Last updated: December 10, 2024
Article
Green Beetle Hangers, the “Banana-Balloon” Microfungi of the Boston Harbor Islands
A dive into the vibrant life of the bright yellow, banana-balloon-shaped fungi that flourish in the Boston Harbor Islands & a journey into the islands’ vast microwilderness
By Elle Bernbaum, Biological Technician
Bizarre, near-translucent, lemon-yellow growths sprout like miniscule bunches of bananas from the deep strawberry hues and patchy dark spots of a ladybug’s back. With tissue taut and poppable, the odd little fungi look almost like banana-shaped balloons – but it’s no party. The parasitic growths are called "green* beetle hangers" (named in spite of their lemon hue), and they’ve made themselves at home in the exoskeletal armor of the harlequin ladybug.
Gilles San Martin
As the name might suggest, beetle hangers are a group of fungi that spend their lives "hanging" from the exoskeletons of their arthropod hosts – beetles, flies, mites, and the like. While they hang, they feed off of the nutrients in their hosts’ bodies. And some have specific cravings – green beetle hangers, for instance, only have appetites for harlequin hips.
These green beetle hangers are the stars of a February 2024 publication[1] by the fungus fan and principal investigator of the Boston Harbor Islands fungi inventory, Dr. Danny Haelewaters. Haelewaters has studied green beetle hangers for over a decade now, and with the help of community volunteers, he has developed a new, one-of-a-kind database that will shed light on the lives of these zany organisms.
Unknown credit. Permission from Danny Haelewaters.
The quest to study green beetle hangers has historically been hampered by species misclassification[2] and gaps in communication across science disciplines. But, after decades of inaccuracies and inattention, Haelewaters’ niche work finally clarifies our view of beetle hangers while closing the chasm between the worlds of mycology and entomology. The project tracks both green beetle hanger fungi and their ladybug beetle hosts simultaneously across the globe.
Tantalizing conclusions about this relationship and interactions with humans and environments are now – for the first time – starting to emerge. And the story of that work starts here, in Boston. But before we dig into the beetle hanger project, let’s talk more about just how these wild little creatures exist.
Let’s talk microfungal aliens.
Green beetle hangers are sort of the aliens of the microfungi world. Developmentally, evolutionarily, morphologically, these edgy little banana-shaped ladybug-lovers just don’t fit in.
Cost Calma
They don’t form customary hyphae – those thread-like branches that absorb nutrients from soils and substrates. Instead, beetle hangers get right to it and just grow spore-producing structures called thalli. Those thalli are compact and oblong – they're the banana-balloon-looking parts of the equation – and they are absolutely miniscule at only 100 to 300 micrometers tall. That’s just about as thick as one or two human hairs!
In lieu of hyphae, for sustenance, the microfungi grow little root-like straws called haustoria to slurp up nutrients from ladybug blood flowing beneath the exoskeleton. It’s a pleasant little harlequin slushy for a hungry vampiric fungus.
For context, that's pretty odd stuff in the world of fungi. Imagine a flower with no roots beneath the soil – just a straw slurping away. Better yet, imagine a flower growing on a chicken’s ear, slurping potassium from its blood!
Do harlequin ladybugs, Harmonia axyridis, mind their vampiric tag-alongs? Probably.
Previous work by Dr. Haelewaters shows that this fungal infection can reduce ladybugs’ lifespans by up to 50%.[3] Infected ladybugs move more slowly, sense more slowly, mate less, and struggle more to survive over winter.
But before you start rooting against these microfungal outsiders, you might like to know one of the reasons why they’ve attracted some interest outside of the pure pursuit of knowledge.
Throughout most of the world, excepting Asia, harlequin ladybugs are invasive and pesty. They out-compete native ladybugs[4] [5] as they hunt for aphids, and they eat other insects’ eggs and larvae, like those of the endangered and much-applauded monarch butterfly[6] (Lepidoptera: Nymphalidae). Harlequins also love to munch on nectar, pollen, fruit, and young plant tissues, making them unpopular characters among farmers (and native plant stewards).
Andre De Kesel
Some suggest that green beetle hangers may just be the biocontrol that we need to slow the (damaging) roll of these harlequin hoards. It’s the very reason why Haelewaters has done research on ladybug mortality – he's testing the idea.
It seems though that the green beetle hangers on their own just aren’t the killers that some might like them to be. Haelewaters isn’t convinced that the fungi can develop into biocontrol agents. To quote him, "Do I really think that this is going to be possible? Ehhh...I don’t think so."
But, Haelewaters says, partnering beetle hangers with another pathogenic fungus could bring out more of its fatale flare.
Haelewaters and his team have not machinated a long-sought-after ladybug killer just yet, but they have learned quite a lot about beetle hangers.
Unknown credit. Permission from Danny Haelewaters.
