Dashing caterpillars predicting weather and preparing for winter: Banded woolly bear, Pyrrharctia Isabella, Giant woolly bear, Hypercompe scribonia, and Saltmarsh caterpillar, Estigmene acrea

 

Does the wide orange band portend a mild winter ahead?

 

I usually think of caterpillars as rather delicate creatures and sometimes wonder how they survive bone chilling cold in places like Maryland where polar vortices sometimes visit. A fascinating study by Jack Layne and his colleagues revealed that woolly bear caterpillars survive winter’s cold through a process called supercooling. As temperatures drop in autumn and early winter, woolly bears and many other species of insects produce cryoprotectants, antifreeze-like compounds including glycerol and sorbitol, that prevent the formation of lethal ice crystals in their bodies. This brew of Mother Nature’s antifreeze allows caterpillars to survive even when ambient temperatures dip well below freezing.

The banded woolly bear turns into the pretty Isabella tiger moth. Image: Karin Burghardt

One of the most interesting and commonly encountered caterpillars of late autumn is the banded woolly bear. This dashing caterpillar began life in spring when it hatched from an egg laid by its mother, the Isabella tiger moth. Eggs deposited on nutritious vegetation, maybe a dandelion or an aster, hatch into leaf-munching caterpillars that feed during spring, summer and autumn on a broad range of plants. However, the caterpillar fails to transition to a pupa during the growing season. The partially grown woolly bear passes the winter, or, in bug-geek-speak “overwinters”, as a larva. In spring with the return of warm temperatures and arrival of fresh leaves, it feeds a short while before spinning a cocoon and completing the transformation to an adult moth. The pretty orange moth is rather unremarkable as tiger moths go, but the caterpillar certainly catches one’s attention with its alternating bands of black and orange.

A popular folktale has it that the woolly bear can forecast the harshness of an approaching winter. A wide orange or brown band in the middle bordered by black bands at head and tail indicates that a mild winter is at hand. Conversely, a narrow band of brown or orange means that a long, severe winter is on the way. A noted entomologist from the American Museum in New York City, Dr. C. H. Curran, tested this idea by collecting woolly bear caterpillars from nearby Bear Mountain Park each year between 1948 and 1956. He used band-width observations to forecast the severity of the upcoming winter and his observations gained notoriety when published in the New York Herald Tribune. Several other entomological experts around the country have used various clues garnered from the woolly bear to predict the winter weather. Claims of 70-80% accuracy are not uncommon.

A banded woolly bear races across my driveway to find winter refuge.

A bit earlier this season, I discovered a tiger moth caterpillar dressed only in orange and was delighted at the prospect of an incredibly mild winter. I imagined paltry fuel bills and fantasized about how I would spend the extra money. Unfortunately, a little research revealed this pretty orange caterpillar to be the saltmarsh caterpillar, Estigmene acrea. The saltmarsh caterpillar lacks black bands and, apparently, any ability to predict weather.

The gorgeous and very hairy saltmarsh caterpillar fattens up on weeds in preparation for its wintry respite.

A bit later in the season a viewer sent me an image of a solidly black tiger moth caterpillar, one completely devoid of the hopeful orange band and obviously the herald of a dreadfully long and bitterly cold winter. But once again, a little digging proved this to be not a banded woolly bear, but the larva of the giant leopard moth known as the giant woolly bear, a.k.a. black woolly bear. Like its cousins the banded woolly bear and saltmarsh caterpillars, caterpillars of the giant leopard moth eat a wide variety of woody and herbaceous plants, such as dandelion, plantain, violets, cherry, and honeysuckle, to name a few. Its magnificent coat of stout, black hairs is a formidable defense. When disturbed by a predator or bug geek, the caterpillar curls into a tight round ball of prickly black spines. What an unappetizing meal for a would-be predator! The adult is a fantastic large moth with a white coat adorned with black circles, bars, and dots.

When threatened, the giant woolly bear caterpillar presents a phalanx of stout spines punctuated by crimson rings between body segments – a strong warning to would-be predators and bug geeks. It’s astonishing that such a fearsome appearing creature should pupate into the delicate and gorgeous leopard moth.

The mother of the giant woolly bear, an adult leopard moth, is a thing of beauty with black patterned white wings.  Image: Chris Sargent

In the waning days of autumn, enjoy these caterpillars as they dash about and please leave them undisturbed if you discover them beneath a pile of leaves or under the loose bark of a tree where they are chillin’ out for winter.  

