Greenhead Fly

The greenhead fly is a species of horse fly in the genus Tabanus, named for its large, iridescent green compound eyes that stand out against a dark, robust body. It is best known along the Atlantic coast of North America, where dense populations emerge from salt marsh nursery grounds in mid to late summer and become a seasonal nuisance on nearby beaches and dunes.

Like other horse flies, only the females bite, feeding on the blood of a host to develop their eggs, while males restrict themselves to nectar and plant fluids. Greenheads are strong, fast fliers capable of covering long distances from marsh to beach in search of a blood meal.

Ecologically, greenhead larvae are voracious predators within the marsh mud, feeding on other invertebrates, and the adults in turn serve as prey for birds and dragonflies, making them one link in the coastal marsh food web despite their reputation as a summer pest.

• Stout, robust fly, 10–13 mm long, larger than a house fly
• Large, iridescent green to gold compound eyes, often patterned
• Dark gray to black body with a somewhat flattened abdomen
• Wings are clear to smoky, held flat over the back at rest
• Lookalikes: deer flies are smaller with patterned wings and a narrower body; other Tabanus species lack the vivid green eye coloring

Greenhead flies are tied closely to Atlantic coast salt marshes from the Canadian Maritimes down through the Gulf coast, where their larvae develop in marsh mud. Adults are most abundant from roughly late June through August, moving from the marshes onto adjacent beaches, dunes, and coastal towns, especially on warm, calm, sunny days.

Female greenheads are persistent daytime fliers that use vision and heat cues to locate hosts, often approaching in a fast, direct flight and returning repeatedly if driven off. Males do not bite and instead feed on nectar and plant sap. Larvae live in the anaerobic mud of salt marshes, where they are active predators on smaller invertebrates including worms and insect larvae, making them a significant predatory presence within the marsh benthic community.

Females lay egg masses on marsh vegetation just above the waterline, and larvae drop into the mud upon hatching. Larvae overwinter in the marsh substrate, feeding and growing through the cooler months before pupating in spring in slightly drier marsh soil. Adults emerge in a synchronized burst in early to midsummer, producing the dense, short-lived swarms characteristic of the species, with generally one generation completed per year.