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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Cankerworm conundrums: Fall cankerworm, Alsophila pometaria


Fall cankerworm caterpillars come in two color morphs, light green and dark brown.


On a recent sojourn to western Maryland, I stopped in the historic town of Boonesboro for a cup of coffee and delicious pastry at a remarkable small bakery called Kristi’s. While enjoying my muffin, I noticed a story in the local newspaper heralding the occurrence of record numbers of cankerworms, eruptive native caterpillars that strip trees of their leaves. The onslaught was underway at several locations in Washington and nearby Frederick counties.  Beautiful Weverton Cliffs, just a short drive from Boonesboro, was reported to be a hotbed of cankerworm activity, and as I arrived at the scene a short while later impressive levels of defoliation were obvious on maples, oaks, birch, and other hardwoods up and down the mountainside. Caterpillars were dripping from shredded leaves where fall cankerworms had hatched in spring just after buds opened.

Shredded leaves are the hallmark of cankerworms.

Earlier in the season, these tiny caterpillars fed on tender young leaves creating a type of damage called shot hole damage. Shot hole damage is so named because infested leaves appear to have been blasted with a shotgun. As the caterpillars grow and develop, they remove ever–increasing amounts of leaf tissue. The name cankerworm derives from the shredded, cankered mess caterpillars make of plant leaves as they feed. These green or brown caterpillars dangle from leaves on a strand of silk when disturbed.

It’s easy to see why another name for cankerworms is inchworms.

Caterpillars of fall cankerworms and other members of their clan are also known as inchworms. They have multiple legs on their front and rear ends. By alternating their grasp between front and rear legs and arching their body into a loop, they move along twigs and leaves as if measuring the world an inch at a time. Their larvae reach phenomenal numbers in some locations and years, and devastate many shade trees such as oaks, maples, elms, and lindens. 2015 is such a year and the Maryland Department of Agriculture has reported outbreaks in Anne Arundel, Calvert, Charles, and St. Mary’s counties in addition to the ones in Frederick and Washington. After pillaging trees in spring, caterpillars move to protected locations on the forest floor to pupate. Late in the season as autumn turns to winter, fall cankerworms complete their development and adults emerge from the earth.

The female fall cankerworm is a strange creature with non-functional mouthparts and no wings. She walks rather than flies to the tree tops to lay eggs.



The little gal moths are remarkable in that they have lost their ability to fly. Many entomologists believe that the cankerworm has found a clever way to leave more offspring behind. By shifting bodily resources from equipment needed for flight, such as wings and the muscles to flap them, and redirecting these resources to the production of eggs, the female cankerworm may be able to lay more eggs, thereby bringing more little caterpillars into the world and ultimately enhancing her lineage’s odds for survival. Wingless females move from the soil and climb vertical structures such as trees and buildings. Shortly after sunset, on milder winter nights, female moths release a chemical signal called a sex pheromone that attracts the male moths. Fall cankerworm males have functional wings and are good fliers. The male moths track the pheromone to its source and the chilly couples mate. After this interlude, females climb high into the trees and place their eggs on the bark of twigs and branches. Females do not live to see their offspring. Unlike other species of moths that have tubular mouthparts used to sip nectar, the female fall cankerworm lacks functional mouthparts. She cannot feed and shortly after depositing her eggs, she dies.

Fall cankerworms lay eggs in late autumn and early winter.

In addition to the fall cankerworm, other members of their clan, such as the spring cankerworm and linden looper, are active in winter and have flightless females. A close relative of the fall cankerworm called the winter moth has been making most unwelcome appearances in cities and suburbs near Boston, where it is wreaking havoc on many deciduous trees. Members of the nefarious cankerworms clan are making their presence known up and down the east coast where communities and individual homeowners are scrambling to deal with this problem. Many insecticides are available to thwart this pest, including the microbial insecticide Bacillus thuriengienses (Bt) and reduced risk products containing spinosad, as well as many more potent insecticides. However, the sheer magnitude of the outbreaks makes control difficult and widespread applications of insecticides may harm non-target organisms. Some citizens believe they found relief by placing barrier bands around the trunks of trees. These bands snare female moths as they attempt to reach the tree tops to lay eggs. In one published account on the use of barrier bands, the authors trapped thousands of moths but defoliation in the tree tops was unaffected.

Repeated defoliations by cankerworms weaken trees causing dieback and death in some cases.

While short term eruptions of cankerworms are thought to have little lasting effects on trees, long term outbreaks of cankerworms in cities including Charlotte, North Carolina, have increased mortality of forest and shade trees. Trees in urban areas may face increased risk of death when cankerworms defoliate them due to inherent stress related to water deprivation, elevated temperatures, pollutants, and infestations of other debilitating insects and diseases. Fortunately, in natural settings Mother Nature’s checks and balances usually bring outbreaks of cankerworms to an end after a few years. Next week, we will meet some awesome predators that help put a beat-down on cankerworms and their kin.


Older cankerworm larvae shred leaves as they feed.