Acknowledgements

Bug of the Week thanks Sheri, Finn, and Iggy for inspiring this episode, Chris Sargent for the nice image of the leopard moth, and Karin Burghardt for providing images and identifying featured caterpillars. David Wagner’s remarkable book, “Caterpillars of Eastern North America”, was used to prepare this story, as was the interesting article “Cold Hardiness of the Woolly Bear Caterpillar (Pyrrharctia isabella Lepidoptera: arctiidae)” by Jack R. Layne Jr, Christine L. Edgar, and Rebecca E. Medwith.

What do insects do in winter? Migrants in the DMV: Common Buckeye, Junonia coenia, Variegated Fritillary, Euptoieta Claudia, Painted Lady, Vanessa cardui, Large Milkweed bug, Oncopeltus fasciatus

 

As my zinnias deliver their last blossoms, this pretty common buckeye sips nectar in preparation for its journey south to escape a chilly winter in the DMV.

 

In last week’s episode we explored the wonders and perils of migratory Monarchs in the eastern and western regions of the US. Although the journeys of the Monarchs are remarkable, these are not the only migratory visitors to the DMV. This week we meet several other sojourners that make annual warm-weather visits to our temperate region in spring and summer, and then beat a hasty retreat when old man winter warns of chilly days and nights ahead. For the past several weeks in my rapidly fading perennial flower bed, bedraggled zinnias provided some last-minute meals for peripatetic pollinators before they get out of town in advance of a killing frost. One of my absolute favorites is the beautiful Common Buckeye. It arrives in the DMV in the spring and early summer as a migrant from its overwintering redoubts in southern states, where it has three generations each year. During the warmth of summer, females lay eggs and attendant caterpillars consume plantains, foxgloves, figworts, and verbenas. After completing two generations in our region, adults tank-up on nectar from late blooming flowers like my zinnias before heading south.

Late in autumn, bedraggled zinnias provide much needed nutrients for migratory butterflies before they embark on journeys to overwintering grounds in the south. Recently, this pretty Common Buckeye made daily visits to some fading zinnias in preparation for its journey. Not one to wait for Halloween, an orange and black Variegated Fritillary stopped by earlier this season to sip nectar from a cone flower. This pretty Painted Lady also visited the garden before heading south for the winter. And butterflies aren’t the only migratory insects found in your garden. Did you know that Large Milkweed Bugs you see on your milkweeds also head south for the winter before returning to the DMV next spring?    

When not visiting flowers, variegated fritillaries often bask in the bright autumn sun before taking flight.

Another lovely visitor to our region is the pretty Variegated Fritillary. This member of the orange and black Halloween ensemble also arrives in later spring and early summer in the DMV. It enjoys as many as three generations before heading south for the winter. Unlike larvae of the buckeye, I often find variegated fritillary caterpillars in my gardens. Several years ago, I willingly surrendered the battle to maintain a lawn as a monoculture of exotic grasses and now, floristically speaking, my yard has become quite diverse. Among the winners in the ground cover competition, particularly in shady spots and landscape beds, violets rule. With regularity, I notice significant nibbles and bites at the margins of the omnipresent violets, and regularly discover glorious larvae of the Variegated Fritillary.

Whether munching leaves of hooded violets or petals of a pansy, variegated fritillary caterpillars find these members of the Viola clan delectable. Adults love to nectar on cone flowers in the summer and can be seen in late autumn basking in the sun among fallen leaves before heading south for the winter.

One of the most astounding migrants of the insect world, this pretty painted lady stopped by the flower bed to carbo-load before escaping to warmer climes in advance of winter.

Last week I also had the good fortune to spot a Painted Lady on my zinnias. This lady has been called “the planet’s most cosmopolitan butterfly” by virtue of its worldwide distribution. It is found on every continent except Antarctica. Painted Ladies also colonize the DMV each year from their overwintering grounds in Mexico and the southern US. Painted Lady caterpillars can be found on members of the Asteraceae like thistle and burdock, mallows including hibiscus, and a few other herbaceous plant families.  I’ve often thought that the eastern Monarch butterfly sets the standard for long distance migrations with its annual 3,000-mile trip to Mexico. Scientists in Europe discovered that the Painted Lady travels some 9,000 miles on its multigenerational annual migrations between Africa and Northern Europe. Wow!

Large milkweed bugs are also seasonal migrants spending the summer in the DMV before retreating to warmer regions to escape northern winters.

Are butterflies the only migratory six-legged sojourners in the DMV?  Not at all. Several other insects conduct annual migrations to our region in spring and head south from the DMV to escape a frosty, wintery death. One common migrant familiar to milkweed enthusiasts is the Large Milkweed Bug.  This red and black harlequin arrives from the south in late spring and early summer. Adults lay eggs on milkweed pods and immature stages called nymphs use sucking mouthparts to obtain vital nutrients from developing milkweed seeds. As milkweed wane in autumn, Large Milkweed Bugs migrate to warmer zones. With record warmth here in the DMV and some flowering plants still providing rewards of nectar and pollen, you may yet have a chance to see some of these migratory wonders before they hit the road or, more accurately, take wing for refuge in the sunny south.          

