Green Beetle Hangers: The Tiny Fungus Riding Invasive Ladybugs, and the Boston Park Where Its Story Began

On the red, spotted back of a harlequin ladybug, you may see what looks like a tiny bunch of lemon-yellow bananas — taut, translucent, almost like balloons. They are a fungus, and a parasite: the green beetle hanger, Hesperomyces harmoniae, named "green" despite its color. Its story as a subject of science runs through Boston Harbor Islands National and State Park.

A fungus that doesn't act like one

Beetle hangers — Laboulbeniales, to mycologists — spend their lives hanging from the outer shells of beetles, flies, mites and other arthropods, feeding on their hosts. The green beetle hanger lives only on harlequin ladybugs.

• No hyphae. Most fungi spread thread-like branches through soil or wood to absorb nutrients. Beetle hangers skip that and grow only compact, spore-producing bodies called thalli — the "bananas" — just 100 to 300 micrometers tall, about the thickness of one or two human hairs.
• A drinking straw. Instead, root-like haustoria pierce the shell and draw nutrients from the ladybug's blood.

The ladybugs pay: earlier work found the infection can cut their lifespan by up to 50%, and infected beetles move and sense more slowly, mate less and struggle to survive the winter.

A biocontrol that probably isn't

That matters because outside Asia, the harlequin ladybug is invasive. It outcompetes native ladybugs for aphids, eats the eggs and larvae of other insects — including the monarch butterfly — and damages crops by eating nectar, pollen, fruit and young plant tissue. Some hoped the fungus could keep it in check. The researcher at the center of this work, Dr. Danny Haelewaters, has tested the idea and doubts the fungus alone will ever be an effective control agent, though pairing it with another disease-causing fungus might work better.

From a park inventory to a world database

Haelewaters came to the park as a Harvard mycology PhD student. The park, created in 1996, had begun inventorying its smallest residents in 2005 — unusual for a park, and more so for an urban one. A six-year all-taxa biodiversity inventory, funded by the James and Cathleen Stone Foundation with the Boston Harbor Islands Partnership and Harvard's Museum of Comparative Zoology, found 1,732 species of snails, slugs, spiders, beetles, bees, ants, millipedes and more on 18 of the park's 34 islands and peninsulas — far more than expected.

Having found beetle hangers on some of those insects, Haelewaters led the park's first fungi inventory, begun in 2012. In three and a half years his team collected nearly 1,000 samples, including 20 beetle hanger records and four species new to science.

For a century, beetle hangers had been lumped together as one species with regional quirks, which made them hard to study. In 2019 Haelewaters split them into a complex of distinct species, and the green beetle hanger became a species of its own. At the same time, iNaturalist and other participatory science platforms took off, and photographs of the fungus poured in from around the world. The result, published in February 2024, is a database tracking the fungus and its ladybug hosts together, worldwide.

What the records show

• Cities have far more sightings than suburbs or countryside. Urban warmth may keep aphids alive and ladybugs active through winter; and ladybugs pack together for warmth in winter shelters, knocking spores from one to another like a winter flu.
• In late spring and early summer, the fungus spreads by mating, turning up on females' backs and males' undersides.
• The ladybug's range is larger than the fungus's — news to mycologists, who had assumed they matched, and relevant to control efforts.

Community volunteers after a Stewardship Saturday at World's End, December 2023. NPS photo / E. Bernbaum.

Anyone can help

There aren't enough mycologists to watch every ladybug on Earth, and Haelewaters credits volunteers with filling the gaps — he wants them as partners, not just data collectors. Green beetle hangers suit it: though a fraction of a millimeter tall, their color makes them visible without a microscope. Haelewaters spotted his first on a harlequin on the wall of a Kentucky men's room. The park invites visitors to look for them on its Stewardship Saturdays and nature walks.

Sources

Based on "Green Beetle Hangers, the 'Banana-Balloon' Microfungi of the Boston Harbor Islands," by Elle Bernbaum, biological technician, National Park Service, Boston Harbor Islands National and State Park, drawing on de Groot and others (2024), Haelewaters and others (2020, 2024) and Rykken and Farrell (2013, 2018); a work of the United States government in the public domain; rewritten in hubnx's own words.