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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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Several weeks ago, on June 21, Bug of the Week said farewell to Brood X periodical cicadas in the DMV. However, this week there is one more tale to tell about these iconic invertebrates. As interest in Brood X developed this spring, one of the most frequently asked questions was “how many cicadas will there be?” Although it was and is impossible to answer this question precisely, estimates of billions were often bantered about and those prone to hyperbole guesstimated numbers in the trillions. If that sounds like a lot of cicadas, ruminate on this for a moment. Recall that female cicadas lay eggs in the treetops and upon hatching, cicada nymphs tumble to earth and enter the soil to feed on tree roots. It is widely reported that each female cicada has the potential to lay from 400 to 600 eggs during the course of her lifetime. Suppose that only 10 billion female cicadas lived to reproduce fully and each laid a modest 400 eggs. This would translate into a several trillion cicada nymphs falling from trees in the eastern half of the United States courtesy of Brood X. Yikes!

This week an eminent member of the press corps inquired about potential showers of cicadas raining down on citizens in the DMV as billions of cicada nymphs hatched from eggs imbedded in small branches and tumbled to the earth below. The ancillary question was, of course, would people notice the cicada shower? Recalling a recent dewy shower of cicada pee while studying cicada behavior, I anticipated a repeat performance while working under trees wearing scores of flagging branches and hundreds of healthy branches bearing thousands of cicada egg nests. To date I have not experienced a deluge of cicada nymphs dropping from trees, despite the fact that egg-hatch is in full swing as of this writing.

During most of their embryonic development, little detail can be seen of the cicada nymph within the egg. Within days of hatching however, eyes become visible in hues of red or brown. Using muscular contractions, cicada nymphs slither from the confines of their eggshell, ready to skedaddle in a matter of minutes. After tumbling from the treetop to earth, they join thousands of brood mates entering the soil in search of plant roots that will sustain them for the next 17 years.

Part of the explanation why I and most other people will not notice periodical cicadas as they hatch and fall to earth is the minute size of cicada nymphs. At only a couple millimeters in length they are very hard to spot. Their small body mass may allow breezes to carry them beyond the dripline of trees from which they hatched. Although siblings from a single egg nest hatch somewhat synchronously, each cicada takes a few minutes to escape from an egg, thus causing hatching to be dispersed in time. Remember too that egg laying was distributed over several weeks in May and June. This further spreads out the rainfall of hatchlings in time. As evidenced by the massive numbers of trees with flagging branches, whether or not you encounter a shower of cicadas falling from trees, rest assured that barring some unforeseen catastrophe Brood X will return in force in 2038 here in the DMV. 

Acknowledgements

Bug of the Week thanks Paula Shrewsbury for contributing images and commentary for this week’s episode. We thank the writers of the Washington Post for providing inspiration for this episode and for their enthusiastic, continuing in-depth coverage of Brood X. Much of the background information for this episode and others was taken from “The ecology, behavior and evolution of periodical cicadas” by K. S. Williams and C. Simon, and references therein.

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The DMV abounds with some of the best nature centers in the country. Two of my favorites are Woodend Sanctuary, nestled inside the Beltway in Chevy Chase, Maryland and home of the Audubon Naturalist Society, and the Howard Conservancy, a 232-acre wildlife sanctuary in Woodstock, Maryland. Summer is always an exciting time to take a suburban safari at these refuges when some of Mother Nature’s wildest flowers are at their best. On a recent trip to the Howard Conservancy, large patches of Heliopsis, a.k.a. false sunflower and oxeye, were in dazzling full bloom. Oxeyes are full bodied members of the aster clan and our native species are dynamite attractors for many interesting insects.

While immersed among some oxeyes, I was surprised to find hordes of gorgeous seed bugs busily sucking nutrients from the developing flowers and engaging in their buggy mating rituals amongst the blooms. These little beauties are close relatives of other true bugs like the milkweed bugs and scentless plant bugs we’ve met in previous episodes. In fact, early records often confused this species with small milkweed bugs and recorded it as a denizen of milkweed. Careful observations by one Reverend James M. Sullivan of St. Louis, Missouri, helped clarify the true pattern of food choice of false milkweed bugs. However, the name false milkweed bug is a bit of a misnomer, as these little rascals actually are connoisseurs of members of the aster family.

