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An explosive season for fall webworms finds trees throughout our region festooned with silken webs.

We’ve had a lot of fun this season enjoying Lepidoptera, members of the moth and butterfly clan, where swallowtails, brush footed butterflies, and silk moths large and small have been featured. In this year of caterpillar glory, several of the not-so-nice moths, including Fall Webworm, have enjoyed a renaissance as well. Actually, the crescendo of Fall Webworm has been building over the past several years and this Bug Guy is hoping they have reached the climax in our region. Fall webworms are causing quite a mess. Complaints about their numbers have been coming in from Baltimore to Washington as they denude trees of leaves and festoon them with their messy, persistent webs.

Eastern tent caterpillars are spring feeders whose tents wind-up in the crotches of branches on preferred hosts like cherry.

 As I visited with folks this summer, several asked about these ubiquitous “tent caterpillars”. Let’s set the record straight: tent caterpillars, webworms, what’s the difference and how do you tell them apart? It’s easy. Eastern Tent Caterpillars (ETC) appear in March and early April in our area and build tents around branches in the crotches where branches diverge, or where limbs join the trunk. The common hosts of ETC are trees in the rose family such as cherry and apple. ETC leave the safety of their nests to feed on foliage along the branches, returning to their nests by evening after eating their fill. They will abandon their nests to pupate on the ground and emerge as adult moths in June and July. By contrast, Fall Webworms first appear in May and June and build expansive nests on the terminals (ends) of branches, incorporating the foliage they will feed upon. A second generation follows the first and in this year of moth and butterfly nirvana, they and their nests are super abundant through July, August, and September.

Inside the webs, fall webworm caterpillars devour leaves and deposit tiny mounds of frass.

Fall Webworms are one of the most catholic of all caterpillars and eat several common trees in our area including cherry, crabapple, maple, sweet gum, sycamore, and walnut. Fall Webworms are known to feed on more than 200 species of plants. Two distinct tribes of Fall Webworms have been described, one with black heads and others with red heads. In the latter half of August, we near the finale of webworm activity for the year. The webworm caterpillars in the nests will soon vacate their shelters to form pupae in the leaf litter and duff beneath trees, where they will spend the autumn, winter, and early spring. Next spring and early summer, adult Fall Webworm moths will emerge from the soil, find mates, and lay hundreds of eggs on the undersides of leaves. Tiny caterpillars hatch and spin small webs, first around their natal leaf and later around small clusters of leaves on the same branch. Inside the relative safety of these webs, larvae munch on leaves. As they grow, larvae enlarge the web to include ever-greater expanses of leaves and branches. When Fall Webworms are abundant, entire trees may be enclosed in these webs. After the first generation of webworms has developed, they move to the ground to pupate, produce new adults, and the cycle repeats. In Maryland, two generations of Fall Webworm are common annually, while in southern states such as Georgia and Texas as many as four generations occur each year.

 In most years and most locations, Fall Webworms do not cause serious damage to trees. However, their nests create a bad case of “ugly” that persists well into the winter. Of course, one easy way to be done with these messy guys is to simply remove them by hand or with pruners when you first see the nests. If nests are far above the ground, a pole pruner may be handy. And don’t just toss the nests aside or you may learn that the caterpillars can march right back up the tree. Place the nest in a bag and discard it. Fortunately for us, Fall Webworm cycles wax and wane in a given area with years of hordes followed by years of few.

A look inside a webworm nest often reveals caterpillars twitching nervously, and why not? A Polistes wasp has landed on the web, extracted a webworm, and turned it into a ball of protein ready for transport back to the nest, food for developing wasp larvae.

Larvae of parasitic wasps emerge from the body of a fall webworm after devouring its internal organs.

