Don’t let this insect worm its way into your fall


Meteorologically fall begins Sept. 1 — Thursday — and lasts through end of November.

This marks the return of the Fall Webworm — although they have been here since July. The common name is based on when we typically see the largest nests.

Often people say they have Bagworms. When I ask people to describe what they are seeing, they say bags of worms encased in webbing hanging from their trees.

Another good example of common name problem is the Red Maple. When I hear the common name Red Maple (Acer rubrum) I am referring to maples that have leaves that turn red in the fall. This may not be what the person I am talking to is referring to — they may be referring to maples that have leaves that are dark red to purplish through the summer. Those maples are actually Norway Maples, scientifically known as Acer Platanoides.

Acer rubrums and Acer platanoides have different pathological or disease and insect issues when it comes to diagnostics.

The same is true when we use the common name Bagworm (Thyridopteryx ephemeraeformis), an insect pest found mainly on conifers. The bags of worms encased in webbing hanging from trees are commonly known as Fall Webworms (Hyphantria cunea).

Fall Webworms (Hyphantria cunea) are now in their second generation in Ohio. The first generation appeared once the overwintered eggs hatched normally in July and the caterpillars began to construct their silk nests. These insects undergo complete metamorphosis. Complete metamorphosis in insect development is egg, larva or interchangeably called worms, pupa, then adult. The female moths that eventually arise from these nests tend to lay their eggs on or near the nests from which they developed.

Thus, second-generation caterpillars expand the nests once occupied by first-generation caterpillars. The second-generation nests typically reach their maximum size in the fall.

Fall Webworm Caterpillars may be found on a wide variety of woody ornamental trees and shrubs as well as fruit trees. Some online references list over 90 tree species as fall webworm hosts. Fall Webworm caterpillars feed on the leaves enveloped by their silk nest. Early instar caterpillars feed primarily as leaf skeletonizers with later instars consuming all leaf tissue except for the petioles and coarse veins. An instar in caterpillars is a stage of growth. Normally caterpillars go through five stages of growth. As a caterpillar grows, it molts or sheds it skin and becomes larger. Each time it molts the caterpillar progresses to the next instar (first instar, second instar, etc.) When caterpillars reach the fifth instar they pupate. As caterpillars of the fall webworm increase in size , they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites.

Fall Webworm Caterpillar have two biotypes: the red headed and black headed. The size of their nests is dependent on the biotype caterpillar. Caterpillars of both types are very hairy but differ in body coloration, nesting behavior, dates for spring adult emergence and to some extent, host preferences.

Both biotypes produce communal nests occupied by caterpillars from multiple nearby egg masses. However, black-headed Fall Webworm nests appear to include caterpillars from only a few egg masses. They tend to produce small, wispy nests that envelop only a dozen or so leaves, but it is common for several of these small communal nests to be found on the same branch. Red-headed fall caterpillars are far more cooperative; their communal nests may include caterpillars from many egg masses. Thus, they can produce some truly spectacular multilayered nests enveloping whole branches.

Fall Webworms are native to North America. Also, Fall Webworms follow the “three P’s”: predators, parasitoids and pathogens. These have evolved with the webworms and ultimately reduces the webworms populations.

Consequently, heavy populations seldom reappear in the same location from year to year meaning that tree and shrub hosts rarely suffer consecutive years of heavy defoliation.

Also, most of the damage occurs in mid-to-late summer after established trees have acquired and stored enough carbohydrates through photosynthesis to support next season’s new growth. Despite the tree’s appearance, the caterpillars cause no significant harm to the overall health of healthy, established trees. Insecticide applications should be used sparingly since insecticides may kill the three P’s that help keep population densities in check.

If the nests are on small or newly planted trees the best control tactic is to apply digital management. Simply remove the silk nests and caterpillars by hand; gloves are optional. Throw nests on the ground and stomp on them. Thus far, no populations have become resistant to this handy pest management tactic.

For more information on Fall Webworms refer to this Ohio State University website:

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