CLUSTER FLIES BREED IN THE GROUND OUTSIDE OF STRUCTURES DURING THE WARM WEATHER (LATE SPRING INTO EARLY SUMMER) USING EARTHWORMS AS A FOOD SOURCE FOR THE IMMATURE LARVA (MAGGOTS). THE FLIES LATER PUPATE (GO INTO A COCOON STAGE) AND THEN LATER HATCH AS ADULT FLIES. IN TEMPERATE AREAS OFTEN IN LATE AUGUST THESE FLIES BEGINS TO MIGRATE INDOORS FINDING ANY SMALL CRACKS OR CREVICES THAT PERMIT ENTRY INDOORS.,THESE MAY INCLUDE WINDOWS,DOORS,EAVESETC. ENTRY SEEMS TO BE ON THE SAME WARM SUNNY SIDE OFTEN THE SOUTHERN OR WESTERN EXPOSURE OF THE STRUCTURE AS THE FLIES LATER EMERGE FROM.
DURING THE FALL, WINTER OR SPRING MONTHS THESE FLIES MAY EMERGE PARTICULARLY ON WARM SUNNY DAYS.THE FLIES APPEAR AT WINDOWS BUZZING AND CLUSTERING AROUND THOSE AREAS TO THE DISMAY OF THE OCCUPANTS.