Vertical-looking entomological radar over southern England, combined with ground sampling, allowed researchers to estimate the seasonal flow of migrant hoverflies at somewhere between one and four billion individuals a year, with Episyrphus balteatus and the related Eupeodes corollae dominating the movement. The flies cross the Channel with the wind but are not simply blown along; they select flight conditions and orient.
The consequence for farmland is direct. Adults are pollinators, visiting a wide range of open flowers, while the larvae are aphid predators, each consuming hundreds of aphids during development. Aggregated across the incoming population the estimated consumption runs to trillions of aphids in a season.
The adult's yellow and black banding is Batesian mimicry of stinging wasps and bees. The fly has no sting and no bite, and can be told from the models it copies by its single pair of wings and by the hovering, darting flight that gives the family its name.