When your ash tree begins developing strange growths, avoid the instinct to become overly alarmed. While these growths, known as galls, are cosmetically less than desirable, they rarely harm ash trees. The growths are simply a symptom of an underlying infestation of mites which, though frustrating, fail to cause severe damage. However, for continued health and enjoyment of the visual beauty of your ash trees, gaining an understanding of the infestation and potential control methods will ensure a longer, happier life for your landscape.
Maintain healthy ash trees in your landscape as a method of protecting them against pest infestations and diseases. Additionally, your properly cared for trees will be more likely to overcome the effects of problems such as mites and galls when compared with unhealthy trees, particularly those experiencing stress or deficiencies. Care for your ash trees by planting them in areas exposed to full sunlight to partial shade. Though ash trees tolerate a wide variety of soil conditions including occasional drought and poorly drained soil, maintain rich, moist, well-drained soil for greatest health. Grow ash trees in U.S. Department of Agriculture plant hardiness zones 5a through 8a.
Inspect your ash trees closely. Though you will not be able to see the ash flower gall mites without the aid of a microscope, these pests display bodies reminiscent of the shape of a cigar or carrot with two pairs of legs. During the summer, you will notice the development of galls on male ash trees; mites feed on male flowers. Galls are abnormal growths that appear round or lumpy and typically display the same green color as the ash tree on which they develop. These swollen areas change to a brown color and vary in size measuring one-half to 1 inch in diameter.
As the mites feed on male flowers, the tree reacts with a hormonal response, resulting in the abnormal and excessive growth of its flowers into galls. Galls develop during the spring season, and mites lay their eggs within these tumors. Throughout the summer, eggs hatch, releasing several generations of mites that continue to feed on your ash tree. Galls are unsightly but do not pose much of a threat to the health of your tree. The only potential problem is a tree so covered with galls that they weigh down a branch; however, this issue is rare and easily avoided through pruning.
Since ash flower mites cause little to no harm to the health of your tree, you can choose to not control the infestation. However, if the galls bother you, killing the mites before they feed on your tree will prevent the formation of growths. You may apply an insecticide with the active ingredient carbaryl or bifenthrin in the spring, just as new leaves begin to open, though the use of chemicals is not recommended unless absolutely necessary. A natural alternative is horticultural oil or insecticidal soap. Contact a licensed professional for assistance. For control of galls, carefully prune them from your tree by the beginning of July.
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- University of Wisconsin Cooperative Extension: Ash Flower Gall Mite - Eriophyes Fraxiniflora
- University of Minnesota Department of Entomology: Ash Flower Gall Mite
- Michigan State University IPM: IPM Scouting in Woody Landscape Plants
- University of Wisconsin Cooperative Extension: Ash Flower Gall
- Iowa State University Extension and Outreach: Insect Galls on Trees and Shrubs
- University of Florida IFAS Extension: Fraxinus Excelsior: Common Ash