The caterpillars are varied in color, hairy, with paired dark spots along the back.  The adults are small white moths.  When fully mature, the larvae seek a protected location to pupate, and spend the winter in a cocoon in bark, building crevices, or debris on the soil.  The adult moths begin emerging in late spring and then lay eggs.  In this area, there may be a few generations through the summer.

The heaviest and most obvious incidences seem to be at the end of the summer, when webs as long as 3 feet wide can be seen.  Since they affect deciduous plants

Fall webworm on Kwanzan Cherry tree

that will soon lose their leaves, the damage is mostly aesthetic, but bad infestations can defoliate trees.   Since it is a native pest, predators such as various tiny wasps, often keep the population at bay.  To encourage predators, make sure you have favorable host plants, such as those in the daisy family, in your landscape.  If the damage is extensive or bothersome, the best action is simply to prune out the tents, destroy (crush) the caterpillars, and dispose of them in the trash (not recycling).   As with many other caterpillar species, the bacterial insecticide Bacillus thuringiensis (Bt), can be effective when the larvae are still small, and is an organically acceptable approach, as long as you can break the nest open to spray the disease onto the pests.   Additionally, as coated leaves are incorporated into the tents and eaten, the Bt will be ingested. Photo on right of Kwanzan Cherry Tree shows more mature instars of the catepillar, which may not be impacted by insecticide.

In sum, the fall webworm does not typically cause extensive damage to trees and shrubs, and since it is a native pest, it is susceptible to natural control by local pests.  The defoliation that they cause is often late enough in the season not to be terribly detrimental.  If you find the webs at a reachable area, then best to remove and destroy them to prevent their overwintering and causing more damage next year.  Otherwise, let them be and, instead, look out for the many more destructive non-native pests that plague our landscapes.

All photos by Connie Bowers

Fall webworm on Forest Pansy Redbud leaf

Large Fall webworm tent

Smaller caterpillars in this Fall webworm tent

Fall webworms appear at tree branch tips

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It is important to have a licensed tree expert – arborist  — evaluate your prized trees to recommend pruning or other care to ensure they maintain good health and vitality, but as I’ve experienced, trees have a mind of their own.  Be mindful that, of late, serious diseases have affected major groupings of trees, such as Ash, Oak, and Walnut;  therefore, it is imperative to have such trees (and others) checked for signs of disease.

It is important to look out for your trees and keep them in good shape.  One issue that I’ve witnessed time and time again – an issue that could have been prevented or can be corrected if caught early – is the issue of tree root girdling.  This occurs when roots at the base of a tree begin to grow around the main stem (trunk) of the tree and appear to be “strangling” the trunk.  This “self-choking” will cut off or restrict the movement of water and essential nutrients to the roots of the tree.  Certain trees are more prone to this issue – e.g., Magnolias, Pines, Lindens, and certain Maples.

Affected trees will not die suddenly, but will weaken over time.  They may exhibit symptoms such as early fall color, unusually small leaves, and dead interior branches.  Tree root girdling will likely promote insect infestation and disease due to the weakened structure of the tree.

What causes root girdling?  While not fully understood, this problem is thought to be the result of certain conditions – most likely due to improper planting techniques — that prevent roots from growing out and spreading in a normal manner.  One typical example would be a container-grown tree where the roots are forced to grow in a circular fashion inside the container.  If the roots of these trees are not correctly pruned at the time of transplanting, this circular growth pattern will continue and could cause girdling   The correct pruning method at time of planting such container-grown plants, is to score, or make several slashes vertically down the rootball to an inch deep into the rootball, and a couple of slashes across the bottom of the rootball 3-4 inches deep.

Alternatively, failure to dig a large enough hole for a bare-rooted tree, at time of planting, or planting a tree too deep, can also cause roots to grow in a circular pattern too close to the trunk.

As indicated, the problem usually starts when the tree is young, and symptoms are not evident for quite some time.  As the tree grows larger and the trunk expands, pressure against the base of the tree limit water and nutrients from getting to the leaves, and symptoms become apparent.

These issues speak to the importance of ensuring that trees are planted only by trained horticultural professionals who use proper methods, even if it takes more time (i.e., expense).    For example, it is important to identify the root flare and ensure it is at ground level when the tree is planted.  The width of the planting hole should be two to three times as wide as the rootball.

For existing mature trees, it is important to look for cases of girdling.  Take note that while girdling roots may be visible, they are often found underground.  If tree root girdling is noted, remedial action should be taken.  While removing a girdling root is itself wounding, if the tree is young, removing the root is preferable to taking no action, which will increase the likelihood of the tree dying.  Consult a tree expert if in doubt.

Score roots encircling root ball at time of planting to prevent girdling.

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