eGarden © 2008 Ed Mues

September 10, 2008

Unsightly, but Not Very Serious

Driving home from work the other afternoon, I stopped to take a picture of a tree branch that had a web spread out all over it to its very tip. The leaves that should have been there were dead and mostly gone, eaten, I surmised, by a caterpillar.

The ragged remains as well as the caterpillar droppings dotted the dirty looking webby structure making it unattractive against the lush green that remained.

This is the outward manifestation of the Fall Webworm, Hyphantria cunea, often confused with the Eastern Tent Caterpillar and also a native to the U. S. The date was August 15th and it was right on time as for its arrival. The larvae of this moth feed on over 85 species of trees including pecan, fruit trees, walnut, elm, maple, hickory and almost any broad leaf shade trees.

garden

This fall webworm nest is only seven feet above
ground in a relatively young cherry tree.

Because this pest appears late in the season, it is not regarded as a serious threat to most trees, though in a very long summer a second generation can result in more defoliation. The theory is that by this time of year most of the work the tree needs to fulfill has already been accomplished as to photosynthesis and food production and storage for the winter. What little damage is done is of little consequence as days continue to shorten, and temperatures begin to cool.

The caterpillars remain inside the web and as food runs out, new leaves are encased. As temperatures drop their activity will begin to slow, and the amount of damage they do will be mitigated by the time colder weather arrives. Like so many caterpillar infestations, there are cyclical occurrences with periodic population explosions and then declines. The other forces of nature generally control outbreaks.

The biggest offense of the fall webworm is to the human aesthetic sensibility. The very presence of a dirty, silken fall webworm platform-like nest is disfiguring. Where we live, we should be grateful that there is usually only one generation a year.

A note to future sightings: Trees that are most vulnerable are those deprived of light, air drainage, or stressed from some other cause such as drought, deicing salt at the roots, damage to the bark from lawn mowers or weed whackers, or excessive lime in lawn areas. A stressed tree is most likely to be attacked.

Mature caterpillars overwinter as cocoons on the ground in leaf litter, in the surface soil, or on trees in cracks, branch crotches or crevices. The adult moths begin emerging in mid-June and lay their eggs on the undersides of leaves. The eggs hatch in a week or so, and the small caterpillars begin to feed and simultaneously spin the silken web over the foliage as they grow.

I mentioned the forces of nature above. Many infestations of the fall webworm are controlled by a variety of insect predators, parasitic wasps, and birds. I'm not an advocate of turning to chemical sprays to control every damaging insect outbreak. In this case, if the web-like structure can be reached, simply prune it out of the tree and destroy it. If a tree has multiple nests and some are too high to get to and remove, a powerful stream from a garden hose will usually break up the nest and open it to a host of beneficial predators. Never try to burn a nest while it is in a tree. Much damage to the tree can result.

If a principal landscape tree is severely infested, say it's a prize ash, contact an arborist or a Certified Pesticide Applicator. A few different strategies for control are available, but remember, most of the tree's important work of generating food stores for winter is complete.

Gardening is any way that humans and nature come together with the intent of creating beauty.
- Tina James

From The Garden of Ed. Submitted for publication in The Towne Crier on September 10, 2008

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