By Mike Wilson
ISA Certified Arborist®
September 12, 2024
As a regional manager, I have been hearing many concerns from homeowners about fall webworm over the last few weeks and have seen even more in landscapes throughout central Ohio. Fall webworm (Hyphantria cunea) is a species of moth known for its distinctive and conspicuous webs, which are commonly found on a wide variety of trees across North America. While not considered a major threat, fall webworm can cause significant defoliation in late summer and early fall, leading to reduced vigor and unsightly trees.
Life Cycle of the Fall Webworm
The life cycle of fall webworm is a fascinating process that begins with the emergence of adult moths in late spring or early summer. The adult moths, characterized by their white wings often dotted with black spots, are nocturnal and are seldom seen during the day. After mating, the female moth lays her eggs on the underside of leaves. These eggs are small, white, and laid in clusters, often numbering several hundred.
Within a week or two, the eggs hatch into larvae, which are the most destructive stage of the fall webworm life cycle. The caterpillars are initially light green or yellow with two rows of black spots along their bodies. As they mature, they develop long, silky hairs. The larvae immediately begin to feed on the leaves of the host tree, creating a communal web that serves as both a shelter and a feeding ground. As the larvae grow, the web expands to cover more of the tree's branches. This feeding occurs from late spring to mid-summer. By late August, they typically stop feeding.
The larvae go through several instars (or developmental stages) over the course of four to six weeks. Once they reach maturity, they leave the webbing to pupate in the soil or under loose bark, where they remain until the following spring.
Habitat and Distribution
Young fall webworm insects are highly adaptable caterpillars that feed on a wide variety of plants, including more than 100 species of deciduous trees. Some of their preferred hosts include hickory, walnut, cherry, and persimmon trees.
The webs of the fall webworm are commonly found in late summer and early fall, which is how the species earned its name. These webs can cover entire branches and, in severe infestations, can even envelop small trees entirely.
Ecological Impact
While fall webworm is a pest and nuisance, its overall impact on the urban forest is generally minimal. Most healthy trees can withstand the defoliation caused by these caterpillars, and the damage is typically more cosmetic than lethal. The webs themselves, though unsightly, do not cause direct harm to the tree. The unsightly webs can diminish the aesthetic value of ornamental trees, leading to concerns among tree managers.
Management and Control
Controlling populations of fall webworm can be challenging due to the species' high reproductive rate and wide host range. However, several management strategies can help reduce their impact.
Mechanical Control: One of the simplest methods is physically removing the webs from affected trees. This can be done by pruning infested branches or tearing open the webs to expose the caterpillars to predators.
Biological Control: Natural predators and parasitoids can help keep fall webworm populations in check. Birds, spiders, and wasps are all natural enemies of fall webworm. In some cases, introducing or encouraging these predators can be an effective control measure.
Chemical Control: In severe infestations, insecticides may be used to reduce fall webworm populations. We generally only recommend this if the infestation is extreme or if the tree is of high aesthetic value.
Cultural Control: Maintaining healthy trees through proper watering, fertilization, and pruning can help them better withstand defoliation. Additionally, removing fallen leaves and other debris from around the base of trees can reduce overwintering sites for pupae.
Conclusion
In conclusion, although the presence of fall webworm can be unsightly and concerning, they are generally not a serious threat to the overall health of trees. We generally do not recommend treatment of this pest unless populations are excessive, or if a young tree is infested to the point where serious defoliation is a concern. Should chemical or physical treatment be warranted, one of our 20+ ISA Certified Arborists® can provide a free tree quote and determine the best care for your trees at RussellTreeExperts.com/Quote or at (614) 895-7000!
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Sincerely,
Mike Wilson I Regional Manager, Russell Tree Experts
Mike joined RTE in 2013 and has been in the green industry for over 14 years! He is an ISA Certified Arborist® and a graduate of Oregon State University. Mike is an outdoor enthusiast and a US Army Medic Veteran!