Acknowledgements

References for this week’s episode include “Caterpillars of North America” by David Wagner, “A Swift Guide to Butterflies of North America” by Jeffrey Glassberg, Maryland Biodiversity Project, “Featured Creature, common name: common buckeye, scientific name: Junonia coenia Hübner (Insecta: Lepidoptera: Nymphalidae” by Jaret C. Daniels, “Direct and correlated responses to selection among life-history traits in milkweed bugs (Oncopeltus fasciatus)” by J. O. Palmer and H. Dingle, and “Multi-generational long-distance migration of insects: Studying the painted lady butterfly in the Western Palaearctic” by Constanti Stefanescu and 26 other scientists.  

Catching up with magical monarchs: Monarch butterflies, Danaus plexippus

 

Western monarch butterflies cluster on a Monterey Pine in the Monarch Sanctuary at Pacific Grove, California.

 

The Beavertail lighthouse at Beavertail State Park in Rhode Island is a great place to watch Eastern migratory monarchs heading south to their overwintering grounds in Mexico.

A recent road trip took Bug of the Week to Beavertail Lighthouse, iconic guard to the entrance of Narraganset Bay, Rhode Island. This historical lighthouse, located in Beavertail State Park, is a stopover point for eastern migratory monarch butterflies as they wend their way from the northernmost breeding grounds in eastern North America en route to their winter retreats thousands of miles away in the mountains of Mexico. Recently, scientists have added Beavertail State Park to a growing list of locations where monarchs are recorded and tagged to track the movement of these peripatetic wonders. Our arrival late in October missed the peak of the monarch stopover, but during the course of our visit, we watched a dozen or so monarchs heading south across Narraganset Bay. On a windswept afternoon, we were lucky to see one lonely voyager hunker down for a rest in a thicket of forlorn perennial flowers. A recent status report provided by Dr. Paula Shrewsbury of the University of Maryland brings us up to speed on the status of monarchs in North America. Here is the report in its entirety. 

The Beavertail Lighthouse at Beavertail State Park in Rhode Island is a great place to watch Eastern migratory monarchs heading south to their overwintering grounds in Mexico.   

“Monarch butterflies, Danaus plexippus (family: Nymphalidae), are one of, if not, the most well-known butterflies in North America. There are few children who make it through school without learning the lifecycle of these beautiful and interesting iconic butterflies. There are community science projects and numerous other programs with the goal of monitoring and conserving monarchs. Everyone loves monarchs and rightly so. They have one of the most fascinating lifecycles and migratory behaviors of all insects. In addition, monarch adults provide pollination services and the caterpillars are food for other organisms. Given all of this I think they qualify as a “beneficial” even if the caterpillars make milkweed plants look a little ratty. 

If you can, plan a trip to Pacific Grove, California to visit the Monarch Sanctuary to witness one of the greatest wonders of the natural world.

Monarchs have an amazing lifecycle that involves multiple generations and migration across miles. In North America (NA), in general, there are three populations. There is the eastern NA Monarch population that overwinters in Mexico, and in the spring, around mid-March, begins its seasonal migration to the north toward southern Canada traveling a few thousand miles. Monarchs undergo multiple generations during this long journey to Canada. In the late summer–fall months, monarchs begin their migration back to their overwintering habitat in the oyamel fir forests in central Mexico. So basically, the adults that return to the overwintering roost in Mexico at the end of the season are several generations later than those that began the journey (ex. their great, great,… grand-butterflies). There is also a western NA Monarch population (west of the Rockies) that similarly migrates between sites in California and Canada. They overwinter or roost in coastal regions of California, migrate to Canada, and back again to California to overwinter. A few winters ago, in December, I was fortunate enough to visit one of the overwintering roosts of monarch adults in Monterey, CA. A truly amazing site that I recommend you all put on your bucket list to experience. A third, more recently founded population that is non-migratory, is in Florida and Georgia.  

