Swarms of an insect over Lake Erie is so huge that they continue to show up on the National Weather Service radar over Lake Erie.

The weather service office in Cleveland late Thursday tweeted a radar video showing the mass of mayflies and the following message:

On Wednesday, Cleveland and Detroit doppler radar picked up the swarm over the western part of the great lake.

The insects pay an annual visit to the North Coast and can be so numerous that they cause problems for drivers.

"Many people don't realize these flies create a safety issue because their carcasses contain an oily substance that makes the surfaces they coat very slick, especially when it rains," said Nick Katsaros, an external affairs consultant in FirstEnergy's Lake Erie service area.

At times they have been so thick on bridges that they had to be cleared with snowplows.

As a result, FirstEnergy works with lakeside communities to discourage the swarms by turning off streetlights over the next few months.

The life cycle of a mayfly starts out as a larva that resides on river or lake bottoms.  After 3 months to 2 years, depending on the species, they emerge as winged adults and fly in swarms to mate. Once they emerge, they only live for a few minutes to a few days.

According to the Tennessee Wildlife Resources Agency, adult mayflies do not bite, sting, or carry disease-causing organisms.