Cankerworm chaos: Fall cankerworm, Alsophila pometaria

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

Last week while driving eastbound on Route 70 from western Maryland, I was amazed to see a broad swath of leafless trees stretching for miles on the western face of South Mountain. A quick hike up the side of the mountain confirmed the handiwork of thousands of caterpillars whose hungry jaws completely stripped oaks, hickories, maples, birch, and assorted other trees and shrubs of their foliage. A recent post at the Maryland Department of Agriculture’s website implicated a late season frost as one cause of woes for trees here in the DMV, however the real culprits of severe defoliation in Allegany, Garrett, and Washington counties were several species of native caterpillars collectively known as cankerworms and loopers. A decade ago, in a previous episode we visited fall cankerworms as they laid waste to oak trees along the Appalachian Trail near Weverton, Maryland. By the time I scaled the slopes of South Mountain last week, denuded trees were all that remained of the caterpillar onslaught save a few shredded leaves of small saplings and a few tall trees. Cankerworm caterpillars had finished feeding and moved to the soil to continue their metamorphosis.

So, what are these culprits called cankerworms? The name cankerworm derives from the shredded, cankered mess caterpillars make of plant leaves as they feed. Another name for cankerworms is inchworms. Here’s why.  Fall cankerworms 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. They belong to a family called Geometridae, literally “land measurer” in Greek.

Cankerworms move by alternating the positions of front legs and hind legs called prolegs. It’s easy to see why another name for cankerworm is inchworm.  Early in the season cankerworms make holes in leaves and nibble at the margins. Later in the defoliation cycle only shredded leaves, leaf veins, or no leaves may remain.

This season of evil began in early spring when eggs deposited on branches last fall and winter hatched near the time that buds of hardwood trees sprang to life, creating a new crop of tender leaves. Tiny caterpillars fed on 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. These green or brown caterpillars dangle from leaves on a strand of silk when disturbed. Their larvae reach phenomenal numbers in some locations and years, and devastate many shade trees such as oaks, maples, elms, and lindens. The Maryland Department of Agriculture has reported periodic outbreaks in Anne Arundel, Calvert, Charles, and St. Mary’s counties, in addition to the ones in Frederick and Washington counties in 2015 and again in 2020. After pillaging trees in spring, caterpillars move to protected locations on the forest floor to pupate. Later in the season as autumn turns to winter, fall cankerworms complete their development and adults emerge from the earth.

Fall cankerworms lay eggs in treetops in late autumn and early winter.

These strange moths are remarkable in that the females 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 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 to attract a mate. Fall cankerworm males have functional wings and are good fliers. The male moths track the pheromone to its source and the chilly moths mate. After this interlude, females climb high into the trees and place 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.

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

In addition to the fall cankerworm, other members of their clan, such as the spring cankerworm, half-winged geometer, and linden looper have flightless females. A close relative of the fall cankerworm called the winter moth has been making most unwelcome appearances in New England where it is wreaking havoc on many deciduous trees. Members of the nefarious cankerworms clan make their presence known up and down the east coast where communities and individual homeowners scramble 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 treetops to lay eggs. In one published account on the use of barrier bands, the authors trapped thousands of moths but defoliation in the treetops was unaffected.

Repeated defoliations by cankerworms like this one on South Mountain, Maryland 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 two awesome predators that help put a beat-down on cankerworms and their kin.

References

The great references “Caterpillars of Eastern North America: A Guide to Identification and Natural History” by David L. Wagner, “The Effect of Sticky Bands on Cankerworm Abundance and Defoliation in Urban Trees” by Chanthammavong Noukoun, Gregory Bryant, and Steven D. Frank, and “Fall Cankerworm. Forest insect and disease leaflet 182. USDA Forest Service, Pacific Northwest Region (R6), Portland, OR” by W. M. Ciesla and C. Asaro were consulted to prepare this article. To learn more about caterpillar outbreaks in the DMV this year please visit this link:  https://news.maryland.gov/mda/press-release/2026/05/18/public-notice-maryland-department-of-agriculture-monitoring-frost-damage-and-caterpillar-outbreaks-in-western-maryland-hardwood-trees/

From the Bug of the Week mailbag: Song of the Bess beetle, Odontotaenius disjunctus

Look who is recycling a dead tree of heaven branch - a family of bess beetles.

