is covered on the emerald ash borer page.

Look-alikes the sources name

Virginia Cooperative Extension (publication ENTO-146NP, June 2, 2022) names the ash midrib gall, made by a midge on the midveins of ash leaves, so it sits on the leaf rather than the flower. Other insect and mite galls are on the tree galls page, and growths with other causes are on the odd tree growth page.