Heliopsis is a great plant to spot false milkweed bugs. Watch as the bug on the left nonchalantly grooms its antennae while its mate taps its left hind leg somewhat impatiently while dining on a floret. Bugs are pretty entertaining.

After choosing a handsome mate and consummating the relationship, the female false milkweed bug lays eggs in batches of 15 to 50. Eggs hatch and bright red nymphs use their elongated sucking mouthparts to sip fluids from the plant. Adults can live two months, and along with their nymphs these beauties can be seen on oxeye from June through August in many parts of the country. Bug of the Week recommends a trip to the meadows and gardens at Woodend Sanctuary and the Howard Conservancy to glimpse members of the aster clan. If you encounter glorious oxeyes, take a few moments to hunt for false milkweed bugs.

Acknowledgements

We thank Woodend Sanctuary and Howard Conservancy for providing inspiration for this episode. The interesting article, “On the Biology and Food Plants of Lygaeus turcicus (Fabr.) (Hemiptera: Lygaeidae)“ by James A. Slater, was used to prepare this episode.

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Homonyms, words with two or more meanings, are interesting constructs of any language. For example, take bark, it has a couple meanings. It can be the corky tissue protecting the outside of a tree or the vocalizations of furry, four-legged mammals that like trees for, well, you know. My dictionary defines squash as a noun meaning “vegetable of the gourd family” and as a verb meaning “to crush something with pressure.” Let’s look at the first meaning of squash. Squash, zucchini, pumpkins, and gourds are all delectable and nutritious members of the cucurbit family. However, humans are not the only ones that find these prickly plants scrumptious. In a previous episode of Bug of the Week, we met the dastardly squash vine borer, a caterpillar with the power to wilt even the toughest pumpkin vine. While admiring some squash vines in a community garden, I noticed several plants with wilted, yellow leaves. A closer inspection failed to find bad borers, however, the leaves of the squash were loaded with immature and adult squash bugs merrily sipping sap.

Squash bugs are members of the true bug clan, meaning they have an elongated beak for sucking liquid food, wings that are part membranous and part leathery, and, as juveniles, they are known as nymphs. We met other members of this cantankerous clique including bed bugs, brown marmorated stink bug, boxelder bug, and wheel bug in stories past. Both nymph and adult squash bugs consume fluids from their cucurbit hosts and problems arise when dozens of feeding squash bugs jab so many beaks into the vascular system of the plant. Inserting the beak damages the plant’s vascular system. In addition, squash bugs can transmit a nasty bacterium, Serratia marcescens, causing what is known as cucurbit yellow vine disease. These insults and removal of vascular liquids cause plants to wilt. When squash bugs are abundant, their damage can reduce the bounty produced by your squash and zucchini vines.

Who doesn’t love pattypan squash? But look out pattypan, those clusters of bronze eggs will soon hatch into tiny green and black squash bug nymphs. As they grow and molt, white wax cloaks their bodies. If you see adults roaming on your cucurbits, or discover eggs and nymphs, snatch them off and dispose of them in whatever way you like.

Fortunately, there are a few tricks you can use to foil the squash bug’s shenanigans:

1)       At the end of the year, rid your garden of decaying vegetation and remnants of vines and leaves. These refuges are used by adult squash bugs to survive the wild winter.

2)       In spring, plant varieties such as Butternut, Royal Acorn, or Sweet Cheese that are more resistant to squash bugs.

3)       I have spoken to gardeners who place floating row covers over their plants early in the season to help keep these buggers from colonizing their plants. If you go this route, remember to remove row covers when blossoms first appear. If you don’t, then pollinators cannot do their job. No pollination means no pumpkins, squash, or zucchini.

