Iowa Insect Information Notes

Ash Spider Mite Webbing on Trees

Ash Spider Mite Webbing on Trees

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 circumstances the webbing is so pronounced on the majority of the trees that it appears that they are shrouded in a fog. 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.

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.

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.

Updated 09/29/2008 - 3:42pm