Month: August 2021

 

 

While much of the nation roasts through stifling heat waves this summer, it is hard to believe that any creature could struggle with cool temperatures. But remember from your high school biology days that many animals are poikilotherms, cold-blooded beings whose body temperatures largely depend on the ambient temperature of the environment they occupy. Terrestrial creatures including amphibians like frogs, non-feathered reptiles like snakes, and, of course, insects fit into the category of cold-blooded animals. However, to perform fundamental activities of locomotion either by legs or wings, muscles of cold-blooded insects must reach a minimum temperature to function. For example, take butterflies – in cool montane habitats of the Sierra Nevada, flight muscles in a butterfly’s thorax may require temperatures in the 90’s to sustain flight. One way to generate this level of heating is to bask in the strong mountain sunshine. Basking is a thermoregulatory behavior used by many insects and other poikilotherms to gather energy and warmth to facilitate metabolic processes and sustain activities such as walking and flight.

As homeotherms, that is, warm-blooded creatures, we have behaviors and the metabolism to help keep our body temperatures at optimal levels. One familiar way to keep warm is shivering, rapid involuntary contractions of our muscles that expend energy and generate heat to warm our bodies when we are cold. Do insects employ shivering or something akin to shivering as a way to warm up? You bet! Enter a chilly dobsonfly. Each year about this time, Bug of the Week receives requests to identify a large creepy looking insect found on the side of a building or on a plant, often near a porch light that attracts flying insects. These grotesque marvels belong to an order of insects known as the Megaloptera – “huge winged” insects – and go by the name of dobsonflies. They are among the largest winged insects found in the DMV, ranking in size with large moths and butterflies.

On a recent chilly morning on Cacapon Mountain, West Virginia when temperatures had dropped into the low 60s the night before, I spotted a magnificent female dobsonfly resting on a railing near a porch light. Apparently, temperatures in the upper 70s at nightfall were warm enough to enable this behemoth to fly toward the light and come to rest on the nearby railing of a deck. With morning’s first sunbeams just creeping over the mountains but not yet intense enough to warm flight muscles, the dobsonfly found a way to generate its own heat as a bug geek approached with a camera. Rather than a mammalian-style shiver, a burst of rapid wing-fluttering ensued. In just under two minutes, with flight muscles warmed and ready to rock, the dobsonfly escaped the probing lens of the paparazzi.

On a chilly mountain morning, watch as a creepy female dobsonfly flutters her wings to warm flight muscles in preparation for takeoff. In less than two minutes she is ready to escape the probing lens of the camera.

As you can see, female dobsonflies are magnificent, but males are really something special with their enormous sickle-shaped mandibles. Careful observations of mano-a-mano encounters between male dobsonflies reveal that their super large jaws are used in combat to dislodge competitors from substrates where potential mates might be present. These mandibles are useless in capturing prey and both male and female dobsonflies, which have powerful jaws, are not predatory as adults. As adults their diet is likely a liquid one.

Juvenile dobsonflies go by the name of hellgrammites and live a life aquatic. These fierce predators roam the interstitial spaces between stones and vegetation at the bottom of rapidly flowing streams, where they capture and dine on immature mayflies, stoneflies, and caddisflies. Experience tells me that their powerful jaws can deliver a memorable bite to unsuspecting humans attempting a capture. Hellgrammites are a key indicator of stream health and not found in polluted waters. Fish adore them and they are excellent bait. Like many aquatic insects, hellgrammites have gills lining the margins of the abdomen enabling them to extract oxygen from their watery habitat. In an unusual developmental twist, they also have spiracles, breathing ports, which allow them to obtain air on land. This adaptation is critical to their amphibious life style as they climb out of the water to build pupal chambers on land beneath stones, logs, or other moist protected structures. You may encounter adult dobsonflies in the morning near lighted buildings, as both sexes are attracted to light.  After mating, female dobsonflies deposit eggs on vegetation overhanging water. Hatchlings drop to the stream below to roam the benthos in search of prey. Larval development can take from one to three years. Over the next several weeks as you wander the banks of freshwater streams and rivers in our region, or visit structures with nighttime illumination near waterways, keep an eye open for these marvelous giants of the insect world.     