Been waiting for the second meaning of squash? If you see squash bugs, squash the squash bugs. Really, if you have just a few plants, it is relatively easy to inspect plants and when you find the golden-bronze eggs, green or whitish nymphs, or tawny adults, crush them.

If squashing squash bugs isn’t your thing, remove them from the plant and drop them into a vessel of soapy water. Squash bugs are poor swimmers and when they have expired their tiny bodies can be placed in the compost heap to nourish your garden next season. Have a squashing good time!  

Acknowledgements

Bug of the Week thanks community gardeners of Montgomery and Howard Counties for providing the inspiration for this episode and the backdrops for observing squash bugs.

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Echinacea, a.k.a. cone flower, renowned for its medicinal qualities as well as its beauty, is blooming at full throttle this week in gardens here in the DMV. Just after sunrise, the fragrance of cone flowers is delightful not only to humans, but also to a raft of pollinators. Bumble bees are usually the first arrivals in the morning, but shortly thereafter industrious bees, members of the halictid clan, arrive to collect nectar and pollen for youngsters in their colonies. The name “sweat bee” is a little goofy and somewhat misleading. These bees don’t sweat, but like sweat bees we visited at Bug of the Week some years ago, some halictids alight on humans and imbibe salt-rich perspiration. Guess this is why they are called sweat bees.

Unlike mason bees and plasterer bees we met in previous episodes, which are solitary with every female caring for her own young, halictid bees vary in their social structure. Some species like Agapostemon virescens adopt the solitary life style, while others like Halictus ligatus are eusocial (truly social) with queens producing non-reproductive daughters known as worker bees, tasked with foraging for nectar and pollen and tending the brood of their mothers. Colonies are founded in spring by females, survivors of winter’s ravages. The sweat bees we’re visiting this week adore cone flowers as a source of food, and both build nests in soil. Founding queens of Halictus ligatus gather pollen and lay eggs that hatch into daughters, worker bees, destined to help with caring for the young and gathering food for the colony. Some of these daughters will eventually become reproductively active and produce daughters and sons of their own. A fascinating study of Halictus ligatus by Miriam H. Richards and Laurence Packer discovered that when weather conditions were favorable, worker bees survived at greater rates and grew larger. Some of these large workers produced their own offspring. However, under less favorable conditions of temperature and food availability, workers were smaller and produced fewer offspring of their own, submitting to larger, more aggressive queens that produced the majority of offspring. Shifting conditions of temperature and food availability govern the social dynamics of halictid bees like Halitcus ligatus.

Cone flowers are a gold mine for many kinds of pollinators. Watch as female halictid bees, a.k.a. sweat bees, gather and store huge loads of pollen in pollen baskets, called corbiculae, on their hind legs. The smaller bee in the first two clips is Halictus ligatus or its sibling species, Halictus poeyi, and the last clip is the gorgeous Agapostemon virescens.  

Beautiful Agapostemon virescens, sometimes called “little green bees”, favor loamy soils with sparse vegetation as prime real estate to build their nests. Individual nests may contain more than 100 brood cells and, while more than one female has been observed in a single nest, this species is considered to be solitary. Once they find favorable plots of land, many Agapostemon virescens queens may move in and form large aggregations of nests. To enjoy these delightful native pollinators, consider letting part of your lush lawn go a little thin in a sunny spot to provide nest sites for nurseries of these bees. Both species of sweat bees and many others of their clan are generalist pollinators and a diversity of flowering plants provide food. However, on this week of Independence Day celebrations, Echinacea and other members of the Asteraceae are dynamite attractors of these beauties to your garden.  

Acknowledgements

Bug of the Week thanks Dr. Paula Shrewsbury for planting cone flowers that served as the backdrop for these delightful bees. Great references consulted for this episode include “Bees of Northwestern America: Agapostemon (Hymenoptera:Halictidae)” by  Radclyffe B. Roberts, “The Socioecology of Body Size Variation in the Primitively Eusocial Sweat Bee, Halictus ligatus (Hymenoptera: Halictidae)” by Miriam H. Richards and Laurence Packer, “The Insect Societies” by Edward O. Wilson, “The bees of the world” by Charles D. Michener, and “Bees, wasps, and ants” by Eric Grissell.  Special thanks to Sam Droege for identifying the halictids featured in this episode.      

