Greenhead Fly FAQ | Northeast Massachusetts Mosquito Control and Wetlands Management District

What is a “Greenhead Fly”?

The Greenhead flies of Essex County, MA are insects that are called horseflies. They belong to a family called Tabanidae. The name comes from the large iridescent compound eyes which appear as dark green.

Why are Greenheads a problem?

Because they occur in great numbers, the females bite during daylight, they can fly for miles, attack persistently domestic animals and people, and they interfere with the enjoyment of coastal areas throughout much of the summer.

Why do Greenheads bite people and animals?

Only the females are equipped to pierce the skin to take a blood meal. They seek out a blood meal to provide a rich protein source necessary for egg development. Protein for the first egg mass is obtained when the predaceous larva eats other insect larvae or small animals.

Where do Greenheads come from?

Greenhead flies are produced from our coastal marshes. Early settlers of the area reported their problem with biting insects and had to protect their stock. Adult flies mate on the open marsh. Within a few days without seeking a blood meal, the female lays her first (of up to four) egg mass consisting of 100 to 200 eggs on marsh grass.

The eggs hatch into worm-like larva, with as many as 80 larvae in a single square meter of marsh sod at maturity. Developing larva concentrate along the upper vegetation zone reached by daily high tides. The larvae forage around surface muck, through wet thatch and vegetation. The predaceous larva attack and devour a variety of invertebrates, including some of their own kind. The larvae overwinter beneath the frost, form a pupa in early summer, forage around briefly (10 days or so) and then depending on environmental conditions emerge into adult fliers.

Are Greenheads harmful?

As a nuisance and economically, greenhead flies can be very harmful. In Essex County they affect real estate values, beach use, golfing, outdoor recreation, and so on. They are not considered a health problem to people as they have not been found to transmit disease. However, some people have strong allergic reactions to their bite.

When are Greenheads at the highest volume?

It varies – generally the farther south, the earlier the “peak” of activity occurs, e.g. Saugus, Manchester. The environmental conditions dictate the emergence and development e.g. tides, light, temperature. The season is from June to September with the highest densities occurring the last two weeks of July to the first week of August.

How can Greenheads be controlled?

Because of their predaceous nature as larva a significant reduction in number could produce a trade-off in species produced. For example, if greenheads did not keep the deerflies in check that problem could possibly become greater. Conventional methods of biting fly control are either environmentally undesirable or economically impractical. Because of the large size of the larva and adult it would take a higher concentration of fairly toxic pesticides to control them and thus adversely affect other non-target organisms. Burning the marshes, water management, ditching or impoundments have been found ineffective and costly. Therefore at this time the most environmentally and economically solution to the greenhead fly problem is the proper trapping on the salt marsh.

What are some of the things a person can do to reduce fly nuisance?

Biting flies prefer dark over light objects. Wear appropriate apparel with this in mind.
Dry off after being in water
Wear long sleeved shirt for protection
Timing – female flies are most active from 10:00am to dusk.
Screen in pools, porches, patios, etc.
Keep windows on cars closed. If some flies enter, brush them out – do not take a chance, the distraction could cause you an accident.
Remember – sprays, perfumes, and increased metabolic activity serve to attract the biting female.

How does the North Shore Greenhead Fly Project Control the flies?

For a number of years our research and development has been directed toward abatement of nuisance levels along beaches, recreational areas, and housing. In 1968 we began developing a box trap which has been modified many times and will probably be subject to other changes from year to year as we attempt to increase its effectiveness. In fact, traps capture large numbers of blood seeking flies, and if such traps are located on the marsh may serve as a control for greenheads. Each year we place over 400 traps on the marshes of coastal towns and gather and remove millions of biting flies from circulation.