More than 50 species of parasitoids and 30 species of predators are known to attack and kill Fall Webworm in North America. People often ask if this is yet another invasive species from another land causing trouble in the US. Nope, this pest is a home grown native that loves our native trees. One issue that really makes me peevish is the ever-increasing influx of exotic and invasive pests we find in our landscapes and gardens. Just think about the misery caused by Japanese beetles, gypsy moths, and emerald ash borer. I am often asked if this is a one-way street, with the USA the recipient of nasty bugs and the rest of the world getting off scot-free. Absolutely not! One of our pesky “gifts” to the rest of the world is Fall Webworm. In the 1940’s this insect was accidentally introduced to Hungary from North America. It has now spread throughout large parts of Europe and Asia. In China, Fall Webworm is known as the North American White Moth and it has become one of the most devastating of all forest pests by damaging thousands of acres of trees each year. Alas, in a world with a global economy we seem destined to share a global biota.  

And that’s not all; deadly assassin bugs like this female wheel bug find fall webworms delectable even while giving a ride to a complacent mate. Yellow jackets also plunder fall webworm tents to bring juicy caterpillars home to feed their young.

Acknowledgements

 Bug of the Week thanks Dr. Shrewsbury for spotting the paper wasp extracting webworms from their nests. ‘Insects that Feed on Trees and Shrubs’ by Warren Johnson and Howard Lyon was used as a reference for this episode.

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Orange racing stripes and a pair of fleshy horns behind the head make the oakworm one cool looking caterpillar.

Orangestriped oakworms grow up to be handsome moths.

The spring and summer of 2019 have witnessed a renaissance of charismatic moths and butterflies. This season we visited beautiful brush-footed butterflies, spectacular swallowtails, and a bevy of stupendous silk moths including cecropia, promethea, and luna. Most of these wonderful moths and butterflies never become common enough to be considered pests and tend to consume plants that are not economically important. Of course, exceptions do exist. One gourmand shared her sad story of the disappearing dill, consumed by a hungry horde of black swallowtail caterpillars. This week we meet a duo of wild native silk moths that have a predilection for two of our most common and important landscape trees – oaks and maples. Periodically, oakworms and mapleworms cause a bit of trouble when Mother Nature presents conditions that favor survival of these caterpillars. Outbreaks can occur and caterpillar depredations are not uncommon throughout the range of these insects. Fortunately outbreaks are usually short lived and brought to closure by naturally occurring parasites and predators – Mother Nature’s hit squad.

In some years oakworms will strip oaks of leaves.

In Maryland, adult orangestriped oakworms first appear in May from pupae that survived winter and spring in the soil. These moths mate and females lay eggs on the underside of leaves of favored hosts including oak, hickory, and birch. Eggs hatch and the tiny larvae feed gregariously, first as skeletonizers removing small bites of leaf epidermis, but later by devouring leaves right down to the mid-vein. After completing development, caterpillars move to the soil to pupate and a second generation of adults issues forth to repeat the cycle. 

Telltale pellets of frass (insect poop, a technical term) beneath a tree can mark the presence of caterpillars feeding above.

The life cycle of the greenstriped mapleworm is similar to its cousin the orangestriped oakworm with respect to overwintering in the soil and emerging as adults in spring to colonize trees, where its eggs are deposited on leaves. Throughout much of its range there are two generations, but in the deep south there may be three or more. A critical part of the life cycles of both of these beauties is to drop from the tree when larval development is complete, burrow into soil to pupate, chill-out through autumn and winter, and emerge as an adult in spring ready to find a mate and lay eggs on leaves. Normally, the exodus-from-trees part of the game is relatively straightforward and merely involves dodging ground-dwelling predators like spiders and predatory beetles, and excavating a cozy chamber in which to pupate. However, humans have created a new challenge for earth-bound caterpillars in the form of paved walking and cycling trails. Last week while enjoying one such trail circling a small lake in Columbia, Maryland, I was astonished by vast numbers of oakworms and mapleworms running for their lives to escape the athletic shoes and bicycle tires of fitness-bound humans. Now, don’t get me wrong, these trails are treasures bringing adults and children into contact with the natural world, but oy vey, trails, sidewalks, and roadways are virtual no-bug lands for many six-legged creatures whose lives depend on soil!

Stripped leaves tell us oakworms are nearby and sure enough the cluster has moved several meters away to hide from enemies. After leaving the relative safety of the tree, the pathway beneath becomes a graveyard for unlucky oakworms. Further along the trail, a mapleworm searches for its next leafy meal. But on the ground, death is only a footstep away. Run for your lives caterpillars!