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Last week scorching temperatures in our region provided an excellent reason to head for the hills for a cool hike. Forest trails of the Catoctin Mountain Park provide both the perfect location to escape summer’s heat and an opportunity to visit some of Mother Nature’s most interesting recyclers of organic compounds, millipedes. Millipedes are detritivores, creatures that consume organic matter including mosses, algae, and decaying vegetation that carpet the forest floor. Millipedes belong to the subphylum of the arthropods called Myriapoda, those with “many feet.” Do they really have a thousand feet? Nah, they don’t really have feet, but they do have legs and the record number of legs for a millipede is somewhere around 750. However, most millipedes have fewer than 400 legs. Young millipedes have only a few body segments, each of which bears a single pair of legs. As millipedes molt and grow, body segments with two pairs of legs each are added. Millipedes live two to seven years and can produce hundreds of offspring during their lifetime. Millipedes do not bite or sting, but several species secrete noxious chemicals from glands lining the margins of their body.  

On a stony forest trail, we happened upon two remarkable members of the millipede clan. The first was Narceus americanus-annularis, one of the true giants of the millipede world in North America. Its otherwise dull brown body was accented with beet-red legs and red bands encircling the body at each segment. These colors might serve as a warning of noxious defenses ready to be unleashed by the millipede. Unable to resist handling Narceus earned me an acrid reward of benzoquinones, foul smelling droplets of the millipede’s chemical defense. A bit further down the trail, we encountered one of the flat-backed millipedes, perhaps Apheloria virginiensis. Blending in with the forest floor was clearly not this creature’s game as it sported lemon-yellow legs, alternating bands of yellow and black along the back, and pink-hewed margins.  

Many legs of the millipede work in a wavelike fashion to propel these timid grazers as they search for organic matter to eat. Watch as it munches moss on the surface of a boulder. Nearby, another millipede dashes across a forest trail. Bright contrasting colors may warn predators not to mess with these chemically defended detritivores. Defensive compounds released by this millipede smell like almond extract.

When plucked from the ground, the smell of almonds filled the air and brought me back to my days of organic chemistry when benzaldehyde was the unknown compound on the lab practical. Benzaldehyde is the compound found in bitter almonds and is used as a flavoring in almond extract. Ah, but benzaldehyde is not the only compound found in Apheloria’s defensive secretion. The other more lethal moiety released by the millipede is hydrogen cyanide, a highly toxic irritant. Glad no one sniffed this millipede too deeply! Millipedes, important recyclers of organic matter, are well-protected by potent toxins and irritants that could make a bird, lizard, or toad think twice about messing around with these denizens of the forest floor.

Acknowledgements

We thank the intrepid hikers of the Catoctin Mountain: Erin, Ellie, Abby, Maggie, Jo, Paula, Laurie, and Kevin for providing inspiration for this episode. Thomas Eisner’s books “The Love of Insects” and “Secret Weapons” were used as resources for this story.

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Over the last three months, we explored the ecology, behavior, and interactions of Brood X periodical cicadas with humans, other animals, and plants in the eastern United States.  We visited cicadas underground, watched the emergence of cicada nymphs from their subterranean crypts, discovered how male cicadas make noisy and ethereal sounds, learned how to tell the boys from the girls, watched as hungry predators enjoyed the cicada bounty, witnessed mating and oviposition by females, shuddered at their lethal STD, and witnessed havoc wrought upon young trees when females laid their eggs. In the waning weeks of June here in the DMV at locations that had formerly been zones of rollicking cicada choruses a few weeks ago, there is now an eerie silence. The reason for the hush is immediately apparent. Beneath trees and shrubs, the ground is littered with thousands of spent exuviae, the shed skins of cicada nymphs, and the bodies of adult cicadas that survived the onslaught of hungry sparrows, blue jays, hawks, mocking birds, foxes, raccoons, squirrels, dogs, and dozens of other predators. Also scattered about are wings and fragmented body parts of cicadas that were not so lucky. Macabre half cicadas with abdomens transformed into fungal fruiting bodies litter the ground, victims of Massaspora. Hungry ants and maggots repurpose cicada protein and return it to the food web. Along neighborhood roadways and sidewalks greasy black patches mark the spots where cicadas found their final resting place beneath the tires of vehicles or shoes of humans.