You have likely heard discussion regarding monarch butterfly decline – in both the Eastern and Western monarch populations. Scientists studying monarch butterflies implicate several factors that come together to threaten populations of monarchs. Illegal logging of the forests in Mexico has reduced optimal overwintering habitat for monarchs. Weather events associated with climate change also threaten monarchs. In 2002, unusual weather in the mountains of Mexico killed an estimated 75% of monarchs that were overwintering; in 2015-2016, a winter storm killed more than 7%. These weather events killed tens of millions of overwintering monarchs. Within the U.S., Eastern monarchs have lost an estimated 165 million acres of breeding habitat along their migration route northward. Although monarch butterflies feed on nectar from a diversity of flowering plants, monarch caterpillars only eat milkweed foliage. Scientists believe that critical milkweed resources used by caterpillars have been dramatically reduced due to the use of herbicides and genetically modified crops that tolerate direct spraying of herbicides and kill milkweed. Other threats to their populations include habitat fragmentation and destruction, urban development, and pesticides, along with other climate change related phenomena.  

What is the status of Monarch butterfly populations now? The 2023 annual accounts of the Eastern monarchs that spend the winter in Mexico showed a 22% decline from 2022 counts. The Eastern monarch populations have declined by around 90% since the mid-1990’s. Western monarchs overwinter in forested groves on the coast of California. Western monarch overwintering “Thanksgiving” counts were conducted from November 11 through December 3, 2023 with a total count of 233,394 butterflies across 256 sites in California. The 2023 Thanksgiving count was slightly lower than the 2022 counts, but similar to those of 2021 (Xerces, Jan. 2024). The 2023 Thanksgiving count was followed by a “New Year’s” count (2024) that indicated the highest seasonal decrease on record, likely due to severe winter storms on the west coast. We will have to wait to see what the 2024 Thanksgiving count shows. The Western monarch overwintering population remains at about 5% of what it was in the 1980’s. Scientists say that monarchs are at risk of extinction in North America. 

Amidst the eucalyptus and pines, western monarchs brave the maritime chill on the Monterey peninsula.

Research indicates that milkweeds planted along the perimeter of butterfly gardens had 2.5-4 times more monarch eggs and larvae than those planted in the interior. So, if you want lots of monarch caterpillars and butterflies, plant the milkweeds at the edges of pollinator gardens.

What can be done to help monarchs? Globally, efforts to slow (and hopefully stop) climate change, conservation of food resources for adult and larval monarchs, and improvement of habitats for monarchs will help. Actions to influence policy on climate change, pesticide use, and the placement of monarchs on the Endangered Species Act list will also assist monarchs. At a local level, providing habitat with milkweeds for caterpillars and nectar resources for adults should improve monarch reproduction and survival. There are 73 species of milkweed in the U.S., monarch caterpillars use about 30 of these as hosts. Be sure to consult references to learn what milkweed species work well in your geographic region. Here in Maryland, species including common milkweed (Asclepias syriaca), swamp milkweed (Asclepias incarnata), butterfly weed (Asclepias tuberosa), whorled milkweed (Asclepias verticillata), and poke milkweed (Asclepias exaltata) are good choices. Research has determined that tropical milkweed, Aesclepias curassavica, is not a good choice and can actually be detrimental to monarchs. Adult monarchs feed on nectar from a diversity of plants, but not all nectar is created equally. Consult references to learn what plants provide the best nectar for monarch butterflies and their bloom times. Regional references for milkweed plants can be found at this link https://xerces.org/milkweed and references for monarch nectar plants can be found at this link https://xerces.org/monarchs/monarch-nectar-plant-guides. 

Research from the lab of Dan Potter (with student Adam Baker, UKY) shows that garden design is important and suggests how to build more effective monarch butterfly gardens. For example, monarch eggs and larvae were 2.5 to 4 times more abundant in gardens with milkweeds planted around the perimeter as opposed to gardens in which milkweeds were surrounded by or intermixed with the other non-milkweed plants. They also found female monarchs laid significantly more eggs on standalone milkweed plants as opposed to milkweeds that were visually "camouflaged" or physically blocked by adjacent non-milkweed plants. Although planting any milkweed and nectar hosts for monarchs will be helpful, these studies provide guidelines to design gardens that are more effective. Start planning for how to include milkweed and monarch nectar plants in your perennial gardens next spring. We have a critical role to play in conserving these remarkable travelers.”

Research indicates that milkweeds planted along the perimeter of butterfly gardens had 2.5-4 times more monarch eggs and larvae than those planted in the interior. So, if you want lots of monarch caterpillars and butterflies, plant the milkweeds at the edges of pollinator gardens.

 Acknowledgements 

Bug of the Week thanks Dr. Shrewsbury for granting permission to reproduce her story for this week’s episode. The great study “Configuration and Location of Small Urban Gardens Affect Colonization by Monarch Butterflies” by Adam M. Baker and Daniel A. Potter was a key resource for this episode.  Thanks also to Carol and Brian for helping us observe migrating monarchs at Beavertail State Park.