Visitors to Riverfest along and on the Susquehanna River at Columbia over the weekend strolled along paths littered with the bodies of thousands of mayflies, signaling that the annual whitefly storm has begun its upriver march for 2022.

The river’s world-famous whitefly hatch sees millions of the insects emerge for mating.

They also swarm light sources, like street lights and neon signs, but that is not part of their purpose during the emergence. Like many night-flying insects they are drawn to the lights after they’ve emerged for other reasons.

The attraction that artificial lights hold for the small insects led to international attention for the bridge – the Route 462 Veterans Memorial Bridge – where in 2014 the lighting was replaced with historic reproduction lanterns that the whiteflies found even more enticing.

They swarmed the lights in such numbers that they caused problems for motorists and accumulated in masses of dead insects on the bridge. News stories about the situation spread around the globe.

Eventually the borough shut off the lights until the local phase of the whitefly hatch had passed, a measure that’s been repeated each year in subsequent whitefly seasons.

The whitefly emergence begins at the southern end of the river in early summer and gradually moves upriver before petering out in mid-August.

At many spots along the Susquehanna, and some of its tributaries, swarms of the insects can be expected to block out the lights in fast-food parking lots and slam into windshields in numbers sufficient to make drivers pull over in coming weeks.

Mayfly is the common name for the ephemeral insects, but many different species and populations emerge spring through fall. The name of mayfly first appeared in the “Epic of Gilgamesh,” a tale of the life of a Babylonian ruler written around 2000 B.C. on clay tablets.

In 1834 British entomologist John Curtis published a paper titled “Description of some nondescript British species of mayflies of anglers.”

The term appeared again in 1883 in a paper titled “A revisional monograph of recent Ephemeridae or Mayflies” by the Rev. A.E. Eaton, a leading expert on Ephemeroptera at the time. That was the first entomological publication to label the entire Ephemeroptera as mayflies.

The order Ephemeroptera includes more than 3,000 species of mayflies worldwide, more than 700 species in the U.S. and more than 300 species in Pennsylvania.

Several species of mayflies call the Susquehanna River and its tributaries home.

The whiteflies (Ephoron lukeon) that swarm the Harrisburg Senators’ FNB Field on City Island and parking lots all along the river and ignite a feeding frenzy in fish across the river, are known worldwide.

Small, short-lived (as adults) insects, as adults most mayflies emerge from their nymph form, take flight, mate, lay eggs and die in just a day or two.

Mayflies in the Susquehanna are singular. The insects are a burrowing nymph that lives in the silt of the river or stream bed. They begin hatching at dusk, with the nymphs quickly rising to the surface of the water to emerge as a dun and then take flight. The male duns quickly molt into spinners with two tails. The females then lay their eggs on the water. And all that takes place in one night. The next night will see an entirely different batch of mayflies.

Water quality has improved in waterways like the Susquehanna River, launching a boom in the size of mayfly hatches in those waterways.

Warming streams pose threat to mayflies, study finds

With sensitive external gills, mayflies cannot survive in polluted water. Because of that, they are considered an indicator of good water quality. However, that does not mean the Susquehanna is a river without its problem.

Agricultural runoff, chemicals passing through municipal waste treatment plants, other sources of contamination, invasive species and other factors continue to impact the waterway.

Fly fishermen look forward to the whitefly hatch on the Susquehanna River because the abundance of food floating on the waters where the hatch occurs “turns on the bite,” sending fish into a near frenzy of feeding.

The best fly-fishing is found from about 8 to 10 p.m., which is also prime viewing time for whitefly swarms under lights and on lighted windows all along the river.