Combing the Boston Harbor Islands for Little Creatures
The beetle-hanger-and-beetle duo have taken up residence all around the globe, with sightings recorded from Helsinki to San Diego, but Boston is particularly well represented among those sightings. That’s because for about a decade, the leader in green beetle hanger research, Danny Haelewaters, conducted his research here at the Boston Habor Islands National and State Park.
Haelewaters first joined park science efforts as a PhD student in mycology at Harvard University. He started off partnering on missions to find and document insects and shell creatures (invertebrates) since fungi foraging wasn’t yet on the park radar. The still-young park, only just created in 1996, was in the early beginnings of its microwilderness exploration.
Unknown credit. Permission from Danny Haelewaters.
The launch of the park’s invertebrate inventory in 2005 marked its first serious venture into inventorying the small stuff in the Boston Harbor Islands. That was an unconventional move for the park and remains an uncommon practice today. Most national and state parks focus on more traditionally spotlighted kingdoms of life. (Ladybugs don’t demand attention like a seven-foot bear does.)
With funding from the James and Cathleen Stone Foundation, collaborators from the Boston Harbor Islands Partnership and the Museum of Comparative Zoology at Harvard University launched the six-year-long all-taxa biodiversity inventory (ATBI) for invertebrates without much expectation of finding rich biodiversity. The park had a keen interest in learning about its littlest residents, but it was, after all, an urban space with a rich history of human alteration and habitat disturbance. How much could they really find?
A lot, as it turns out – exponentially more than most would expect from an urban park.
Unique species of snail, slug, spider, clam, beetle, bee, flea, weevil, ant, millipede, and kin numbered 1,732 by the end of the survey,[7] which covered eighteen of the park’s thirty-four islands and peninsulas. The results were a window into the beautifully varied and colorfully tiny world around. Simple species counts transformed into new and fascinating insights and bolstered park staff to keep focus on the islands’ little creatures.
Haelewaters, whose eyes had been trained on beetle hanger research for years, jumped at the opportunity to spearhead the Boston Harbor Islands’ first fungi inventory. He had already found beetle hangers on the backs of some of the beetles in the invertebrate inventory, but he aimed to find more. In 2012, the park began its mushroom-gazing era.
Unknown credit. Permission from Danny Haelewaters.
After three and a half years, Haelewaters and his team collected nearly 1,000 samples across a handful of islands. Among those records were 20 sightings of beetle hangers and four species new to modern-day knowledge – never before formally described.[8] [9]
The records doubled as early data for Haelewaters’ burgeoning beetle hanger research. By the end of his time in the field, he had made leaps and gains in unraveling the poorly understood nature of beetle hangers.
Sorting Out Species
For a long and confused time, all beetle hangers, or laboulbeniales for the mycologists in the room, were considered just one species that happened to show slight differences across regions and beetle sexes. It was an oversimplified and poor classification of the misunderstood collective of various beetle hanger species. Paired with technical obstacles, this century-old misunderstanding has historically hindered inventory of beetle hangers and made for very slow and somewhat messy study.
That changed in 2019 when Haelewaters reclassified the microfungi as a complex of various unique species. Hesperomyces harmoniae, the green beetle hanger, broke from its muddled umbrella category to become its own species under his watch.
Around the same time, more people were embracing Participatory (a.k.a. "Citizen") Science platforms like iNaturalist, which make it possible for anyone to participate in species discovery and documentation. The joint impacts of Haelewaters’ reclassification and growing science tech led to an explosion in documentation – a mushroom cloud of records of green beetle hangers across the globe, which energized the field of beetle hanger research with unprecedented data.
Joseph Warfel
Today, Haelewaters continues to study different forms of beetle hangers but is particularly keen on green beetle hangers – the eclectic species with ladybug-specific tastes.
So what have we found? Ladybugs cuddling and STDs.
Haelewaters and his team may not have machinated a long-sought-after ladybug killer just yet, but they have found fascinating conclusions about these freaky little fungi. Tracking sightings of the fungus-ladybug duo alongside information like local human population, land use, and vegetation cover have given Haelewaters and his team a sense of how these disease-like fungi spread.
To start, researchers have found that there are far more recorded beetle hanger sightings coming from cities than from rural and suburban areas.
This could affirm the "urban island heat effect" on ladybugs. Cities tend to be warmer than rural and suburban areas because clustered buildings and pavements retain heat well. This warmer microclimate means that aphids can better survive in the cold winter, and ladybugs can continue snacking on urban-specific foods throughout an otherwise sleepier, less active season of hibernation-like snoozing. More activity could mean more opportunity for beetle hanger sightings or more opportunity for spreading spores.
The disparity in city-suburbia-boonies sightings could also be chalked up to the ladybugs’ social lives. Winter is cuffing season…or at least, cuddling season for ladybugs. Human-built shelters are plentiful in cities and can serve as haunts for urban ladybugs in search of warmth. Nests of nestling beetles can double as fungal spreading grounds, as spores are knocked from one beetle body to another – almost like spreading a winter flu through close social contact.
