What Is a Bugnado? When Insect Swarms Look Like Tornadoes
A vortex of whirling bugs? A bugnado seems like something straight out of a sci-fi movie! See how and why they form and what to do if you see one coming your way!
Quick Reference
- What it is: a swarm of insects (most often midges or mayflies) packed densely enough that the column resembles a tornado funnel from a distance.
- What is actually happening: a male-mating swarm called a “lek.” Male midges form a tower of pheromone-rich air to attract females.
- Where they form: over water or wet farmland, where the larvae grew up. Lakes, marshes, flooded fields, and slow-moving rivers are the hot spots.
- How big: a typical bugnado runs 10 to 100 feet wide and can extend 50 to 200 feet vertically. Some over the Great Lakes have been measured at 500+ feet tall.
- Hazard level: harmless to people. Bugnado-type swarms can reduce visibility for drivers, leave a slick coat of insect bodies on the road after mating ends, and produce a very memorable bug-coated windshield.
The word “bugnado” sounds like a movie villain. The reality is somewhere between a science-class lesson and a viral video. Every late spring and early summer, swarms of insects gather over water and wet fields, packed densely enough to look (from a distance, on a phone screen) like small tornadoes. Storm chasers, farmers, and lake-country drivers have been watching these columns for decades. The swarms themselves are mating leks. The funnel shape is one part wind, one part biology, and one part the way a billion small bodies cluster in a thermal updraft. Here is what is actually going on inside a bugnado, why the funnel shape, and how to tell one from a tornado at a glance.
What a Bugnado Actually Is
A bugnado is a swarm of small flying insects (most commonly midges, mayflies, and certain species of gnats) packed so densely that the swarm itself takes a tornado-like vertical column shape. The swarm is real and alive. The funnel shape is partly geometric (insects tend to gather around an updraft) and partly social (insects flying in a swarm align their flight to other swarm members, producing a coordinated mass).
The swarms are almost always made up of male insects. The reason is reproduction. Males of many midge and mayfly species form what entomologists call a “mating lek”: a dense, pheromone-rich flying column that attracts females from a wide surrounding area. The denser the column, the stronger the pheromone signal, and the better the chances of attracting mates. Once a female enters the swarm and chooses a partner, the pair drops out of the column and mates in the air or briefly on a nearby surface.
Why the Funnel Shape
Three forces produce the tornado-like shape:
- Thermal updraft: warm air rising over a sun-warmed surface (especially asphalt, fields, or shallow water) creates a column of rising air. Insects use the column to gain altitude with minimal flapping effort.
- Gentle wind shear: light wind near the ground, lighter wind aloft, can twist the swarm column into a tapered shape that mimics tornado morphology.
- Swarm geometry: insects in a swarm orient toward each other. The densest concentration sits in the column’s center. The lighter outer fringe creates the soft tapered edges that read as a tornado funnel from a distance.
Bigger swarms with more wind shear get taller. A perfectly still, calm-air swarm tends to be more spherical or pillar-shaped. A swarm in light wind elongates into the funnel shape that looks (and photographs) most dramatically like a tornado.
When and Where Bugnadoes Form
Bugnadoes are seasonal. The peak window across most of North America runs from late May through early August, during the breeding cycle of the dominant midge and mayfly species. The geography is predictable:
- Great Lakes: midges and mayflies hatch in massive numbers off Lake Erie, Lake Michigan, and Lake Superior. The Lake Erie shoreline (Cleveland, Toledo, Detroit) gets some of the most spectacular swarms in the world. Mayfly hatches on Lake Erie have been visible on Doppler radar.
- Mississippi River and major tributaries: mayfly hatches in late June and early July produce swarms that can blanket bridges and small towns.
- Plains lakes and reservoirs: midges hatch from shallow waters, with peak swarms in late afternoon and early evening.
- Flooded farmland: after spring rains, flooded fields produce midge hatches that swarm in funnel formations the next clear evening.
- Ponds and marshes throughout the temperate U.S.: smaller-scale bugnadoes form daily during the warm season.
The time of day matters. Most swarms form late afternoon to early evening, when ground heating produces strong thermals and males begin their mating displays. A swarm that has formed at 7 PM may dissipate by 9 PM as the thermal collapses with sunset.
The Mayfly Hatch on Lake Erie
The annual mayfly hatch on Lake Erie is the most spectacular bugnado source in North America. Mayflies (the family Ephemeridae) spend most of their life cycle as aquatic nymphs in lake-bottom mud. They emerge from the water as winged adults in massive synchronized waves between mid-June and late July, often peaking around the summer solstice.
The adult mayflies live only 24 to 48 hours, just long enough to mate and lay eggs. During those hours, they are a phenomenal pulse of biomass: a single Lake Erie hatch can produce billions of mayflies in a 48-hour window. The swarms regularly show up on local Doppler radar (the National Weather Service in Cleveland and Detroit posts annual mayfly-hatch radar imagery, sometimes joking about a “mayfly tornado warning”). Streetlights, gas station canopies, and lakeside neighborhoods become coated in adult mayflies for the duration of the hatch. The lake’s fish, especially walleye and perch, follow the schedule and gorge during the brief window.
