Month: August 2019

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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