Like scenes from a biblical plague, lakeside communities in some parts of Canada are being swarmed by insects.

They might not be the locusts of the Book of Exodus, but the fish flies blanketing one city had baseball players fleeing. Young athletes in Windsor, Ont., were mid-play Monday night when the game was called off. The bright lights of the diamond attracted a swarm of fish flies, also called may flies, interfering with play.

The insects are an annual occurrence in communities near the water, carpeting streets and covering beaches from Winnipeg to Ontario. The popular Pelee Island ferry in Ontario was docked over the weekend after an infestation.

Their emergence, which happens en masse once water temperature hits 19 or 20 degrees Celsius, came late this year in some communities after historic high-water levels. But while they are a nuisance to some humans, the fish flies aren’t around for long and are actually a positive sign for the health of the water, experts say.

“They’re the aquatic canaries in the [coal] mine,” says Ken Drouillard, a researcher at the University of Windsor’s Great Lakes Institute for Environmental Research.

Seeing them means the water is healthy, since the flies are sensitive to low oxygen. The more pollutants in the water, the less oxygen in the water -- and more dead fish flies.

“We almost drove them to local extinction in the 1950s because we weren’t treating our sewage in the way we do now,” he says.

Still, the annual swarm is an irritant to many in lakeside communities. Officials in the town of Lakeshore, Ont., were forced to remind residents not to disconnect street lights to thwart the bugs. “This act could be fatal and cause visibility issues for pedestrians and motorists,” they said in a tweet.

Drouillard says there is no need to worry about bites from the fish flies either.

“Because they emerge and live only two days, they don’t eat at all. They have no mouth parts, so they can’t bite,” he said. They simply end up being lunch for other species.

“Everybody is feeding on this large protein package that is coming out of the lakes,” he says.