How to Tell a Bugnado from a Real Tornado
From a distance, a bugnado can superficially resemble a tornado. Several quick checks distinguish them:
- The sky overhead. Tornadoes form under towering supercell thunderstorms with dark, rotating wall clouds and visible mesocyclone structure. Bugnadoes form on calm or partly cloudy days with no parent storm cell.
- Movement. Tornadoes move with their parent storm at 20 to 60 mph. Bugnadoes hover over a single spot or drift slowly with light wind.
- Color and texture. Tornadoes are gray, brown, or sometimes nearly black, with visible debris cloud at the base. Bugnadoes have a softer, gauzier look, often with a slight gold or amber color from the insects themselves.
- Scale. Most bugnadoes are 10 to 100 feet wide and 50 to 200 feet tall. Tornadoes are typically 100 to 1,000+ feet wide and extend continuously from cloud to ground.
- Ambient noise. Tornadoes are loud, often described as a freight train. Bugnadoes are silent unless you stand close enough to hear the wing buzz.
Storm chasers and trained spotters distinguish bugnadoes from gustnadoes, scuds, dust devils, and other tornado-like phenomena routinely. Most bugnadoes get caught on phone video and shared on social media as “what is this thing in my yard?” once a year, somewhere in the Great Lakes basin.
The Drive-Through Bugnado
For drivers along Lake Erie, the Mississippi, or other big-water bugnado country, the swarms are a known summer hazard. Three things to know:
- Visibility drops. A dense swarm crossing a road can reduce visibility to a few car lengths for 15 to 30 seconds. Slow down and stay in your lane.
- The road gets slick. After mating ends and the male insects die, their bodies coat the road in a thin oily film that can be surprisingly slippery. Roads near major bugnado zones (especially bridges over bugnado lakes) sometimes need pressure-washing or sand-spreading after a major hatch.
- Your windshield will be coated. Some Lake Erie drivers report stopping every 5 to 10 miles during peak hatch to clean the windshield enough to see. Carry a microfiber cloth and a bottle of windshield wash.
Most bugnadoes are not directly hazardous beyond visibility issues. The insects themselves do not bite during their adult phase (most midge and mayfly adults do not have functional mouthparts) and pose no allergy risk in normal exposure. People with severe insect allergies should still close car windows when driving through dense swarms.
Why Synchronized Hatches Are a Mystery
One of the more puzzling features of bugnado-producing insect populations is how synchronized the hatches are. Mayflies that have been developing as nymphs for months or even years emerge from the water within a 24-hour window across a 1,000-square-mile lake. Midges in flooded farmland synchronize across an entire field. The mechanism is not fully understood, even by professional entomologists.
Current research points to a combination of cues: water temperature crossing a species-specific threshold, day length passing a key value, lunar cycle alignment, and possibly even chemical signaling among the developing nymphs themselves. The synchronization is adaptive: a single huge mating window overwhelms predators, ensuring most of the adults survive long enough to mate and lay eggs. The cost is the brief but spectacular bugnado season that the rest of us see at the surface.
For more on insect-related weather lore (including swallows hunting low ahead of storms), see our animal weather folklore piece. For other tornado-look-alike phenomena, see scuds and gustnadoes. For unusual cloud formations, see crazy clouds. For the science behind mosquito and midge mating swarms, the Entomological Society of America’s research roundup on mating swarms is a useful reference.
Frequently Asked Questions
What insects make up a bugnado?
Most often midges (Chironomidae) and mayflies (Ephemeroptera). Locusts, gnats, and certain grasshoppers can also form smaller swarm columns. The dominant species varies by region and season.
Is a bugnado dangerous?
Not directly. The insects do not bite during the adult mating phase. The main hazards are reduced driver visibility, slick roads from dead insects after mating ends, and a thoroughly coated windshield.
When is bugnado season?
Late May through early August across most of North America. Lake Erie’s mayfly hatch peaks around the summer solstice. Smaller midge bugnadoes happen daily during the warm season throughout the temperate U.S.
Why do insects form a tornado-like shape?
Three factors: thermal updrafts that the insects ride to gain altitude, light wind shear that twists the column into a tapered funnel, and swarm geometry where insects orient toward each other and produce a dense central core with softer tapered edges.
How do I tell a bugnado from a real tornado?
Tornadoes form under dark rotating storm clouds; bugnadoes form in calm or partly cloudy weather. Tornadoes move 20 to 60 mph; bugnadoes hover. Tornadoes are loud; bugnadoes are nearly silent. The atmospheric setup tells you almost immediately.
Are mayfly swarms really visible on Doppler radar?
Yes. The Lake Erie mayfly hatch produces enough biomass to register on the National Weather Service NEXRAD radar. The Cleveland and Detroit NWS offices regularly post the radar imagery during peak hatch in late June and early July.
Why do the hatches happen all at once?
A combination of temperature thresholds, day length, lunar cycles, and possibly chemical signaling among the developing nymphs synchronizes emergence. The adaptive value is overwhelming predators in a single window so most adults survive long enough to mate and lay eggs.
Natalie LaVolpe
Natalie LaVolpe is a freelance writer and former special education teacher. She is dedicated to healthy living through body and mind. She currently resides on Long Island, New York, with her husband, children, and dog.
- Natalie LaVolpe
- Natalie LaVolpe
- Natalie LaVolpe
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Just caught some after a rainstorm here in Central Manitoba
Wow never knew