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Sportsmen: Presence of mayflies is a source of pride

Sunday, July 01, 2001

By Ben Moyer

We made our first visit to PNC Park one night during the recent homestand, but our attendance did not impress the Phillies. They rudely snapped the Pirates' four-game winning streak, and the walk back to the Gateway Clipper was quiet -- until, that is, a nocturnal Pittsburgh happening pushed baseball to the back of our surrounding crowd's concern.

People walking near us began sidling away from the lights along the side of the stadium, their maneuvers accompanied by cries of "Ooh," "Gross," or "Yuck!" Usually alert to such things in less urban settings, I didn't notice what was causing the mild panic around us. But as we passed a lighted wall the cause of the crisis was clear. Mayflies, hundreds of them, were clinging to PNC Park and accumulating on the sidewalk below the lights. They were big mayflies, over an inch long, pale in color and robust in form.

The bustle didn't permit a good look but the flies appeared to be of the genus Hexagenia, a group of similar species that inhabit in their nymph stage a wide range of streams, rivers and lakes across the upper Midwest, most of Pennsylvania, New York and New England.

Mayflies, best known to trout fishermen, spend one to three years living as flat, clawed, evil-jawed, spiny-sided nymphs on the bottom of a stream or river. There, they burrow and clamber about, feeding on plankton, algae and organic debris. When environmental conditions -- primarily rising temperature trends -- are right, the nymphs ascend to the surface, split their nymphal exoskeleton, erect two pairs of fragile wings, and float for a few seconds before taking flight as the elegant insects seen by Pirates patrons. It is during that floating pre-flight period that mayflies are most vulnerable to feeding fish, and most imitated by enthusiastic fly fishermen.

Most species spend a short time clinging to streamside vegetation before again taking flight, this time to congregate over the water, mate in mid-air, lay eggs on the surface, and die. The flies we saw after the game may have been confused in their nuptial flight by the stadium lights, but their presence in Pittsburgh should be met with pride instead of alarm.

Mayfly species do exploit a diverse range of habitats but they never reside in badly polluted water. Mayflies are being studied worldwide to reveal subtle changes in water quality and the concentration of pollutants in bottom sediments. The insects that unsettled the crowd that night were emerging from the Allegheny River, and their presence says the Allegheny is a healthy, hospitable place. They need high levels of dissolved oxygen to survive, and they are a critical link in aquatic food chains, enabling energy in organic debris to be converted to the game fish prized by anglers.

Mayflies -- there are more than 1,500 known species worldwide -- are among the most "primitive" insects known to science. Fossils nearly identical to those alive today date back hundreds of millions of years.

Although linked by angling literature to rapid mountain streams, some species, Hexagenia in particular, favor moderately warm, large and slow-flowing rivers like our Allegheny. Heaviest emergence generally happens from mid-June to early July, but flights can occur into September if conditions are favorable. Once all but extinct in Lake Erie due to pollution, mayflies have returned to the now cleaner lake, so much so that their hatches have shown up in swarms on weather radar.

Nightly mayfly swarms may seem like an invasion of man's exclusive urban space, but that's because large rivers near cities were so badly polluted for so long that the miracle of mayflies was all but forgotten. Once known only to anglers who ventured to pristine headwater creeks, mayflies have returned to our midst as testimony that watersheds can be restored from all kinds of abuse. If vital rivers and the vitality they attract are to be a part of Pittsburgh's future, mayflies will need to be, too.

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