Ash flower gall mite (Aceria fraxiniflora) is an eriophyid mite native to North America that creates unsightly galls on ash (Fraxinus spp.) tree flowers. While the mite itself is microscopic, the galls they create on ash flowers are green, lumpy plant growths that turn dark brown in late summer. Unless there is a heavy infestation, these galls, which are about ½ to 1 inch in diameter, rarely affect tree health.

Green and brown galls made by female ash flower gall mites on male ash flowers.

Ryan Armbrust, Kansas Forest Service, Bugwood.org

×

Green and brown galls made by female ash flower gall mites on male ash flowers.

Ryan Armbrust, Kansas Forest Service, Bugwood.org

Print Image

Dried ash flower galls often remain on the tree even after all the leaves have fallen.

A. Steven Munson, USDA Forest Service, Bugwood.org

×

Dried ash flower galls often remain on the tree even after all the leaves have fallen.

A. Steven Munson, USDA Forest Service, Bugwood.org

Print Image

Ash flower gall mites are aesthetic pests of all ash species. Ash trees are dioecious, meaning that male and female flowers are produced on separate trees. Ash flower gall mites attack male tree flowers.

Female mites feed by sucking sap from flowers, causing the flower’s tissue to swell and galls to form, distorting flowers and leaves (Figure 1). However, the galls do not impact the male ash tree’s ability to generate pollen.

Ash flower galls are initially yellow-green but turn dark brown in late summer as they dry (Figure 2). Many galls remain on the ash tree long after mites have left and leaves have fallen from the tree. In some cases, ash flower galls are visible until the following spring.

Although ash flower galls are unattractive, they do not usually impact an ash tree’s long-term health. In rare situations, severe infestations may cause minor tree stress or the weight of the gall tissue can strain tree branches.

Female ash flower gall mites spend the winter in bark crevices under bark scales. In the spring, females emerge from bark crevices to feed on male ash flowers causing galls to form. Female ash mites lay eggs in the developing galls. Once the eggs hatch, the young mites feed inside the gall tissue. There are several generations during the spring and summer. After mating in the fall, female ash flower gall mites migrate to bark crevices under bud scales to spend the winter.

Ash flower gall mites are microscopic (less than 0.5 mm), cigar-shaped eriophyid mites. They are usually identified by the characteristics of the galls they produce.

Since ash flower galls are predominantly a cosmetic issue, management is not typically necessary or recommended. However, management may be desired if mite infestations are high or galls are an aesthetic concern. Galls can be pruned off the tree in late winter or early spring, before mites become active. Consider replacing highly susceptible trees with female ash trees or an alternate species altogether. This may be an option also appealing given ash trees are also susceptible to the emerald ash borer, an invasive insect that kills North American ash.

If chemical treatments are pursued, timing is critical. Look for initial signs of green in the opening flower buds. Apply horticultural oils or insecticides (avermectin, carbamate, or carbaryl) when blossoms first begin to form to reduce the number of galls. Once a gall starts growing, it is too late to treat the ash tree, as mites are protected within the gall.

Davis, R., and Beddes, T. 2011. Eriophyid Mites.

Hahn, J., and Ascerno, M. 2019. Insect and mite galls.

Hanson, T., and Walker, E.B. n.d. Ash Flower Gall Mite Aceria fraxiniflora Felt.

Kerzicnik, L. n.d. Ash Flower Gall Mite.

Pellitteri, P. 2012. Ash Flower Gall.

The Morton Arboretum. n.d. Ash Flower Galls.

Utah State University Extension. N.d. Ash Flower Gall and Cottonwood Catkingall Mites.