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Fall Cankerworm (What Time is it in Nature)

January 10, 2015

Recently, we’ve spotted several moths on the side of the Prairie Ridge office trailer and other areas of the grounds.  They’re fairly nondescript gray moths with subtle markings and have been active in spite of some very cool nights over the last few weeks.  Upon investigating them further, we discovered what these little gray moths were: Fall Cankerworms.  Today, let’s explore the fascinating biology of these cool weather insects!

Fall Cankerworms are sexually dimorphic, which means that the males and females have very different appearances.  Male moths are drab gray with wingspans reaching just over an inch:

Fall cankerworm male

The wings have indistinct pale brown and white markings, but they are subtle.  Female Fall Cankerworms, on the other hand, have a completely different appearance:

Fall cankerworm female

Female Cankerworms are wingless and rely on their legs to get them from place to place.  They’re up to a half-inch long and are most likely to be spotted on tree branches, sometimes in very large numbers.

Fall Cankerworms are native to the US and are found throughout the east as well as extensive areas of the west.  Though they are native, they are also considered pests.  After mating, female Fall Cankerworms lay neat rows of eggs on the high branches of a variety of trees (maples, ashes, and elms are favorites) in the late fall, typically after the first hard freeze.  The eggs overwinter and hatch in the early spring.  The inchworm caterpillars that emerge then feed on the leaves of the trees they were deposited on.  Periodically, Fall Cankerworms will experience a boom and cause extensive defoliation of trees in an area, eating most to all of the leaves on a variety of tree types.  If these outbreaks occur early enough in the year, the trees can sometimes recover some of their leaves.  Other times, the trees will have to wait until the following spring to replace their leaves.  Either way, Fall Cankerworms rarely kill the trees they feed on.  Once they have eaten enough leaves to grow to full size, they will lower themselves from the trees to the ground via a silk thread and pupate in the ground throughout the summer, then emerge as adults in the fall.

Fall Cankerworms are interesting insects.  Most insect populations peak in the spring and summer, times when the evenings are warm and the days warm to hot.  Because they are exothermic (i.e., rely on the temperature of their environment rather than warming themselves), most insects enter a dormant stage during the cooler months of the year.  Fall Cankerworms, on the other hand, are only active in those cooler months!  The adults generally emerge shortly after the first hard freeze in the fall and can remain active until early February.  The eggs go through a brief dormant period in the late winter and then the caterpillars emerge in mid-spring when it is still chilly.  The pupae are hidden safely away underground through the hot parts of the year until a freeze signals the moths that it is time to emerge as adults.

In our state, the Charlotte area seems to be particularly hard hit by Fall Cankerworm outbreaks and the city has implemented a state-approved control program that involves spraying the trees with a Bt pesticide spray.  Bt is derived from a biological source, the bacteria Bacillus thuringiensis, and rather specifically targets caterpillars of butterflies and moths.  By using Bt during the early spring when pretty much only the caterpillars of the Fall Cankerworm are active, cities or forest managers can target the cankerworm caterpillars without harming most of the other insect species (many of which are beneficial or harmless) in the area. Other control methods include tacking wide strips of sticky paper onto the bark of trees to trap the females as they climb up to the tops to lay their eggs.

Fall Cankerworms blend in with their environment quite well, so it can be hard to find them even when they are abundant.  Next time you’re at Prairie Ridge, look for females on the upper branches and trunks of trees and males on the walls of the office trailer and outdoor classroom.  The males might be a little boring to look at, but their interesting biology more than makes up for any lack of showiness!

What Time is it in Nature is a weekly feature highlighting the current plants, animals, and other wildlife at the Museum’s public outdoor facility, Prairie Ridge Ecostation. Find out more about the natural happenings at Prairie Ridge at our What Time is it in Nature Archive!

(Photos by Chris Goforth)

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