We have received few (very few) reports of a peculiar phenomenon involving woodland walnut trees in Iowa. In each case, the walnut tree leaves are gone, and the tree trunks are shrouded with fine webbing. See Images 1 and 2.
This mysterious defoliation and webbing were reported from Wisconsin and Minnesota in 2020, and again in 2021. (Forest Data Network) Reports in northeast and southwest Iowa occurred in 2021 and again in 2022 (Tivon Feeley, personal communication).
It turns out the defoliation and webbing are caused by a native Tortricid moth, Gretchena amatana. (ForestryNews) Moths in the family Tortricidae are called leafroller moths because of the feeding behavior of some of the caterpillars. Leafrollers roll and tie the leaf into a tube with silk webbing, providing a protective enclosure for the caterpillar while it feeds on the leaf. Other members of this large and diverse family are not leaf rollers but instead bore into fruit (codling moth), roots, or seeds.
Pictures of the moths are online at Insects of Iowa. A closeup photo of the caterpillar and the tree trunk webbing is shown below.
The shrouded tree trunk and appearance of the webbing by Gretchena amatana are striking. So far, the defoliation does not appear to have significant consequences to the affected trees. To date, there have been no indications that applied controls are warranted. Otherwise-healthy trees can tolerate defoliation especially when the damage occurs later in the season, and black walnut mortality is not expected.
The few, isolated instances of defoliation by Gretchena amatana in Wisconsin, Minnesota, and Iowa present an anomaly. Why haven’t we noticed walnut tree webbing previously? Will defoliation and walnut webbing become more widespread or remain isolated?
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