Andre DeKesel
The fungal infection also looks different in warm weather. When ladybugs get frisky in late spring and early summer, green beetle hangers spread more like pesky little STDs. They pop up on females’ backsides and males’ undersides after ladybug boot-knocking.
Separate from the mechanisms of fungal spread, Haelewaters and his team have furthermore learned about the habitable range of green beetle hangers and ladybugs. The range for ladybugs stretches further than that of the fungus, and that's news to mycologists. Previously, researchers thought the two to be the same. This could inform how scientists choose to move forward with invasive ladybug control measures.
Participatory Scientists: MVPs of Beetle Hanger Research
Haelewaters stresses that none of these conclusions would be possible without Participatory Science initiatives. Platforms like iNaturalist play a key role in the research that he and his team do. There aren’t enough mycologists to focus solely on green beetle hangers across the globe, so non-scientists who enjoy ladybug-spotting on their sunny Saturdays play a critical role in the success of the research.
Community volunteers help professional scientists who are almost always financially and logistically limited and who rarely have enough personnel to do big, sweeping fieldwork worldwide, Haelewaters says. They do a tremendous job of filling geographic gaps in field data collection.
Haelewaters hopes to encourage enthusiasts to get involved in the science. He’s looking for fungi foragers who are "not just servants of data but partners in crime."
NPS Photo/E. Bernbaum
Green beetle hanger research is particularly well suited for anyone to get involved. A green beetle hanger may only be a fraction of a millimeter in height, but because of its bright color and contrast against most beetle backs, bellies, heads, and hairs, we can see it without the help of a microscope.
From the walls of a Kentucky men’s room, where Haelewaters spotted his first green beetle hanger on a harlequin, to the grasses of Boston, where Haelewaters’ green beetle-hanger research took off, and all around the rest of the world, the beetle-hanger-harlequin duo can be found galumphing through our everyday lives.
Join in on the next Boston Harbor Islands Stewardship Saturday or one of our nature walks if you’d like to try to share a glimpse at a beetle’s back fungus with friends. Or, go beetle-hanger spotting anytime on your own. Danny Haelewaters and his team of researchers will thank you. For more dumbfounding details and images of photogenic fungi, check out Haelewaters’ beetle-hanger website. To read more about the species and science of the Boston Habor Islands, visit our Boston Habor Islands Science Stories page.
Gilles San Martin
*(The researcher responsible for naming the green beetle hangers is rumored to have sported blue-tinted sunglasses during the naming ritual.)
Special Thanks...
to Danny Haelewaters for his time, enthusiasm, and ongoing dedication to exploring the vast diversity of Earth’s fungi.
Footnotes
[1] de Groot, M.D., Christou, M., Pan, J.Y. et al. Beetlehangers.org: harmonizing host–parasite records of Harmonia axyridis and Hesperomyces harmoniae. Arthropod-Plant Interactions 18, 665–679 (2024).
[2] Haelewaters, D., Matthews, T. J., Wayman, J. P., Cazabonne, J., Heyman, F., Quandt, C. A., & Martin, T. E. (2024). Biological knowledge shortfalls impede conservation efforts in poorly studied taxa—A case study of Laboulbeniomycetes. Journal of Biogeography, 51, 29–39.
[3] Haelewaters, D., Hiller T., Kemp, E., Wielink, P., Aime, M., Nedvěd, O., Pfister, D., & Cottrell, T. "Mortality of native and invasive ladybirds co-infected by ectoparasitic and entomopathogenic fungi." BioRxiv. 2020.
[4] The Wildlife Trusts. "Harlequin Ladybird." The Wildlife Trusts. Pub. Date NA.
[5] Invasive Insects: Risks and Pathways Project. "Harlequin Ladybird." Invasive Insects: Risks and Pathways Project. 2020.
[6] Koch R.L., Venette R.C., Hutchison W.D. Predicted impact of an exotic generalist predator on monarch butterfly (Lepidoptera: Nymphalidae) populations: a quantitative risk assessment. Biological Invasions. 2006. 8: 1179.
[7] Jessica J. Rykken and Brian D. Farrell. "Boston Harbor Islands All Taxa Biodiversity Inventory." National Park Service. 2013. Technical Report NPS/BOHA/NRTR - 2013/746.
[8] Jessica J. Rykken and Brian D. Farrell. "Exploring the Microwilderness of Boston Harbor Islands National Recreation Area: Terrestrial Invertebrate All Taxa Biodiversity Inventory," Northeastern Naturalist 25(sp9), 23-44, (1 January 2018).
[9] Haelewaters D.,*, Dirks A., Kappler L., Mitchell J., Quijada L., Vandegrift R., Buyck B., and Pfister. "A Preliminary Checklist of Fungi at the Boston Harbor Islands," Northeastern Naturalist 25(sp9), 45-76), (1 January 2018).