While enjoying my walk and dodging as many caterpillars as possible, I wondered how some of the carnage could be reduced. Perhaps parts of the trail could be closed during these caterpillar migrations or maybe small signs warning of caterpillar crossings could be installed beneath oaks and maples along the pathways. We do this for deer and other wildlife, right? Unlikely. However, toward the end of my walk a trio of mothers with children in tow stopped to admire an oakworm scurrying across the path. One remarked about the plight of the caterpillar, bent down, plucked the larva from the trail, and moved it to the safety of leaf litter along the trail’s edge. The children smiled. Maternal instinct and caterpillars creating a teachable moment for the next generation of naturalists. You go moms!            

Acknowledgements

Three great articles, “An integrated pest management success story: orangestriped oakworm control in Norfolk, Virginia, U.S.” by P. B. Schultz and D.B. Sivyer, “Orangestriped Oakworm” by Steve Frank and Stephen Bambara, and “Greenstriped mapleworm, Dryocampa rubicunda (F.), Lepidoptera: Saturniidae” by Steve Frank and Adam Dale, were consulted in preparation of this episode.  BOTW thanks Dr. Shrewsbury for spotting and wrangling caterpillars for this story.

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Iridescent cobalt blue and gold colors may warn predators not to consider the elderberry borer as a meal.

This week we return to the Shenandoah National Park where we recently met busy swallowtail butterflies sipping nectar and brush-footed butterflies visiting scat and collecting salt from perspiring hikers. We also met the fascinating flower longhorn beetles as they foraged for pollen from blossoms of plants in gardens and forests. This week we meet another member of the flower longhorn clan, perhaps the most beautiful beetle of all. In a patch of herbaceous vegetation along a trail, a gorgeous elderberry borer settled in for a snack just in time to be captured by the cameras. The startling hues of this blue and yellow beetle serve as a warning to predators not mess with this attention-grabbing large insect.

Elderberry, the plant host for elderberry borer larvae, grows in marshy meadows.

Adult elderberry borers feed on leaves and pollen from the flower heads of elderberry, and female beetles lay their eggs near the base of the elderberry plant.  Upon hatching, the tiny larvae bore into roots where they feed and develop on nutritious plant tissues. However, elderberry is chemically defended by potent cyanogenic glycosides, toxic compounds that are believed to be passed along from larvae to adults. A taste of these noxious chemicals during an attack may teach a predator that large blue and yellow insects are not to be included on the menu.

After dining on a tender leaf, it’s up, up, and away for the gorgeous elderberry borer (end of video at 15% actual speed).

Net-winged beetles are members of a mimicry ring bearing contrasting patterns of light and dark colors.

This warning coloration, a.k.a. aposematic coloration, of contrasting colors of blue and yellow is also employed by other distasteful insects, including beetles and moths in a mimicry ring known as Müllerian mimicry, so named for the great German naturalist Fritz Müller.  One group of beetles in the mimicry ring are those known as net-winged beetles. These conspicuous yellow and bluish black beetles ooze a repellent mixture of acetylenic acid and lycidic acid from their joints when attacked by predators. These potent antifeedants have been shown to dissuade attacks by birds, spiders, and beetles. The distributions of several species of net-winged beetles overlap broadly with that of the elderberry borer. A summer hike in forest or meadow may reveal one of these intriguing colorful beetles, pretty to observe but not for consumption.       

Acknowledgements

Bug of the Week thanks Dr. Shrewsbury for spotting the gorgeous elderberry borer and capturing an image. Two intriguing articles, “The effect of the Argentine ant on the threatened valley elderberry longhorn beetle” by Gary R. Huxel, and “Defensive Chemistry of Lycid Beetles and of Mimetic Cerambycid Beetles that Feed on Them” by Thomas Eisner, Frank C. Schroeder, Noel Snyder, Jacqualine B. Grant, Daniel J. Aneshansley, David Utterback, Jerrold Meinwald, and Maria Eisner, were used in preparation for this episode.

