![]() ![]() The native range of the species includes the southeastern United States and the Mississippi River watershed. Prior to the occurrence of large flights several years ago, Hexagenia bilineata was not well known among the species in the mayfly communities of the lower Susquehanna and its tributaries. But some caution is in order in this case. Generally, assessments of aquatic invertebrate populations are considered to be among the more reliable gauges of stream health. Normally within 18 hours of leaving the water and beginning flight, it will molt into an adult, ready to breed during its final night of life.Īfter so many years, why did the swarms of these mayflies suddenly produce the enormous concentrations seen on this particular bridge across the lower Susquehanna? Let’s have a look.įollowing the 2015 flight, conservation organizations were quick to point out that the enormous numbers of mayflies were a positive thing-an indicator that the waters of the river were getting cleaner. The sub-adult (subimago or dun) stage lasts less than a day. A sub-adult (based on the translucence of the wings) female burrowing mayfly (Hexagenia species). Adult mayflies are also known as spinners. An adult (imago) male Great Brown Drake (Hexagenia bilineata) burrowing mayfly. Since then, to prevent attraction of the breeding insects, lights on the bridge have been shut down from about mid-June through mid-July to cover the ten to fourteen day peak of the flight period of Hexagenia bilineata, sometimes known as the Great Brown Drake, the species that swarms the bridge. The slimy smelly bodies of dead mayflies, probably millions of them, were removed like snow from the normally busy Lincoln Highway. In 2015, an emergence on a massive scale prompted the temporary closure of the mile-long Columbia-Wrightsville bridge while a blizzard-like flight of huge mayflies reduced visibility and caused road conditions to deteriorate to the point of causing accidents. Mayflies rise from their benthic environs where they live for a year or more as an aquatic larval stage (nymph) to take flight as a short-lived adult (imago), having just one night to complete the business of mating before perishing by the following afternoon. ![]() Fishermen and others who frequent the river are familiar with the phenomenon. Within the last few years, the early-summer emergence of vast waves of mayflies has caused great consternation among residents of riverside towns and motorists who cross the bridges over the lower Susquehanna. ![]()
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