Ash Spider Mite

Description ash spider mites 

The ash spider mite (Tetranychus homorus) can produce a peculiar sight on ash trees in the fall of the year (mid-September). The trunks of ash trees may be covered with silver webbing giving the appearance that the trees have been “wrapped with cellophane.” Under some circumst ances, the webbing is so pronounced on the majority of the trees that it appears that they are shrouded in a fog.

Ash spider mites and webbing.  Photo by Neric Smith.
Ash spider mites and webbing.  
Photo by Neric Smith.

Life cycle ash spider mites

Upon close examination, the webs are found to contain thousands (if not millions) of tiny orange “bugs” that are the female spider mites. Webbing may completely cover the trunks of ash trees as well as other species nearby. Some trees may have only scattered patches of webbing.

Shrouds by the ash spider mite have been reported only in northeastern and north central Iowa and only in 4 of the past 10 years. This phenomenon had not been observed in the state prior to 1996 to the best of our knowledge. The same experience has been reported in Ohio and in a few other places. The conditions leading up to shroud formations are unknown.

Damage caused by ash spider mites

Very little is known about this mite. The following represents our best synopsis of what is known. The ash spider mites spend the summer on the tree foliage and move down the trunk in September to relocate into protected hiding spots for the winter. Although the spider mites feed and develop on the foliage of ash trees there is no apparent damage to the leaves.

Ash spider mites and webbing.  Photo by Neric Smith.
Ash spider mites and webbing.  
Photo by Neric Smith.

Management of ash spider mites

No control is needed. Webbing on the tree trunks is harmless and eventually weathers away during late fall. One observer has reported that the webbing was nearly eliminated by an overnight rainfall of 0.5 inch. In general ash spider mite sightings have been short-lived curiosities.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.  

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