It's a yearly occurrence
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> “In Michigan, around lake St. Clair we have fish flies. So thick that cars trying to drive on streets covered in them have skidded and crashed just as if you were driving on snow.”
They have the same flies in the Central New York Finger Lakes area, and yes they get thick. Some of the towns have to get out the snow removal equipment to get the slippery bodies off of the pavement. As I recall they could be a half inch thick on the street. In a week though, they’d all be gone until the next year.