Lately many of the tress in the Village have been looking “distressed.” Many are infested with fall webworm (Hypanthia cunea) caterpillars. The preferred hosts of these caterpillars include hickory, elm, ash, poplar, willow, oak, apple and other deciduous and fruit trees.
We are now seeing the second -generation caterpillars, present from August through October. The larvae spin unsightly light grey webs, starting at the tips of the branches and slowly extending down the branch towards the trunk, feeding on the foliage enclosed by their web. The dried webs will probably be seen, hanging on terminal braches, into the winter. Although the web enlarges as it grows, the webworms inside only feed inside the web, and rarely consume enough terminal growth to affect the tree according to the University of Maryland Extension. Damage to the host plant is primarily aesthetic, as leaves are usually eaten late in the season and is not usually a threat to the health of the tree.
To get rid of the webbed branches prune them to remove the tents in the trees. Horticultural oil usually containing the bacterial pesticide Bacillus thuringiensis (‘Bt’) or insecticidal soap is recommended to control young larvae. Some natural predators are social wasps, birds, predatory stink bugs and parasitic flies and wasps.
Although the webs are considered unsightly, the fall webworm is not considered a serious pest by the Maryland Department of Agriculture and so chemical intervention a rare requirement. There are no management programs for county street trees at this time.