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Note the dusting of yellow pollen on the back, legs, and mouthparts of this pretty longhorn beetle.

Over the past several weeks we met several spectacular butterflies visiting flowers to obtain carbohydrate-rich nectar, the rich source of energy that fuels flight. Well, insects cannot live by sugar alone. Like you and me, insects need sources of protein to develop, grow, and complete vital developmental processes, including the production of eggs in the case of females. Plants don’t disappoint in this regard. In fact, the earliest pollinators of plants were likely beetles. Unlike butterflies or bees whose mouthparts are adapted to sipping nectar from blossoms, beetles have chewing jaws designed for biting and munching food. Early on in the pollination game, beetles learned that pollen was a rich source of protein and for more than a hundred million years, beetles have been pollinating flowering plants. While visiting gardens and meadows this year, I have frequently stumbled across several species of longhorn beetles in the subfamily Lepturinae, beetles commonly known as flower longhorn beetles. One look at their antennae, which often exceed the length of their body, lets you know why they are called longhorns. The fact that they are regularly found foraging in flowers for pollen gives you the other part of their common name.

In this year of abundant pollinators, several species of flower longhorn beetles can be seen on blossoms in gardens, meadows, and forests.

While adults visit gorgeous blossoms in broad daylight and consume pollen as a primary food source, the immature stages, the larvae, live a boring life quite literally. You see, immature stages of these interesting pollen eaters are equipped with powerful jaws capable of cutting galleries in hardwoods, including oaks and maples. As they bore beneath the bark of dead and dying trees, potent enzymes in their gut digest refractory polymers including cellulose, which is a major component of wood. Studies have found that this remarkable ability is due in part to the microbiome found in the gut of longhorn beetle larvae. It seems that certain fungi found in decaying wood are ingested along with the wood consumed by the larvae. These fungi breakdown cellulose, thereby making the nutrients available to the developing beetle larvae.  What an interesting partnership.

Next time you visit a coneflower, take a moment to look for curious flower longhorn beetles and enjoy a six-legged wonder that delivers double duty providing ecosystem services as both a pollinator of flowering plants and a recycler of dead and dying trees.  

Acknowledgements

Bug of the Week thanks Dr. Shrewsbury for providing images of longhorn flower beetles and the inspiration for this episode. Two interesting articles, “Records of Cerambycidae (Coleoptera) in Massachusetts with Notes on Larval Hosts” by Josef Vlasak and Katerina Vlasakova, and “ The Role of Ingested Fungal Enzymes in Cellulose Digestion in the Larvae of Cerambycid Beetles” by Jerome J. Kukor, David P. Cowan, and Michael M. Martin, were used to prepare this episode.

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Red-spotted purple butterflies are often seen in light gaps in the forest. Are they simply basking in the sun or foraging for salt deposits in the soil?

From insects to humans the element sodium is needed for physiological processes ranging from the contraction of muscles and firing of nerves to maintaining the fluid balance of body tissues. For many species of butterflies one consequence of consuming a diet rich in nectar is a need to find supplemental sources of sodium to make up for the relative lack of sodium in the nectar of plants. While carbohydrates abound in nectar, it turns out that nectar is notoriously low in other essential nutrients, one of which is sodium. To solve this sodium shortage, butterflies have evolved clever strategies to acquire the sodium they need. One of these behaviors is known as puddling. Puddling involves placing their proboscis on a substrate and imbibing sodium rich fluid found on substrate’s moist surface. Typical puddling substrates include bird droppings, animal dung, and mud – salt of the earth.

A shoulder glistening with perspiration provides much needed salt to a fearless tawny emperor butterfly.