Acknowledgements

Bug of the Week thanks Nolan for providing images that were the inspiration for this episode. The wonderful publications “Behavioral Observations on the Dobsonfly, Corydalus cornutus (Megaloptera: Corydalidae) with Photographic Evidence of the Use of the Elongate Mandibles in the Male” by T. J. Simonsen, J. J. Dombroskie, and D. D. Lawrie, and “Featured Creatures, common name: eastern dobsonfly (adult), hellgrammite (larva), scientific name: Corydalus cornutus (Linnaeus) (Insecta: Megaloptera: Corydalidae: Corydalinae)” by D. Hall, and “Comparative Thermoregulation of Four Montane Butterflies of Different Mass” by Bernd Heinrich, were used as references for this episode. Many thanks to Nolan Jenkins for providing the cool image of the huge-jawed male dobsonfly.

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The last time we visited the magnificent monarch butterfly was August 27, 2018. That particular August turned out to be a rather good season for monarchs at my home in Columbia, MD. Monarch season this year got off to a pretty slow start with the first monarch arriving in the last week of July. Since then, on a daily basis, monarchs have been gliding about the flower beds, sipping nectar from the zinnias and cup plants during daylight hours. Their journey to grace my garden began months ago in early spring. In the Eastern US, these remarkable vagabonds depart their winter refuge in Mexico and continue their journey north for months, reaching milkweed patches in northern states and Southern Canada. In the Western US, a similar migration occurs as monarchs depart overwintering grounds in coastal California and head north to exploit patches of tender milkweeds as they become available in spring and summer. As summer wains days grow shorter and temperatures cool, and these travelers return to their hibernal redoubts. Eastern monarchs embark on an epic journey thousands of miles south to Oyamel fir forests in the mountains of Mexico. Western monarchs return to eucalypt and pine forests of coastal California from summering grounds in Washington, Oregon, Idaho, and several other western states. While 2018 was a rather good year for monarchs in my garden in Maryland, the outlook for migratory monarchs in Eastern and Western North America isn’t so rosy.

In a previous episode, we communed with thousands of western monarchs in Pacific Grove, CA, also known as “Butterfly Town, USA.” During the brief six years since our last visit to Pacific Grove in 2014, overwintering populations of monarchs in California nose-dived from tens of thousands to a shocking 1,914 counted in the 2020 winter census, a decline of more than 99.9% from historic levels. Eastern monarchs have fared better but still their numbers have declined dramatically to around 20% of historic high levels. Many believe it may already be too late to save the Western migratory monarchs and Eastern monarchs may not be far behind. Attempts to have monarchs declared an endangered species by the US Fish and Wildlife Service failed in December of 2020 as resources were needed to focus on “higher-priority listing actions” according to agency officials. The case for endangered species status will be revisited in upcoming years.

Scientists studying monarch butterflies believe multiple factors conspire to threaten populations of monarchs. Illegal logging of trees in the mountains of Mexico has reduced the critical overwintering habitat for monarchs. Without this refuge monarchs cannot survive winter. Scientists suggest that more severe weather events associated with climate change may also threaten monarchs. Freakish winter weather in the mountains of Mexico in 2002 killed an estimated 75% of overwintering monarchs. In the winter of 2015–2016, a late winter storm killed more than 7% of overwintering monarchs. These events translated into tens of millions fewer monarchs making their way north for the annual migration.

In the US Midwest and Pacific Northwest, record summer heat in 2012 and 2015 killed untold numbers of monarch caterpillars. Throughout the US, urban sprawl and the use of herbicides in agricultural production greatly reduce populations of milkweed plants that are vital for the survival of monarch caterpillars. Increasing levels of atmospheric carbon dioxide may change the chemical composition of milkweeds, altering levels of pharmacological chemicals that help monarch caterpillars thwart disease-causing microbes. There is also concern that planting the exotic tropical milkweed, Asclepias curassavica, may interrupt the primal migration behavior of monarchs and cause them to take up residence in areas where tropical milkweeds are planted, derailing their age-old migrations. Another study suggests that when tropical milkweed grows at high temperatures, it becomes more toxic to monarch caterpillars. As the world warms and the growing season get longer, monarchs may expand their range further northward into Canada in search of high-quality milkweeds. This may increase risks of starvation or predation as monarchs attempt an already very long and perilous journey to overwintering grounds in Mexico.  

