Tuesday, October 12, 2021

Fall Webworms: Spinning their way through the season

webwormsAs fall sets in, it’s not difficult to identify the tiny creatures called fall webworms.  This time of year, these masses of larva have been busy recreating scenes from sleepy hollow as they prepare to over winter in the pupa stage.

This display of web weaving starts when the adult tiger moth lays her eggs on the underside of leaves in ‘hair’-covered clusters of a few hundred.  Host plant selection is dependent on factors like the plant’s degree of sun exposure, age, environmental stress undergone, toughness, and nutritional quality. For an insect that needs energy for processes like dispersal or diapause, consuming plants that provide a lot of carbohydrates could is beneficial; for a female insect that is producing eggs, consuming plants that provide a lot of protein is beneficial. In the eastern U.S., pecan trees, black walnut, American elm, hickory, fruit trees, and some maples are preferred hosts.

After 3-5 days the larva will hatch and rapidly grow in ideal conditions inside the web. The caterpillars are variable in color, ranging from a pale yellow to dark grey, with yellow spots and long and short bristles. There are two cream stripes along the sides with a maximum length of 35 mm. These tiny insects are not as eye-catching as the home they build for themselves.

The webs from the fall webworm are concentrated to the tips of the branches.  Larvae feed inside the tents expanding the web until the late instars. Very young larvae feed only on the upper surfaces of leaves then later they consume whole leaves. These webs allow for temperature regulation. Fall webworms experience behavioral thermoregulation in which the self-created web is able to trap heat. Due to this, the fall webworm is able to maintain a warm temperature of about 40-50 °C, which allows the larvae to grow and develop faster.  Their web also acts as a dome of protection from predators.  When a web is approached by a human or animal these caterpillars have several strategies to defend against threats.  They utilize a variety of protective behaviors:  shaking and jerking together, repellant scent, and irritants on hairs or spines in an attempt to ward off predators

The larval stage lasts about four to six weeks at which time the webworm caterpillar descends to the ground seeking refuge in the bark and leaf litter at the base of the trees.  If it is not eaten by toads or other predators, it transforms into a brown pupa, wrapped in the loose hairs from its outer skeleton, and undergoes metamorphosis.  These creatures will overwinter in their insulated cocoons and will emerge again later the following Summer.

webwormsAdult webworm moths emerge from their cocoon, court, mate and the fertilized females continue the moth life cycle by placing her eggs on the underside of deciduous tree leaves, which hatch, in our area, sometime in early to mid-September, signaling the second generation of larvae; a much larger generation with more clusters of webs adorning the trees.  The adult is mostly white in our region of the Adirondacks and is quite ‘hairy’ with the front legs having bright yellow or orange patches. The underwings will have less marking than the forewings, and the abdomen often has a sprinkling of brown hairs. It has a wingspan with a range of 35–42 mm.  They are night flyers so catching a glimpse of them will be after the Sun goes down and the porch lights come on.

While webworms may not be very glamorous, they are significant contributors to the food cycle and the survival of several migrating bird species. Each autumn many hundreds of millions of songbirds migrate from their northern summer breeding grounds to southern wintering areas; with stops along their path to feed from nature’s menus that include protein-rich insects, especially juicy caterpillars.  The flights of migrating warblers and other insect-eating songbirds are powered by the energy “stored” in the bodies of insects.

If you’re not willing to continue to view these webbed eyesores well into Winter when they will begin to fall out of the trees, there is an easy way to rid yourself of them.  It’s important you know before attempting to remove the web, if there are still occupants there.  If the web is white, it is new and has live larva living in it. If it is tan or brown, there is most likely no larvae there.  If the web is occupied you may want to keep a distance while removing.  The easiest way to remove these creatures and their mess of a nest is to either prune the entire branch they are enclosing or to take a pole or long stick and knock the web out of the tree.  Either way both the branch with the web or solely the web, need to be buried in at least 6 inches of soil.

If you’re not up to the task just think of it as a natural Halloween decoration or a good bye summer treat for our feathered friends flying the cool falls skies.


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Jackie Woodcock was born and lives in the Adirondack Mountains. She is an apiarist, lepidopterist, conservationist, teacher, writer, artist, and a co-owner of SkyLyfeADK. You can find her SkyLyfeADK on Instagram and Facebook.


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