The bizarre-looking Ash Inflorescence (flower) Galls produced by the eriophyid mite, Aceria fraxiniflora (Syn. Eriophyes fraxiniflora) (family Eriophyidae) are gracing ash trees in Ohio. The growths consist of variably sized clusters of distorted flower parts.
Small, clubby, inconspicuous galls may be hidden by this season’s leaves. However, larger galls consisting of a profusion of gnarled, twisted flower tissue look like broccoli florets growing just beneath this season's leaves.
Currently, the galls are light green, but they will turn brown to black later in the season. Old, blackened galls remain attached to the twigs and are evident for a number of years.
Anecdotally, treatments to protect ash trees against Emerald Ash Borer (Agrilus planipennis) appear to have no impact on mite populations. I’ve observed heavy galling on trees that I know are receiving treatments as well as on trees in wooded areas that never received treatment. However, I could not find scientifically produced efficacy data specific to the gall mites.
A study published in 1990 showed that the number of mite galls on ash trees has no impact on root starch storage or tree stem growth. Although the galls may reduce the aesthetic appeal of heavily infested trees, they cause no harm to the health of the tree since they only affect the flower parts.
On a final note, as with many plant galls produced by arthropods, the ash flower galls are commonly cyclical from year to year. The galls appear to be particularly common in Ohio this season. However, we may see far fewer next year.
Wawrzynski, R.P. and M.E. Ascerno, 1990. Effects of ash flower induced by Eriophyes fraxiniflora on tree vitality. J Arbor, 16, pp.182-185