Last week we visited beautiful comma butterflies and their pretty, spiny caterpillars while on a hike in the Shenandoah National Park. During the same adventure, temperatures soared into the upper 90’s and hikers became drenched in perspiration. Human sweat contains relatively large amounts of salt. As the day progressed and sweat moistened our skin and clothing, several species of brush-footed butterflies including commas, red admirals, tawny emperors and red-spotted purples took full advantage of this salt resource as they landed on exposed skin and moistened clothing to enjoy the salty treat. The elongated proboscis of the butterfly provided a slight tickling sensation as it lapped and absorbed small beads of perspiration on the back of my hand. During these encounters, salt-seeking butterflies displayed a complete lack of apprehension while feeding on a human. The drive for salt appears to be strong and capable of overcoming the fear of proximity to a large, potentially dangerous animal. Butterfly experts have discovered that the puddling game is played mostly by male butterflies. It turns out that during copulation, in addition to transferring sperm, a male butterfly may donate a large dose of sodium to his mate, obviating the need for her to seek additional sources of sodium. What a guy!

Tiger swallowtails, painted ladies, red admirals, and commas obtain much needed sodium from salt containing substrates on the ground and from skin or clothing moistened by perspiration.

 With lots of summer remaining and temperatures sure to reach the 90’s, take a hike in the forest, work up a little sweat, and treat some butterflies to the salt of the humans.  

Acknowledgements

Bug of the Week thanks Drs. Szczepaniec and Shrewsbury for posing with salt-seeking butterflies and providing the inspiration for this week’s episode. The fact-filled article “Nutrient acquisition across a dietary shift: fruit feeding butterflies crave amino acids, nectivores seek salt” by Alison Ravenscraft and Carol L. Boggs was consulted in preparation for this story.

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Angled wings with the mottled pattern of a dead leaf on the underside help the comma butterfly escape the searching eyes of predators. Note the bright comma-shaped mark on the hindwing that gives the comma its name.

Stinging nettles and feces don’t make Dr. Oz’s list for the ultimate keto diet, but for caterpillars and adults of the comma butterfly, these foods are just fine. Let’s return to Shenandoah National Park where last week we met tiger butterflies. This week we arrive at the picnic ground at mile post 62.8 for a visit with the clever comma butterfly. Here at the east end of the picnic grounds a trail descends to the spectacular South River Falls, the third highest waterfall in the park. As the trail snakes deeper into the gorge, the understory of a maturing deciduous forest is covered by stinging nettles. These members of the Urticaceae, with their stinging spines and tender leaves, wear the telltale signs of leaf-munching insects: sections missing from the margins, small and large holes riddling leaf blades. On one nettle we spotted the beautiful comma caterpillar basking in the dim light of the forest. Festooned with stout spines, a potent defense against the tender mouths of vertebrate predators, the caterpillar seemed content to rest in plain sight. Nearby, another late instar larva strolled across lichen colored stones, perhaps on its way to find a tender-leaved nettle for dinner. Early stages of the comma are not so bold and hide beneath leaves when not feeding.  

Is this gorgeous comma caterpillar dashing off to find more food?

A little further down the trail we met hikers accompanied by their friendly canine companions. Fortunately, these pooches left deposits of former meals in small and large piles along the trail. Nutrient rich vertebrate scats are used by a variety of insects, including other brush-footed butterflies and broad-headed bugs we met in previous episodes.  Dung is a food source particularly rich in nitrogen, an element necessary for the growth and development of all living organisms, including insects. Commas have added scat to their menu, which also includes rotting fruit and carbohydrate-rich tree sap.

Watch as a pair of comma butterflies battle to see who can be ruler of the dung.

Stout spines help defend the comma caterpillar from attacks from hungry predators.

While comma caterpillars are protected by nasty spines, adult commas use another form of defense – crypsis. Unlike other butterflies and moths whose wings have smooth rounded margins, comma butterflies and their kin have forewings and hindwings bearing deep notches that break-up the typical butterfly outline. This gives rise to the common name for these butterflies that are known as angle wings. In addition to the uncharacteristic body outline, the underside of the comma’s wings strongly resembles a dead leaf. This masquerade helps the resting comma avoid detection by the searching eyes of hungry predators. Fortunately, throughout its range in the DMV the comma has two generations each year so there is still plenty of time to go for a hike, find some scat, and enjoy the antics of the marvelous comma butterfly.

Acknowledgements

Bug of the Week thanks Dr. Shrewsbury for spotting the peripatetic comma larva that inspired this episode and Dr. Burghardt for confirming the identification of the larva. The marvelous web site Butterflies and Moths of North America was used as a reference for this episode.

