Again this year a very small number of Iowans have been treated to a most unusual, pre-Halloween phenomenon: ash trees shrouded in webbing. The webbing is created by the ash spider mite (Tetranychus homorus) on ash trees in the fall of the year (mid-September to mid-October). The tree trunks and branches are covered with silver webbing to the point that the trees look like they have been "wrapped with cellophane." Webbing may completely cover the trunks of ash trees as well as other species nearby. Some trees may have only scattered patches of webbing. Within the webs are thousands (if not millions) of tiny orange female spider mites.
Shrouds by the ash spider mite have been reported only in northern and eastern Iowa and only in 4 of the past 12 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 ash spider mites spend the summer on the tree foliage and move down the trunk in September to relocate into protected hiding spots to spend 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.
Iowa counties reporting ash spider mite webbing:
- 1996 - Emmett, Marshall, Butler, Floyd, Hamilton
- 1999 - Grundy, Wright, Tama, Black Hawk, Carroll and Winnebago
- 2002 - Butler, Mitchell, Buchanan, Black Hawk, Wright, Winneshiek
- 2008 - Winneshiek, Howard, Linn, Benton, Jefferson
Ash spider mite webbing on tree trunk. Photo by Neric Smith.
Ash spider mites and webbing. Photo by Neric Smith.