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Marsh season

Greenhead flies and dogs on Cape Ann marshes

Anyone who has parked beside an Essex marsh in late July with the windows down has met the greenhead. The flies come off the salt marsh in great numbers, and a dog at the edge of the grass gets bitten as readily as you do. This page covers the season, the black box traps standing in the marsh and how to time walks so the flies are not part of them.

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Last reviewed October 10, 2026 · by the Dogtown team

The season

Greenheads breed in the salt marsh and bite from June to September, worst in late July

The Northeast Massachusetts Mosquito Control and Wetlands Management District, based in Georgetown, describes greenheads as horseflies of the family Tabanidae, named for their large iridescent green eyes. Only the females bite, taking a blood meal to develop their eggs. The larvae live in the marsh sod along the upper edge reached by the daily high tides, and the adults can fly for miles, attacking people and domestic animals persistently in daylight.

The district puts the season at June to September, with the highest numbers from the last two weeks of July to the first week of August, and says the peak comes earlier the farther south a marsh lies, naming Saugus and Manchester as examples. Cape Ann's salt marshes along the Annisquam and the Great Marsh around Essex and Ipswich all sit inside that range, so the same few summer weeks are the bad ones from Gloucester to Ipswich.

  • Season: June to September
  • Peak: the last two weeks of July into early August
  • Biting hours: about 10 AM to dusk, females only
  • Southern marshes such as Manchester's peak a little earlier

The traps

The black boxes on the marsh are greenhead traps, and they work best left alone

The four-foot black boxes on legs standing in the marsh grass are the district's traps. Their outline reads as a grazing animal, so the flies go for the underbelly, find a small opening and end up stuck inside, and a single trap can catch hundreds of flies a day. The district began developing the design in 1968, now baits the traps with octenol, a compound found in ox breath and human sweat, and sets out about 400 of them along the creeks each June.

For a dog owner the traps are a handy signal: if they are out, the season is on. They are also equipment doing a job, so keep a dog from marking, chewing or knocking one over, and keep it out of the soft ground around them, where creek edges and ditches are deeper than they look. The district notes that greenheads are fairly resistant to pesticides, which is why trapping is its method rather than spraying.

  • Four-foot black boxes on legs along the marsh creeks
  • Baited with octenol and set out each June
  • Traps in the grass mean the season has started
  • Dogs kept from marking or knocking the traps

Timing

Time marsh walks for early morning, and have a quick way out if the flies find you

Because the biting females are most active from about 10 AM to dusk, the simplest fix is the clock. A 7 AM loop at Cox Reservation in Essex or Stoney Cove on the Annisquam in late July is a different outing from the same loop at noon. Essex County Greenbelt lists dog walking at both, Cox on a tidal estuary of the Great Marsh and Stoney Cove beside the Annisquam's salt marsh, which makes them the likeliest places for a Cape Ann dog to meet a greenhead.

The district's other tips translate to a dog. Biting flies prefer dark objects over light, and a black or chocolate coat is a dark object. Its list includes drying off after being in the water, which applies to a dog fresh from a swim. And it says to keep car windows closed and to brush out any fly that gets in rather than chase it while driving, because the distraction can cause an accident.

  1. 01

    Look for traps on the marsh: if they are out, so are the flies

  2. 02

    Walk marsh-edge trails before 10 AM in July and August

  3. 03

    Dry a wet dog before the marsh, as the district advises for people

  4. 04

    Carry a light towel to brush flies off the ears and face

  5. 05

    Park with the windows closed and brush any fly out before you drive

  6. 06

    At home, check the ears, nose and belly for bites

Bites

A greenhead bite hurts but is rarely dangerous; a swollen face or labored breathing is a vet call

The Merck Veterinary Manual says horse fly bites are painful, that blood keeps oozing after the fly leaves, and that biting flies are usually not dangerous to dogs unless they are extremely numerous or carry disease; the district notes that greenheads have not been found to transmit disease to people. Merck also notes that insect bites on dogs' ears show up at the tips of upright ears and on the folds of floppy ones. Keep the dog from scratching the spots raw.

