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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.

Chilly winter days can’t stop the fall cankerworm, Alsophila pometaria


The fall cankerworm male is a good flyer and follows a scent trail to find his mate.


Last week we learned about fire ants and their northward spread, but with the onset of chilly weather it’s time to visit an insect active in the season of frost and snow. During late December when bug activity ebbs outdoors, you can imagine our delight on a warmish day last week when we had the good fortune to discover several female fall cankerworms resting on a maple tree. The fall cankerworm has a close relative called the spring cankerworm. The monikers fall and spring refer to the season in which cankerworms lay eggs rather than the time of the year their larvae feed. Caterpillars of both species hatch in spring and consume tender young leaves.

The bizarre female fall cankerworm lacks functional wings and mouthparts and does fly or eat.

These little gals are remarkable moths who have lost their ability to fly. Is this some unfortunate twist of fate or the curse a malevolent sylvan fairy? Perhaps, but 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.

Fall cankerworms deposit eggs in the late autumn and early winter.

Regardless of the reason that underlies the mystery of the wingless moth, they are a wonder to see. Beginning in late autumn, adult fall cankerworms emerge from their pupal cases in the soil. The 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 cankerworm caterpillars come in two color morphs, light green and dark brown.

The larvae of fall cankerworms hatch early in the spring after buds open. The tiny caterpillars begin to feed on tender young leaves. These green or brown caterpillars dangle from leaves on a strand of silk when disturbed. Caterpillars of fall cankerworms and other members of their clan are also known as inchworms or loopers. 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 eighth of an inch at a time. The name cankerworm derives from the shredded, cankered mess the caterpillars make of plant leaves as they feed. Their larvae reach phenomenal numbers in some locations and years and devastate many shade trees such as oaks, maples, elms, and lindens. 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 recently appeared in cities and suburbs near Boston where it is wreaking havoc on many deciduous trees and other landscape plants.



Cankerworms and other inchworms belong to the family of moths known as geometrids, Greek for "earth measure." How fitting.


For more information on cankerworms and winter moth, please visit the following web sites: