LUNA PIER - Turn off the lights and bar the doors: Mayflies are on their way back.

They are not yet in the number that lake-shore residents have seen before, but the season is young.

Experts in all things Hexagenia limbata and Hexagenia rigida, which is what people call the critters in Latin, say their peak traditionally is June 18-21.

Cool spring temperatures are blamed for the late arrival this year.

“I have received reports indirectly that [people] have been seeing them in small numbers,” said Dr. Ken Krieger, the state's best-known mayfly researcher and senior research scientist at Heidelberg College's water quality laboratory in Tiffin.

Up along the Michigan shoreline, the story is a bit different.

Fred Snyder, district sea-grant specialist for Ohio State University at Camp Perry, Ohio, and an expert on Lake Erie water quality, tells it differently.

Returning to Fremont last week, Mr. Snyder stopped at the drive-through window of a fast-food restaurant along I-75 near Bolles Harbor, Mich.

Through the window, Mr. Snyder said he could see the wall covered with mayflies that had swarmed inside. Bugs covered the order-taker's hat.

“You just had to see his look of exasperation,” Mr. Snyder said.

Mayflies hatch on the lake bottom and fly inland, where they are attracted to light. The bugs are harmless, but they cause a mess when their bodies pile up on streets and sidewalks at the end of a 24-hour lifespan.

The mayfly invasion typically is over in less than a month. Their appearance dwindled from the 1950s through the 1980s because of high levels of phosphates in the lake that reduced oxygen levels there.

However, better water quality has enabled the mayfly to make a comeback in the last 10 years or so.

Most communities and businesses turn off unnecessary lights and try to keep doors and windows closed during the worst infestations.

Mr. Snyder said he has seen some of them in the Port Clinton area, but nothing unbearable just yet.

“If you're looking, you'll find them somewhere,” he said.

Their late arrival could be just their way of making up for last year's earlier-than-expected appearance, particularly in Port Clinton, where they attached themselves to windows, street lights, and car windshields.

People with long memories say the tiny winged insect - a symbol of Lake Erie's ecological comeback - produce a larger population every other year.

“People have made the point that in even-number years, they always seem to have the resurgence,” Mr. Snyder said.

Dr. Krieger and Mr. Snyder point to the cool spring and colder water temperatures for the apparent delay in mayflies so far.

“When we will have the biggest peak, we can't predict that,” Dr. Krieger said.

“They always start out with just a few showing up, then they more or less increase in numbers,” he said. “Then, one night, they'll be there in great numbers.”

Linda Venier, manager of Brest Bay Marina in Monroe, said mayflies began showing up along the Michigan shore Thursday night.

Most boaters and store owners along the lake resign themselves to the inconvenience, Ms. Venier said.

“They accept it. Especially if you're a boater,” she said.

“You just have to turn off the lights. Then ... hope the wind blows them down to you guys [in Toledo],” she said.

Even when the mayflies hatch in significant numbers, their appearance can be affected by air movement.

A strong hatch might not translate into a significant infestation if the winds are blowing right. The paper-thin bugs are little match for a strong breeze. “Wind plays an effect on any given night because they are weak fliers,” Dr. Krieger said. “If there's a strong wind, they're going to blow downwind.”

When they do hit hard, mayflies can tie up a community's resources and pretty much shut down sport fishing near shore. Shop owners and residents are stuck cleaning up mounds of dead bugs and cities such as Port Clinton in the past have struggled to find places to dump truckloads of bug bodies.

“Other than their nuisance value - they don't sting and they don't destroy crops - they are harmless,” Mr. Snyder said. “They pile up and the fish don't bite.”

First Published June 24, 2002, 3:34 p.m.