A week or so ago, a friend sent me a short video of a beautiful bess beetle (aka, horned passalus beetle, betsy beetle, bess bug) and I marveled at the wonderful sound it made. A quick dive into the literature revealed that the sound is produced when the beetle flexes its abdomen, which causes a file-like structure on the upper surface of the abdomen to rub against a toothed structure, called the scraper, on its hind wings. This remarkable song has been described as chirping or screeking. It is called stridulation and is seen in many members of the insect clan. We met large stridulating beetles in Borneo in a previous episode. More than a dozen distinct types of stridulation have been identified in bess beetles.

Bess beetles sing by rubbing their abdomen on their hindwings. This sound, called stridulation, is used for defense and communication among adults and their offspring.

 Other than amusing humans, does sound production via stridulation serve other purposes for bess beetles? One possibility is that vibrations and sounds made by bess beetles may be a defense against predation. A fascinating study by Dr. Buchler et al. found that stridulation by bess beetles disrupted attacks by crows. Other studies by Dr. Robinson and colleagues found that defense stridulation by one bess beetle caused other bess beetles to have a “freeze” response rather than a “fight or flight” response. In addition to serving as a defense, stridulation in bess beetles is thought to serve other functions including social interactions among other bess beetle adults and larvae. Bess beetles belong to a select group of insects known as subsocial insects. Beneath the bark of trees, they live in closely related groups of adults that cooperate in parental care including feeding, communication, and defense of larvae. And it is not only adult beetles that produce sound. Bess beetle larvae have highly modified hind legs on their thorax that produce sound when rubbed against the middle pair of legs. Stridulation by larvae is used to solicit food from adult beetles.

Dilemma for a bess beetle at a picnic, “Do I go for the hamburger bun or find a dead tree to eat?” Image credit: Ashley May

These powerful beetles are important participants in the great circle of life. No, they do not occupy an exalted place at the top of the food chain like Mufasa, the Lion King. They sit near the bottom of the heap along with fungi and bacteria, where they help decompose fibrous wood. Adult bess beetles use strong jaws to gnaw and ingest wood. After being processed in the beetle’s digestive system and deposited back in the wood, the microbe-packed droppings, aka frass, are consumed by bess beetle larvae. The microbes contained in the leavings of the adult beetles are particularly important for young larvae that require parental microorganisms to help them digest wood. Tough plant tissues such as lignin and cellulose are indigestible to us, but the gut microbiome of the bess beetle and resident microbes found in decaying wood enable bess beetles to capture nutrients as they recycle tough plant polymers. Now is a great time to observe bess beetles as they scramble across the forest floor or recycle wood beneath the bark of fallen trees.

Bess beetles are among the champion recyclers of the insect world. A remarkable microbiome in their gut enables bess beetles to break down tough polymers found in wood and extract nutrients locked up inside. Now is a great time to observe bess beetles as they scramble across the forest floor or recycle wood beneath the bark of fallen trees.

Acknowledgements

We thank Peihan Orestes for providing the cool video of her singing bess beetle that provided the inspiration for this episode. Thanks also to Ashley May for sharing the picture of the bess beetle that joined her picnic. These remarkable references were consulted to prepare this episode: “Super-Protective Child-Rearing by Japanese Bess Beetles, Cylindrocaulus patalis: Adults Provide Their Larvae with Chewed and Predigested Wood” by Tatsuya Mishima, Noriko Wada, Ryûtarô Iwata, Hirosi Anzai, Tadatsugu Hosoya, and Kunio Araya; “Is disturbance stridulation in the passalid beetle Odontotaenius disjunctus a form of social communication?” by K. M. Robinson, Z. C. Mabry, H. Schonekas, K. Y. Robles López, A. N. Johnson, G. Cipriani, A. Nguyen, C. H. Ziemke and K. M. Baudier; “On the functions of stridulation by the passalid beetle Odontotaenius disjunctus (Coleoptera: Passalidae)” by E.R. Buchler, T.B. Wright, and E.D. Brown; and “Disturbance Sounds of Adult Passalid Beetles (Coleoptera: Passalidae): Structural and Functional Aspects” by Pedro Reyes-Castillo and M. Jarman.