What other legacies did cicadas leave behind? For the natural world they provided a bounty for countless predators. This year, birds produced larger clutches more frequently and nestlings were heavier and enjoyed greater survival (Anderson 1977, Strehl and White 1986). As cicadas return to the earth upon death, decomposition of their remains creates a pulse of nutrients to the forest floor boosting microbial biomass, adding nutrients, and enhancing the growth and reproduction of understory plants (Yang 2004). Similar changes in microbial communities occur when cicadas enter streams (Menninger et al. 2008). Young trees took a beating when female cicadas laid eggs in small branches but trees protected by cicada netting were largely spared from this injury (Ahern et al. 2005). Although established mature trees experienced cicada injury, they are able to shrug off this insult in the long run (Clay et al. 2009).

As the door closes on Brood X in 2021, cicadas leave behind billions of holes to aerate the soil, a feast for necrophagous creatures like tiny hungry ants, oily spots along roads where cars were the grim reaper, sidewalks littered with shells and carcasses, and tons of shed skins and decaying bodies that return nutrients to the soil. A solitary late comer to the party wanders amongst earthly remains of its brood mates.  Auf wiedersehen Brood X, until next time in 2038.

For cicadaphiles, this was a season to enjoy a truly remarkable event that happens nowhere else on earth except in the eastern Unites States, only a handful of times in a lifetime. Cicada lovers from as far away as California planned family vacations to enjoy the emergence of Brood X while cicadaphobes tried to cope with the immensity of the event in various ways. Some strove to understand cicadas better and were counseled on how to cope with these red-eyed critters, while others planned a cicada escape. One friend had plane tickets at the ready and when warned that emergence was just around the corner, he departed for a cicada escape to Idaho. For Bug of the Week, it was a welcome break from COVID and other strange events of the past year and a fine time to share a few bug stories. Here in the DMV cicadas had a spectacular run and the teenage offspring of Brood X, 2021 will return in 2038 for their day in the sun.

Acknowledgements

Bug of the Week thanks members of the Cicada Crew, journalists of print and non-print media, neighbors of the Allview community in Columbia, MD, and cicada geniuses that devote their professional careers to studying these strange, whacky, and mysterious insects, who all assisted in the telling of cicada stories. The following references were used to prepare this episode: “Reproductive responses of sparrows to a superabundant food supply” by T. R. Anderson, “Effects of superabundant food on breeding success and behavior of the red-winged blackbird” by C. E. Strehl and J. White, “Periodical cicadas as resource pulses in North American forests” by L. H. Yang, “Periodical cicada detritus impacts stream ecosystem metabolism” by Holly L. Menninger, Margaret A. Palmer, Laura S. Craig, and David C. Richardson,“Effects of oviposition by periodical cicadas on tree growth” by Keith Clay, Angela L. Shelton, and Chuck Winkle, and “Comparison of exclusion and imidacloprid for reduction of oviposition damage to young trees by periodical cicadas (Hemiptera: Cicadidae)” by R. G. Ahern, S. D. Frank and M. J. Raupp.

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With thousands of cicadas chorusing in the treetops this week, male cicadas wooed potential mates and females chose the fathers of their offspring. Within hours of mating, females move to the tips of branches and select soft greenwood terminals about the diameter of a pencil to be the nursery for their offspring. Using a sharp egg-laying appendage called an ovipositor, the female cicada slices narrow slits parallel to the long axis of the branch. She then inserts her hollow ovipositor into the branch and with powerful abdominal contractions, she pumps between 20 and 30 tiny eggs into each slit, creating what is technically known in cicada-speak as an egg nest. Each female has the potential to lay between 200 and 400 eggs during the course of her two to four-week lifespan. In the warmth of summer days and nights, cicada eggs develop and, after six to ten weeks, nymphs hatch and tumble to the earth below.

