The Green Drake is a little mayfly that's only about 4.5 centimetres long, but favoured by trout, which love to gorge on it.

The flies spend several years in a riverbed, in a nymph stage.

When they emerge in late May or early June for action-filled lives lasting about a week, they do so in a gossamer blizzard, numbering in the tens of thousands.

Brian Balunas, 70, a long-time angler, likens this little miracle of nature to "a light snowfall coming off the river."

The disappearance of the Green Drake from the Credit River and other rivers in southern Ontario, first noticed by anglers about five years ago, is raising concerns about the health of the river and the possible impact of unchecked development.

"This mayfly represents the canary in the coal mine," said Henry Frania, an entomologist associated with the Royal Ontario Museum.

"That's what this is really about.

"What they are basically telling us is that quality of the river and other rivers in southern Ontario are declining, and what we've been doing so far to protect the rivers just isn't good enough."

For several years, Frania has been trying to find out what's killing the Green Drake, arguing that while it's the mayfly today, it could be human health affected tomorrow.

There is documentation that on some rivers where the mayflies have disappeared, brook trout disappear a few years later, Frania said.

The insect's sensitivity to pollutants makes it the perfect early warning system. That's why, Frania said, finding out what is killing it is a matter of urgency.

"The nymphs are dying," he said.

"There's something in the river that's killing them. There's no question about that."

Frania has determined from the autopsies he's done on mayflies that the gut linings of the tiny creatures are being destroyed by something unnatural, something toxic in their diet.

The Green Drake used to be found in abundance in the middle to upper stretches of the Credit, a regular denizen of the river as far south as the town of Terra Cotta in the 1980s.

Now it's "functionally extinct," with only tiny pockets remaining in the river's upper stretches.

Frania's theories about what's killing the nymphs range from pollutants and sewage waste to rising amounts of toxins produced by micro-organisms that are living longer because of global warming.

Development, especially in communities like Erin and Orangeville, is a potential contributor and the problem is spreading, Frania said.

Other species of mayflies such as the Quill Gordon have also practically disappeared from the Credit River watershed.

The Green Drake population is rapidly declining in other watersheds, including the Rocky Saugeen River.

Jon Clayton, a biologist with Credit Valley Conservation, called Frania's research indicative of a more widespread development problem.

"As you change from forested areas and wetlands to farm fields or subdivisions, parking lots, malls – that has a dramatic effect on water quality," he said.

Balunas, a retired TTC worker who has been fishing the Credit for more than 40 years, said he clearly remembers when the Green Drake would make its early summer emergence.

It was wondrous, he said, especially when the flies, carrying up to 4,000 eggs each, descended after the frenzied mating ritual and bounced off the water surface to shake their eggs into the river.

"When the Green Drakes came up, it was always fast and furious," said Balunas.

Leafing through his fishing diaries, Balunas said he noted in 2004 that he spotted only one Green Drake in an area where they were once abundant.

"It didn't happen overnight," said Balunas. "It gradually petered out by early 2000."