Cicada surprises in 2026: Straggling periodical cicadas of Broods II and XXII, Magicicada spp.

A lonely periodical cicada awaits a mate on a flowerpot in a garden near Charlottesville, VA. Image credit: Julie Byrd Herbert

A mere five years ago, in the spring of 202, cicada lovers exulted in the arrival of billions of periodical cicadas in the eastern United States. By mid-June as the party wound down, they bemoaned the fact that in most of the DMV these strange and magnificent creatures would not return until the spring of 2038. But guess what, last week I received some amazing images of periodical cicadas emerging in droves just outside of Charlottesville, Virginia.

A surprise of Brood II periodical cicada stragglers greets homeowners in the Charlottesville, VA, region and several states east of the Mississippi. Image credit: Julie Bryd Herbert 

This sighting mirrors more than 400 reports of cicadas in more than a dozen states in the eastern half of the US. These off-cycle sightings of periodical cicadas are part of the ongoing mystery surrounding one of Nature’s most magical creatures. Before local cicadaphiles get their hopes up too high and cicadaphobes start packing to leave town, please know that this is not the full-blown cicadapalooza of 2021 (Brood X) or 2024 (Broods XIX and XIII). According to CicadaMania, early rising cicadas of Brood II may be seen in parts of Connecticut, Georgia, Maryland, North Carolina, New Jersey, New York, Oklahoma, Pennsylvania and Virginia. And in Kentucky, Louisiana, Mississippi, and Ohio, some one-year-early Brood XXII cicadas may be seen. In all of these off-cycle appearances, densities usually are orders of magnitude less than those seen when cicadas emerge with the full complement of their broodmates.  

Cicada stragglers are cicadas that emerge off-cycle from the rest of their massive brood. This poor guy emerged in 2022 and missed the big party in 2021 with billions of his Brood X broodmates that emerged in the Eastern United States that year. Here is what I saw on a spring morning in 2022. Against the background calls of Canada geese and mallard ducks, a male Brood X cicada scales an ancient maple tree in the early morning light. Watch as this lonesome bachelor avoids entanglement in a spider’s web. Instinct drives his quest to find a mate. Little does he know that his chances of passing along his genes to the next generation are between slim and none.

Cicada experts call sightings of cicadas in “off” years, cicada “stragglers.” Stragglers are periodical cicadas that emerge in years prior to or after the year that massive numbers of their broodmates emerge. Often, cicada stragglers emerge four years prior to or after their expected emergence date; however, it is possible for periodical cicadas to emerge between 8 years earlier or 4 years later than expected. Based on historical data, researchers can associate stragglers with their massive parent brood. The map accompanying this episode from iNaturalist provides accounts of actual sightings of periodical cicadas in our region this spring. This wonderful event has entomologists eager to add new information to our knowledge of these inimitable creatures. Recent studies conclude that the remarkable life cycles of periodical Magicicada are the interplay of genetic internal clocks and developmental growth thresholds that synchronize the emergence in 13- and 17-year cicadas. Experts suggest that part of the straggling phenomenon may be related to environmental factors such as variation in thermal regimes or the quality of the host trees immature cicadas dine on while underground. Sadly, densities of stragglers in an area may not achieve a quorum great enough to overwhelm hungry predators and other foes, and their unfortunate off-cycle appearance leads to oblivion for their progeny.

This recent map compiled from data sent to iNaturalist shows locations where Broods II and XXII stragglers have been seen this spring.  Screenshot of iNaturalist May 6, 2026. Modified by M. J. Raupp

So, cicadaphiles, don’t despair, as this spring provides yet another chance to enjoy cicadas and to help scientists learn more about these creatures. You can participate in the highly successful community science project that has resulted in hundreds of thousands of data points by joining the Cicada Safari. To be part of the action, go to the app store on your cellular phone and download the Cicada Safari app. It is free and very easy to use. Download, register, and start snapping pictures of cicadas. Easy as pie. Cicada geniuses will vet your images and add them to a growing database designed to demystify the seasonal phenology and distribution of these charismatic creatures. Over the next several weeks as you enjoy parades, cookouts, and adventures in the great outdoors, keep your cell phones handy, eyes open and ears on the ready, and snap some shots of straggling Brood II and Brood XXII cicadas.

