An enormous pile of dead mayflies was spotted on a bridge in Columbia, Pennsylvania, on June 22, during the annual mayfly hatch.

This video, taken by Tom Johnson-Medland, shows the mountain of mayflies that died on the Veterans Memorial Bridge, on the Susquehanna River.

He estimated that the largest pile was over three feet high. 

The insects hatch in summer months and the swarm is usually so large that the Pennsylvania Department of Transportation will shut the lights off on the bridge from May through late June to reduce the swarm flocking to the lights.

When they die, after just a day or two after hatching, mayflies fall to the ground and are easily squished, leaving a grease-like coating on roads and bridges. Some cities have seen hatches so large they have had to call in snowplows to remove them.

Sometimes the hatch is so big it will even show up on the radar, like in 2019 in La Crosse, WI:

According to the National Weather Service, the life cycle of a mayfly starts out as a larva that lives on river or lake bottoms.  They emerge as winged adults in the warmer weather and fly in swarms to mate. Depending on the species, they only live for a few minutes to a few days.

Along the Mississippi River in the Midwest, one of the largest of the mayfly species generally hatches in late June to early July.

After mating, males will only survive a matter of hours before falling to the ground in large numbers.

However, a large mayfly hatch is a GOOD thing. They are extremely sensitive to pollution in water and their presence means that the waterway is ‘healthy.’

There are about 2,500 species of mayfly worldwide and nearly 630 species within North America. Currently, 109 species are in Iowa.

According to WQAD, last year a large swarm of mayflies took flight on July 3rd in the Quad Cities. The water temperature of the Mississippi River in the area was around 82*. Previous swarms dating back to 2014, also occurred once water temperatures reached above this temperature threshold.

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