And now for the dark side of periodical cicadas – plant injury. Injury caused by adults as they sip xylem fluid is inconsequential. The real insult to woody plants comes from the wounds caused by female cicadas when they slice branches to insert eggs. Where densities are great, egg-laying, a.k.a. oviposition, causes tips of many branches to wither and die. Dying and dead terminals droop, resulting in a type of tree injury called flagging. Some injured terminals break and fall to the ground. Branches that do not break may eventually heal, but the wound-site may form a gnarly irregular swelling on the branch.

With the mating game completed, female cicadas fly to small branches where they deposit eggs in egg nests. Using her sharp egg laying appendage called an ovipositor, the cicada slices the branch and then deposits 20 to 30 eggs into each slit to form egg nests.

Which plants are most likely to be affected? The bad news here is that periodical cicadas are broad generalists. Miller and Crowley (1998) studied 140 genera of trees at the Morton Arboretum and found more than half sustained injury caused by ovipositing females.  Among the most severely affected were ones common to landscapes in the DMV, including Acer (maple), Amelanchier (shadbush), Carpinus (hornbeam), Castanea (chestnut), Cercidphyllum (katsura), Cercis (redbud), Chionanthus (fringe tree), Fagus (beech), Quercus (oak), Myrica (bayberry), Ostrya (hophornbeam), Prunus (cherry) and Weigela (weigela). Another study by Brown and Zuefle (2009) of 42 woody plant species added several new genera to the list and found all but 10 species were used by cicadas to lay eggs. This study found that native and non-native woody plants were equally likely to be used for oviposition by cicadas but alien plants, those with no other known congener (plants of the same genus) in the US, were less likely to be hosts for eggs.  

Dense populations of cicadas spell trouble for young trees. Scores of egg nests damage tender branches causing them to wilt, die, and eventually drop from the tree. Withered branches should be carefully pruned from the tree to promote wound closure and encourage restorative growth from axillary buds.

Factors beyond taxonomic identity of a plant affect decisions made by female cicadas regarding where to place their eggs. Small, compact plants often have fewer egg-nests compared to those with longer more open branching habits. Trees at the edges of forests with rapidly growing branches exposed to sunlight often sustain more cicada injury. These trends are easily seen in tree nursery stock and recently transplanted saplings found in yards, commercial landscapes, parks, and transportation corridors, especially those near established trees that historically support cicadas. While ovipositional injury poses a threat to newly planted trees, for older and well-established trees flagging and limb breakage may occur in the short term, however, studies indicate that the long-term threat to tree vitality is minimal (Miller and Croft 1998). Early successional trees exhibited no clear reduction in radial growth or overall growth rates of trees attacked by cicadas (Clay et al. 2009). For small trees and shrubs injured by cicadas, wait until the cicada season ends and remove the injured branches by carefully pruning them back to the nearest node before the cicada injury.

Acknowledgements

Much of the background information for this episode was taken from “Return of periodical cicadas in 2021: Biology, Plant Injury and Management” by Michael Raupp. Other great references include “Does the periodical cicada, Magicicada septendecim, prefer to oviposit on native or exotic plant species?” by W. P. Brown and M. E. Zueffle, “Effects of oviposition by periodical cicadas on tree growth” by K. Clay, A. L. Shelton and C. Winkle, “Periodical Cicada (Magicicada cassini) Oviposition Damage: Visually Impressive yet Dynamically Irrelevant” by W. M. Cook and R. D. Holt, “Effects of periodical cicada ovipositional injury on woody plants” by F. Miller and W. Crowley, and “The ecology, behavior and evolution of periodical cicadas” by K. S. Williams and C. Simon. We thank Randy and all the good folks in the Allview neighborhood for allowing us to visit, photograph, and film cicadas in their landscapes.

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