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It’s easy to see why it’s called a tiger.

One of the real treasures of the Washington metropolitan region is nearby Shenandoah National Park in northern Virginia. Over the next few weeks we will visit several remarkable six-legged denizens of the park and learn about their fascinating strategies for survival.

A meadow of wild bergamot in bloom becomes an early morning nectar paradise for eastern tiger swallowtails and other pollinators. Both light colored and mimetic dark colored females were common in the meadow.

In recent summers past, insect lovers lamented the scarcity of charismatic moths and butterflies around yards and gardens. This year giant silk moths like Cecropia, Promethea, and Luna moths, as well as several of their butterfly kin, seem to be enjoying a renaissance. While meandering by a meadow filled with bergamot and other native wildflowers, scads of magnificent butterflies, bumble bees, and other native pollinators were busy at work just moments after sunrise. Preeminent among the pollinators were scores of eastern tiger swallowtails, so named for their dazzling wing color pattern of alternating bands of yellow and black. Forces of nature that underlie upswings and downturns in populations of butterflies are not fully understood, but an abundance of high quality food for caterpillars, favorable temperatures for growth of larvae and foraging of adults, and a relative absence of natural enemies benefit butterflies. Butterflies visit flowers for nectar, a primary source of carbohydrates needed to power them through the landscape in search of mates and suitable food plants for their young. Females live only a few days in the wild and deposit small eggs on leaves of suitable larval trees including tulip poplars, cherries, ashes, magnolias, and several other species.

Young tiger swallowtail larvae resemble bird droppings.

Tiger swallowtail larvae are masters of disguise. As you might imagine, plump juicy caterpillars are delicacies for a variety of birds, reptiles, and small mammals. One way to gain protection from these hungry beasties is to engage in the art of deception. The inconspicuous swallowtail egg hatches into a caterpillar that masquerades as a bird dropping. Even hungry predators like birds are unlikely to connect bird droppings with dinner, and dressing as a bird dropping is a very clever disguise for a juicy caterpillar. As the swallowtail grows larger and more apparent, a different subterfuge is used.

Large creepy eyespots of older swallowtail caterpillars frighten would-be predators.

The second trick is to be bold and look dangerous like a snake. The latter stages of the tiger swallowtail caterpillar bear two large eyespots on their back. Eyespots are not true eyes, but only patterns of colors on the skin of the insect. Look closely at these. The eyespots are remarkably complete with pupils and irises. Viewed head-on, the larva of the tiger swallowtail looks like a green viper with very large eyes. Remember, snakes and birds are mortal enemies. Snakes often capture birds for dinner. Confronted by a sinuous creature with very large, scary eyes a bird may opt to seek its dinner elsewhere. The caterpillar’s trickery allows it to live another day.

 The adult tiger swallowtail engages in a ruse of its own. Throughout much of the range of the eastern tiger swallowtail, a second species of swallowtail shares flowers and airspace. The pipevine swallowtail, Battus philenor, has a black dorsal surface that is easy to recognize. Caterpillars of the pipevine swallowtail eat plants in the pipevine family that contain repellent chemicals called aristilochic acids. Larvae store these noxious acids and pass them along to adult butterflies. This imparts a nasty flavor to the butterfly and predators learn to avoid them. In parts of its range where pipevine swallowtails are common, the clever tiger swallowtail evolved a dark color pattern that resembles the coloration of its unpalatable cousin. Ah, but only the female of the species employs the dark colored ruse. These mimics gain protection from predators that have learned not to tangle with darkly colored distasteful butterflies. This week on a bright summer day when you see the gorgeous yellow and black tiger swallowtails in the meadow or your garden, know that you are company to a true master of disguise.

Acknowledgements

We thank Drs. Shrewsbury and Burghardt for providing images for this week’s episode. The delightful books and articles “Secret Weapons” by T. Eisner, M. Eisner, and M. Siegler, “Butterflies” by P.A. Opler and G.O Krizek and “Butterfly Ecology” by L.E. Gilbert and M.C. Singer were used as references for this episode.

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