The district says some people have strong allergic reactions to greenhead bites, and VCA explains that many pets are sensitive to the proteins in insect saliva too. Swelling of the face or muzzle, hives, vomiting or trouble breathing after a marsh walk means calling your vet or the nearest emergency hospital at once; Massachusetts Veterinary Referral Hospital in Woburn is open 24 hours. For repellents, ask your vet, since Merck says horse flies are hard to control and your vet can advise on suitable products.

  • Small bleeding bites on ear tips and ear folds
  • Scratched bites watched for soreness or infection
  • Face swelling, hives, vomiting or labored breathing: vet now
  • Repellents only on your vet's advice

Where we fit

In greenhead weeks, the turf yard and a solo walk from home keep a dog off the marsh at midday

Peak greenhead season falls inside the months most of Gloucester's public beaches are closed to dogs, May 1 to September 30 at the time of writing, and Essex closes its beaches to dogs in June, July and August. That leaves the pools on our turf yard at 210 Eastern Ave as the summer swim for many Cape Ann dogs: a full daycare day is $42 and a half day $33, in a playgroup of 7 to 10 matched by size and temperament.

Our at-home walkers take one dog at a time from your door for 20, 30 or 60 minutes at $25, $35 or $55, with a photo and a note after each walk. Tell us at the evaluation if your street backs onto a marsh, and ask whether we cover your address, since walking depends on the address and the season. Pup Van pick-up costs $10 each way if you live in Rockport, Gloucester, Essex or Manchester-by-the-Sea.

At Dogtown

Summer exercise away from the marsh

Pool days on the turf yard, solo walks from home and the Pup Van in Rockport, Gloucester, Essex and Manchester-by-the-Sea.

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Pick-up & drop-off

The Pup Van comes to you.

Pup Van pick-up and drop-off is available for $10 each way in the Rockport, Gloucester, Essex and Manchester-by-the-Sea area. Not in the area? Ask us for pricing.

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  • Manchester-by-the-Sea
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Asked and answered

Questions people ask about this

Short, complete answers you can act on today. More in the full FAQ.

When is greenhead season on the North Shore?
From June to September, according to the Northeast Massachusetts Mosquito Control and Wetlands Management District, with the highest numbers from the last two weeks of July to the first week of August. Marshes farther south, such as those in Saugus and Manchester, tend to peak earlier. The black box traps on the marsh are a sign the season has begun.
Do greenhead flies bite dogs?
Yes. The district says greenheads attack domestic animals as persistently as people, and the Merck Veterinary Manual notes that female horse flies may feed on any vertebrate, including dogs. The bites are painful and bleed a little after the fly leaves. Walking before 10 AM and away from the marsh edge avoids most of them.
What are the black boxes in the salt marsh?
Greenhead traps. The district sets out about 400 four-foot black boxes on legs each June; their outline looks like a grazing animal, the flies go for the underside, enter a small opening and are trapped. They are baited with octenol and can catch hundreds of flies a day. Keep your dog from knocking or marking them.
Can I put fly spray on my dog?
Ask your vet first. Merck says horse flies are among the hardest biting flies to control and that your veterinarian can advise on the most effective products for your pet, so use only what your vet recommends for your dog. Changing the time of the walk to before 10 AM is the step that needs no product at all.
Where on Cape Ann are greenheads worst?
On and beside the salt marshes, where they breed: the Great Marsh around Essex and Ipswich and the marshes along the Annisquam River in Gloucester. Essex County Greenbelt's Cox Reservation in Essex and Stoney Cove in Gloucester both list dog walking and both border salt marsh, so save those for early mornings in July.

More questions? Read the full FAQ

Sources

Rules and dates change. We re-check every beach, licensing and ordinance page each spring and fall; the source above is the current word.

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