Ash Flowergall Mites are the Least of Our Worries

Southwest Yard & Garden by Dr. Marisa Thompson

with insights from Dr. Carol Sutherland

Ash tree leaves from a sample submitted to the NMSU Plant Diagnostic Clinic in September 2019 look green and healthy. The rough, round, brown bits are galls formed by the ash flowergall mite, but they do not harm the tree itself. Photo credit NMSU Plant Diagnostic Clinic.

Question: Is there a systemic insecticide that can be used to control ash flowergall mites?

-          Question submitted by a Bernalillo County Extension Agent

Answer: If you have an ash tree, you’ve probably seen the evidence of these ash flowergall mites (Eriophyes fraxiniflora). In case you’ve never noticed them before, the ash flowergall mites (aka eriophyid mites) cause deformation of male flowers, which develop into galls, out on the tips of your ash tree branches. In the summer, they’re mostly pale green and are barely noticeable. As the season progresses, they turn an increasingly darker brown. In the fall and winter, they tend to fall from the tree, littering the ground with surprisingly hard, round, popcorn-like pellets that are extremely painful when stepped on with bare feet. Alternatively, these tumor-like galls can stay on the tree for multiple seasons but will eventually drop. They’re not usually a problem except when they cover otherwise walkable surfaces, like driveways and stepping stones.

Galls on ash tree formed by ash flowergall mites are more of an interesting nuisance than a real threat, as they do not harm the tree at all. Photo credit Judy Nickell.

My understanding is that these galls and the mites that form them are just a minor, messy nuisance that only cause aesthetic damage to the tree. NMSU Extension Entomologist and NMDA’s State Entomologist Dr. Carol Sutherland explained, “With these ultra-minute creatures, there’s no control or management strategy. Understand what they are, what they do, and get busy on other projects. They’re not hurting the tree. The tree keeps the little annoyances busy and housed within the gall tissue.”  Sutherland did warn, “If twigs are dying back and snapping off, look for girdling marks made by ash twig beetles. They’re doing a number on lots of ash trees all around the state.”

An ash twig beetle caused dieback on this branch in Belen. At the branching node, the beetle chewed a groove all the way around the branch, girdling it. Photo credit M. Thompson.

This is a great time to restate our collective concern in the tree community about the looming emerald ash borer invasion. Sutherland reported that this dreaded insect, called EAB for short, has already been detected in northern suburbs of Denver and the western edge of Ft. Worth. The EAB is a pretty, narrow-bodied, metallic green, wood-boring beetle that may cause minor foliar damage while in its adult stage. But the larvae feed on the inner bark (vascular system) of ash trees and are so aggressive that even healthy ash trees can die within two years. Another scenario is that the EAB larvae kill an ash tree slowly, taking up to four years before symptoms are even visible. For more info on the EAB, visit the Emerald Ash Borer Information Network (http://www.emeraldashborer.info/) and check out my column on the EAB by clicking HERE.

Adult emerald ash borers are shorter than a penny (Howard Russell, Michigan State University, Bugwood.org).

D-shaped holes where adult emerald ash borers exited the trunk Please note, these “D-shaped holes” in the bark of infected trees are not necessarily oriented like the letter D. They can be sideways or upside down, depending on how the adult emerged from these the exit points. (Daniel Herms, Ohio State University, Bugwood.org).

And that’s not all for our poor ash trees, which Sutherland says are a “virtual magnet for insect pests.” Other common problems include ash whitefly, lilac borers, and abiotic factors such as drought stress and sunscald damage on the trunks. More on these and other “ash issues” can be found HERE and HERE.

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