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How to identify it
- Galls are irregular, fringed, lumpy clusters about 1/4 inch to 1 inch across that start green and turn dark brown to black by late summer.[1][2]
- The dried galls often stay on the tree after the leaves fall, sometimes until the next spring, which is when most people notice them.[1][2]
- The mites are cigar-shaped, under 0.5 mm long, with only two pairs of legs, far too small to see without magnification.[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Other gall mites | Related eriophyid mites make bladder, spindle and felt-like galls on leaves; the ash flower gall mite galls only the flowers of male ash.[1] |
Where it lives
It is native to North America and attacks all ash species, but only the flowers of male trees.[1][2]
Biology and life cycle
Adult females spend the winter in bark crevices and under bud scales, then move to the developing male flower buds in spring to feed and lay eggs.[1][2]
Their feeding makes the flower tissue swell into galls around the mites, and several generations develop inside the galls through the summer.[1][2]
In the fall the last generation produces the females that will overwinter on the tree.[2][1]
Damage and signs
Health risk
It feeds only on ash flowers; NC State and Agriculture Canada mention no risk to people.[1][2]
Control
Prevention
Where galls are a recurring eyesore, consider replacing heavily affected trees with female ash or another species.[1][2]
Traps and physical control
Prune out and destroy newly formed galls in spring if you want to reduce them; there are no insecticides registered for this mite.[2]
Common questions
- What are the brown clumps hanging on my ash tree in winter?
- Most likely ash flower galls, the dried remains of male flowers deformed by a microscopic mite; they can hang on until spring.[1][2]
- Will ash flower galls kill my tree?
- No. They are mostly cosmetic and don't affect the tree's long-term health.[1][2]
Sources
- NC State Extension, “Ash Flower Gall Mite.” content.ces.ncsu.edu/ash-flower-gall-mites (accessed 2026-10-19)
- Agriculture and Agri-Food Canada, “Ash Flower Gall Mite (agroforestry diseases and pests).” agriculture.canada.ca/en/agricultural-production/crop-protection/diseases-and-pests-agroforestry/ash-flower-gall-mite (accessed 2026-10-19)
- iNaturalist, “Aceria fraxiniflora taxon page (order and family).” www.inaturalist.org/taxa/336035 (accessed 2026-10-06)