If it's June, why are there mayflies? Amazing facts about the coming swarms

The Columbia Borough Police Department has announced that mayflies are starting to hatch along the Susquehanna River and when the insect swarms grow too heavy the department will be turning the street lights off on the Columbia-Wrightsville bridge over the river.

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https://www.pennlive.com/wildaboutpa/2018/06/if_its_june_why_are_they_mayfl.html