Acknowledgements

We thank Dr. Julie Byrd Gene Herbert for providing images and commentary of recent cicada sightings that were the inspiration for this episode. To learn more about magical periodical cicadas, please visit the fabulous repository for all things cicada at Cicada Mania and search the archives at Bug of the Week for “cicada’”. The fascinating articles “Advances in the Evolution and Ecology of 13- and 17-Year Periodical Cicadas” by Chris Simon, John R. Cooley, Richard Karban, and Teiji Sota, and “Decoding the periodical cicada clock: field evidence and genomic insights” by Zhenyong Du and Hu Li provided great insights for this episode.

When prickly pear cactus blooms in Texas, busy bees will be there: Leafcutter bees, Megachilidae

A pair of cute megachilid bees are busy pollinating flowers of the prickly pear cactus.

Here in the DMV Mother Nature can’t make up her mind whether it is late winter or early summer, as temperatures fluctuate between the 90s and 20s this spring. To escape this all-to-often chilly weather, let’s travel to sunny San Antonio, Texas, to visit some cool bees on a warm spring day. While wandering along the Texas Native Trail in the amazing San Antonio Botanic Garden, we encountered spectacular patches of Opuntia cactus. Several of these prickly pears were in glorious bloom and pollinators were taking full advantage of nectar and pollen rewards provided by the prickly pears. Among the most entertaining of the pollinators were rambunctious bees in the family Megachilidae, whose members go by many names including leafcutter bees and mason bees, which we visited in previous episodes.

Some bees collect pollen in pollen baskets on their hind legs but megachilid bees like this leafcutter collect pollen on specialized hairs called scopa on the underside of their abdomen.

Peering into a prickly pear blossom, I was surprised to see a roiling sea of pollen-laden anthers encircled by flower petals. After a short suspense, two very cute megachilids surfaced from the anthers and took flight. Nearly every prickly pear blossom was attended by these busy pollinators. Lacking the knowledge of exactly who these beauties might be, I dove into the literature and found a fascinating publication on the nesting behavior of one species of megachilid that frequents Opuntia in the southern US. Lithurgopsis apicalis goes by the common name of orange-tipped woodborer. Other megachilids called mason bees take advantage of existing galleries in wood to raise their young. Their cousins the leafcutters cut circular slices of leaves to create cigar-like rolls which they fill with pollen to raise their brood.

Springtime in Southwestern states brings breathtaking blossoms to a variety of native plants including Opuntia, the prickly pear cactus. Nutritious pollen and sweet nectar attract a wide variety of pollinators, including members of the megachilid. Pollen from pricky pears is gathered by bees on hairs called scopa that cloak their undersurface. Bees in the genus Lithurgopsis build linear galleries in the soft interior stems of the Agave plant and provision the galleries with prickly pear pollen to feed their young. Collecting pollen is serious business and tussles among the rambunctious bees are regular events. 

By contrast, the orange-tipped wood borer builds its brood chambers in the soft tissue of dying flower stalks and stems of the Agave plant. After entering through gaps in the outer layer of the stem, they bore though the inner tissue creating elongate galleries. These galleries are provisioned with prickly pear pollen that serves as food for their developing larvae. Unfortunately, even though prickly pear cacti can be found throughout much of North America, including here in the DMV, entertaining and energetic Lithurgopsis bees occur most frequently in the south and in states west of the Mississippi River. According to iNaturalist, no members of the genus Lithurgopsis are found in Maryland, DC, or Virginia.

Opuntia, the prickly pear cactus, provides nectar and pollen for many pollinators including megachilid bees. Megachilids in the genus Lithurgopsis, provision galleries in the stems of Agave with pollen from prickly pear to raise their young.

 If your travels find you in southern and western states, and you happen upon a prickly pear cactus (don’t laugh, it could happen), take a moment to try and spot one of these beautiful and amusing native pollinators.

Acknowledgements

Special thanks to Dr. Shrewsbury for contributing video for this week’s episode. Thanks also to the San Antonio Botanic Garden for creating a wonderful space to enjoy plants from Texas and around the world. The fascinating article “Nest Site Selection and Nesting Behavior of the Bee Lithurgopsis apicalis (Megachilidae: Lithurginae)” by Jerome G. Rozen, Jr. and H. Glenn Hall was used as a reference for this episode.