TOWN OF NEENAH, Wis. (AP) â Swarms of lake flies are invading the Lake Winnebago area â clinging to everything in sight and leaving behind a green mess when squished. Lake flies typically emerge around Mother's Day but this year's hatch was delayed by the cool spring weather. Lake flies live throughout Lake Winnebago and the upper lakes. Scott Koehnke, a water management specialist for the state Department of Natural Resources, says warmer temperatures this week will result in a bumper crop of lake flies. Koehnke says the current hatch will help migrating birds, which have struggled to find food in the cool spring. Purple martins and swallows had delayed nesting because of the lack of food. Lake flies don't have mouths, so they don't bite or eat vegetation.