What can be done to help save these unique and charismatic creatures? Globally, mitigating climate change, reducing unnecessary pesticide use, and conserving resources and habitats for wildlife will help. Locally, providing milkweeds for monarch caterpillars and nectar plants for adults will facilitate reproduction and survival. 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. Be sure to consult a reference to learn what milkweeds work well in your geographic region. Here in Maryland, species including common milkweed, Asclepias syriaca, swamp milkweed, Asclepias incarnata, and butterfly weed, Asclepias tuberosa, are good choices.

After imbibing nectar, the monarch finds just the right leaf on which to place an egg. Under the watchful gaze of a milkweed leaf beetle, a tiny monarch caterpillar consumes a leaf but soon it will grow into a behemoth capable of consuming milkweed seedpods. Within a dazzling chrysalis, a caterpillar becomes a butterfly ready to feed, fly, and spawn the next generation.

In the waning weeks of summer, go to the meadow and enjoy these beauties. Next spring plan to include milkweed and monarch nectar plants in your perennial gardens. We have a role to play in conserving these remarkable wanderers.

Acknowledgements

The excellent references, “Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?” by Lincoln Brower and colleagues, and “Quantifying impacts of climate change on species interactions while fostering undergraduate research experiences using the Monarch (Danaus plexippus)- Milkweed (Asclepias Sp.) system” by Matthew J. Faldyn, “Monarch butterflies denied endangered species listing despite shocking decline” by Farah Eltohamy, and “ We’re losing monarchs fast—here’s why. It’s not too late to save them, but it’s a question of whether we make the effort” by Carrie Arnold, were consulted for this episode. To learn more about monarchs, their migrations and perils, and how to conserve them, please visit the following websites:

https://xerces.org/monarchs/western-monarch-conservation

https://xerces.org/monarchs/eastern-monarch-conservation

https://xerces.org/blog/monarch-numbers-from-mexico-point-to-declining-population

https://www.nationalgeographic.com/animals/article/monarch-butterflies-near-extinction

https://www.nationalgeographic.com/animals/article/monarch-butterflies-risk-extinction-climate-change

https://www.washingtonpost.com/world/the_americas/climate-change-is-playing-havoc-with-mexicos-monarch-butterfly-migration/2019/12/23/e60c1e0e-21ab-11ea-b034-de7dc2b5199b_story.html

http://www.monarchwatch.org/index.html   

http://www.eeb.cornell.edu/agrawal/2017/02/10/monarch-population-size-over-winter-2016-2017-announced/

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In last week’s episode, we met several truly remarkable caterpillars, larvae of swallowtail and brush footed butterflies, that have evolved the clever defense of looking like bird droppings to fool the searching eyes of hungry predators. This week we travel the muddy banks and floodplains of the mighty Potomac River to visit the gorgeous zebra swallowtail butterfly and its larvae that employ a survival strategy different from the bird-dropping-mimics we met last week.

Along the banks of the Potomac and other tributaries of the Chesapeake grow small forests of beautiful native pawpaw trees. A walk amongst these winsome understory trees sets one to wondering why their luxuriant green leaves go virtually unmolested by leaf-eating insects and vertebrates during the growing season. Even rapacious white-tailed deer shun these plants. Pawpaw has evolved a clever defense, a noxious group of chemicals called annonaceous acetogenins. These bioactive compounds, found in both leaves and bark, likely make them unpalatable to a diverse array of hungry herbivores. In addition to nasty metabolic effects, acetogenins are known to produce a potent emetic response in vertebrates. Ah, but herbivorous insects often discover ways to deal with defenses thrown at them by plants. In previous episodes we learned how monarch caterpillars turned the tables on milkweeds and used defensive compounds produced by milkweeds for their own defense against predators. A similar story holds for the zebra swallowtail butterfly. Sophisticated chemical analysis revealed that zebra swallowtail caterpillars and adult butterflies contained annonaceous acetogenins similar to those found in pawpaws. Scientists believe that these compounds originate in the leaves of pawpaw, are stored in the tissues of caterpillars as they eat leaves, and are passed along to the adult butterfly. The presence of acetogenins likely helps protect both the beautiful butterflies and their larvae from the beaks and teeth of hungry predators.

Along the banks of the mighty Potomac, zebra swallowtails “mudpuddle” to obtain sodium and other minerals necessary for life. Watch as the zebra probes the soil with her proboscis. Caterpillars of the zebra swallowtail are elusive. After searching hundreds of pawpaws, I finally discovered an almost fully developed larva taking a stroll along a pawpaw branch.

