Fall Webworm vs. Bagworm
We’ve received a high volume of calls over the last couple of weeks about “bagworms” in client’s trees. In central Ohio, true bagworm feeds predominantly on evergreens - arborvitae, spruce, and junipers although some deciduous trees can be hosts as well. Generally, this feeding occurs late Spring through mid-Summer and by mid-August they have stopped feeding to go pupate and become a moth. So I initially was confused about this late population of “bagworm” that had taken central Ohio by surprise and was making my appointment schedule grow faster than kudzu.
After visiting with a few customers, I realized the real culprit of concern was actually, Fall Webworm - not Bagworm. It’s easy to understand why a lot of folks call this pest (which resembles a bunch of worms in a bag) bagworm. This article should clear this up. (For information on true bagworm see the postscript at the end of this post). For those of you reading this article, I hope you can help me to rise up and start a movement to correct this awful error in nomenclature. 😉
The Facts about Fall Webworm
Fall webworm on Bald cypress
Fall webworm is a native pest of shade trees and ornamentals and can appear early summer through early fall. It feeds on over 100 different species of trees commonly attacking hickory, walnut, elm, birch, cherry, and willow. In urban landscapes, I’ve observed it daily on oak, sweetgum, redbud, linden, mulberry, and crabapple.
Fall webworm gets most folks attention by the large unattractive webbed nests it makes at the ends of branches. In most cases, Fall webworm is most damaging to plants aesthetically, diminishing the beauty of its host plant. A large nest can contain dozens to hundreds of caterpillars and can measure up to 3 feet across. Even after caterpillars have left to pupate, empty webbed nests can persist for months containing dried up leaf fragments and lots of caterpillar feces.
A fall webworm feed generally lasts for 5 - 6 weeks before the caterpillar leaves its host plant to pupate in the soil. Fall webworm generally has 2 generations per year.
Fall Webworm Management
Because Fall webworm generally causes little to no harm to the overall health of established healthy trees, I generally do not recommend management for this pest. Ohio has dozens of natural predators that make a living on Fall webworm including several species of birds, parasitic wasps, and other beneficial insects and they can generally keep populations of Fall webworm in check without the help of human intervention.
Newly planted trees could be at risk of significant defoliation and heavy feeding could impact fruit or nut yield for crop trees. If management of Fall webworm does become necessary, nests can be pruned out and destroyed or insecticides can be sprayed to kill the caterpillars while they are feeding. The beneficial bacteria "Bt" (Bacillus thurngiensis) can also be used on young caterpillars. This is available at most high-end garden centers labeled as Dipel or Thuricide.
If spraying is your control method of choice, please note that product only needs to be applied directly to the nests (rather than the entire tree) to avoid damage to beneficial insects and other non-target organisms.
If you need assistance managing Fall webworm - we’re here to help.
And Now, Bagworms
Bagworm is a small caterpillar that uses silk and bits of foliage and bark from its host plant to make a small bag around its body to protect itself. Each bagworm has its own individual bag (which often resembles a small pine cone), rather than large webbed nests that protect entire communities of caterpillars like in the case of Fall webworm. Bagworms feed aggressively from late May through July and can quickly defoliate entire portions of trees and shrubs if left unchecked.
Bagworms can be removed from plants by hand and disposed of easily on small trees and shrubs. On larger plants, insecticide applications can be made effectively through June before bagworms have covered their bodies with their bag.
Thanks for reading!
TJ Nagel
ISA CERTFIED ARBORIST® OH-6298A