What are those Webs in the Trees?

Fall webworm is all around on Long Island now.  Our Diagnostic Lab is getting many complaints about a wide variety of trees and shrubs with lots of webbing, defoliation and browning leaves.

According to Sandra Vultaggio, Horticulturist in our CCE Suffolk Diagnostic Lab, this pest has been particularly successful this year in part due to the high humidity we’ve experienced. Since this is a late-season pest it does not tend to affect the health of the tree. It is more of an aesthetic issue when the brown leaves and webbing occur. For this reason we do not often recommend pesticides. Once the caterpillars are finished feeding, which takes roughly 6 weeks, they will fall to the ground to pupate over winter. It may be wise to do a thorough fall cleanup of leaves and debris around the trees this year.

Dan Gilrein, Extension Entomologist adds that some herbaceous plants are affected as well. Some may confuse this with gypsy moth, which doesn’t produce webbing and is not active this time of year, or (eastern) tent caterpillar, which is active in spring mainly on cherry and apple, ornamental varieties of these, and some related plants. Fall webworm is a native insect and its populations go in cycles. It’s wide host range includes over 400 plants. This particular ‘outbreak’ is the largest he has seen, though similar (short-lived) population explosions have been observed elsewhere. Fall webworm levels were high in parts of the Adirondacks last year, for example, but have since collapsed for the most part. Dan is not sure know why these population swings occur, but they probably have to do with direct and/or indirect impacts of environmental conditions on  the insect, its natural enemies, and possibly its hosts.

While the webbing and damage are very ostensible, the actual harm to the plants is probably much less.  At this time of year the foliage has done most of its ‘work’ and will be declining soon. There might be some concern for plants that are being heavily defoliated, were in poor condition, or just recently planted, but generally plants should grow normally next year. The good news is this insect has many natural enemies (one author refers to it as a ‘parasitoid hotel’), so the numbers are expected to be much less next year. Ways to deal with this are:

  • Remove webbing and caterpillars by hand which will improve the appearance immediately.
  • Prune out infested branches. This is the last alternative, as it can damage the plant, spoil the appearance, and open wounds.
  • Contact a consulting arborist or landscape professional for assistance, who can assess and handle the job.

Photo courtesy of Master Gardener Holly Sisti

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