During the next month or so, find a moment for a walk along the Potomac or other nearby rivers where pawpaws grow. Soon the delicious fruits of the pawpaw tree will be ripe. Be sure to keep an eye open for zebra swallowtails and spend a few moments searching pawpaw leaves for magnificent zebra caterpillars.

Acknowledgements

“Chemical Defense in the Zebra Swallowtail Butterfly, Eurytides marcellus, Involving Annonaceous Acetogenins” by John M. Martin, Stephen R. Madigosky, Zhe-ming Gu, Dawei Zhou, Jinn Wu, and Jerry L. McLaughlin, and “Common name: zebra swallowtail, scientific name: Protographium marcellus (Cramer) (Insecta: Lepidoptera: Papilionidae)” by Jerry F. Butler and Donald W. Hall were consulted in preparation of this episode.

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The art of deception has evolved many times in the insect kingdom. In previous episodes we met members of the phasmatid clan from around the globe, walking sticks from the hills in Maryland, jungles of Vietnam, forests of Australia, coastal plains of Florida, and rainforests of Malaysia that did their best to resemble branches, twigs, and leaves of plants. Their clever ruse of resembling parts of plants enables otherwise tasty prey to be overlooked by the hungry eyes of visually astute predators like birds, lizards, and mammals. Similar acts of deception are seen in katydids and other insects with green colors and patterns that mimic leaves of plants.

Ah, but twigs and leaves are not nature’s only items of little or no interest to meat-eaters. Two years ago, while dining outdoors on the patio of a popular restaurant in NYC, we were bombarded with bird droppings from starlings in shade trees overhead. As diners dropped crumbs or abandoned their meals, starlings swooped down to battle over morsels on the ground and scavenge uneaten tidbits from lunch plates. While awaiting the next meal, birds returned to lofty perches and relieved themselves, thereby creating quite a mess on tables and chairs below. While watching the action and dodging birds, not once did I see a bird attempt a taste-test of droppings deposited by the rest of the flock. Forged by eons of selection, many insects have evolved a mien bearing a very close resemblance to a bird dropping. How clever is this? What self-respecting insectivorous bird eats a bird dropping, right?  

Let’s start with a bird dropping. We’ve all seen these. Next, look at the caterpillar of the red-spotted purple, a bird-dropping mimic that turns into a cool butterfly often seen on the forest floor. If you grow parsley, dill, or fennel you may have seen the black swallowtail caterpillar feasting on your herbs. Adult black swallowtails are common visitors to flower gardens. Within a silken shelter on a leaf, you’ll find another bird dropping mimic, the eastern tiger swallowtail caterpillar. Adult eastern tiger swallowtails sip nectar from butterfly weed.

A week or so ago while visiting the spectacular Cacapon State Park in Berkeley Springs, WV, my granddaughter spotted the beautiful wood-nymph, Eudryus grata. Of course, the initial response to this discovery was “don’t touch” but on closer inspection and with a little prodding, what first appeared to be a rather juicy bird dropping took flight and landed on a nearby leaf. The beautiful wood-nymph ranges from Canada to Texas, where caterpillars dine on members of the grape family. In the northern part of its range one generation occurs annually, but in Florida and southern states multiple generations occur each year.

The caterpillar of the gorgeous red-spotted purple butterfly feeds on leaves of cherry, oak, and poplar and resembles a rather large and gooey bird dropping. Larvae of several species of swallowtail butterflies mimic bird droppings in the early stages of development. Those commonly found in our area include dill and parsley munching black swallowtail caterpillars, eastern tiger swallowtail caterpillars which dine on leaves of tulip poplar, magnolia, and several other species, and pretty spicebush swallowtail caterpillars often found on sassafras as well as spice bush. Next time you see what appears to be a bird dropping resting on a leaf, take a second look and you might be treated to the discovery of one of many tiny masters of a frequently overlooked and unusual disguise.

Acknowledgements

Bug of the Week thanks Eloise for spotting the beautiful wood nymph butterfly and providing inspiration for this episode. The wonderful meadows, gardens, and forests of Cacapon State Park were the backdrop for observing several species of swallowtail butterflies. “Insect defenses” by David Evans and Justin Schmidt, and “Caterpillars of Eastern North America” by David Wagner were used as references.

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