Giant Black Fly: All You Need to Know in a Nutshell

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Giant black flies can be an unwelcome nuisance during the warmer months. They are often referred to as gnats and can swarm around their targets, causing itching and discomfort. In this article, we’ll cover everything you need to know about these pesky insects and how to deal with them.

Although there are about 40 species of black flies in New Hampshire alone, only a few of them bite or cause significant annoyance to humans. These insects are relatively small, measuring around 1/8-inch in length, making them smaller than a common housefly. It is important to note that only female black flies will bite as they need a blood meal for reproduction.

Black flies thrive in various environments, with some species found throughout the United States and Canada, including Florida. As the population density determines their level of annoyance, it’s essential to be prepared with some knowledge to mitigate their presence effectively. From preventative measures to treating bites, understanding the behavior and characteristics of these flies can significantly improve your outdoor experiences during black fly season.

Giant Black Fly Overview

Identifying Features

Giant black flies have a few distinguishing characteristics, such as:

  • Their large size compared to common house flies
  • Predominantly black coloration
  • Bigger head and compound eyes

For example, the Simulium spp. is a species of black fly that can have a painful bite.

Habitat and Life Cycle

Giant black flies can be found in various habitats, including:

  • Near rivers and streams
  • Wooded areas
  • Agricultural fields

They typically undergo a four-stage life cycle:

  1. Egg
  2. Larva
  3. Pupa
  4. Adult

Adult giant black flies typically have a short lifespan, while their larvae and pupae stages can span a few weeks.

Feature Giant Black Fly Common House Fly
Size Larger than a house fly Smaller, about 1/8″ in length
Color Predominantly black Grayish-black
Eyes Large compound eyes Smaller compound eyes
Habitat Near rivers, streams, wooded areas, agricultural fields Urban areas, near human food sources
Lifecycle Stages Egg, larva, pupa, adult Egg, larva, pupa, adult

Remember, giant black flies might not only be a nuisance due to their size, but they can also be potential biters, as seen in the case of Simulium spp. black flies.

Behavior and Impact on Humans and Animals

Painful Bites and Associated Symptoms

Giant black flies, also known as buffalo gnats, can be quite a nuisance for both humans and animals due to their painful bites. Their mouthparts are designed to slice through the skin, causing discomfort. Some common symptoms after being bitten by a black fly include:

  • Pain: A sharp, immediate sensation at the bite site.
  • Swollen: Affected area becomes red and inflamed.
  • Itchy: Persistent itching around the bite.

In some cases, victims may develop more severe symptoms like:

  • Nausea
  • Fever
  • Headache
  • Swollen lymph nodes

Allergic reactions can occur in some individuals, potentially leading to serious complications or even death.

Public Health Risks

Black flies pose public health risks as they can transmit diseases and infections through their bites. However, unlike mosquito bites, black flies are not known to spread diseases like malaria or dengue fever. It’s essential to use insect repellents, such as DEET, as a preventive measure.

Effects on Livestock and Wildlife

Black flies can have significant impacts on livestock and wildlife. Their bites can cause:

  • Stress, which can potentially reduce growth and productivity rates.
  • Anemia due to blood loss.
  • Spread of infectious diseases among animals.

Comparison Table

Factor Black Fly Bite Mosquito Bite
Pain Instant, sharp Mild, slight discomfort
Swelling Red and inflamed Small, localized bump
Disease Transmission Less common Common (malaria, dengue, etc.)

Overall, understanding the behavior and impact of giant black flies on humans and animals helps us better manage their presence and mitigate their harmful effects.

Prevention and Pest Control

Protecting Your Home

To prevent giant black flies from entering your home, take the following measures:

  • Install screens on windows and doors
  • Seal cracks and gaps around your home
  • Regularly clean and maintain gutters

For example, a well-maintained screen can keep out not only black flies but also other pests like mosquitoes and house flies.

Managing Infestations in Your Yard

To manage black fly infestations in your yard:

  • Remove standing water to discourage breeding
  • Keep your yard clean and free of debris
  • Use fans in outdoor areas to create air movement

Remember, black flies lay their larvae in running water, such as streams and rivers. However, they can also populate areas with standing water like pools or ponds.

Pros and Cons of Removing Standing Water:

Pros Cons
Reduces breeding grounds Can disrupt wildlife
Fewer pests, such as mosquitoes May require constant maintenance

Personal Protective Measures

When outdoors in areas where black flies are present, take these personal protective measures:

  • Wear long sleeves and pants
  • Opt for lighter colors, as black flies are attracted to dark colors
  • Use fans or electric bug zappers around outdoor seating areas
  • Consider using a head net if black fly population is high

For example, wearing light-colored clothing can not only help deter black flies, but also keep you cooler in hot temperatures. Keep in mind, though, that fans can only provide temporary relief, as black flies can return once the fan is turned off.

In case of severe infestations, consult a pest control specialist to assess the situation and determine the best course of action.

Types of Black Flies and Their Differences

Cluster Flies

Cluster flies belong to the Calliphoridae family, which is different from the common house flies. They are slightly larger and have a tan-colored body. These flies often become a pest in the fall as they search for shelter, entering homes and other structures. They are common in North America and are a nuisance because they tend to swarm in large numbers.

  • Common features: Tan-colored, larger than house flies, swarm in large numbers
  • Habitat: North America
  • Season: Fall

Turkey Gnats

Also known as buffalo gnats, turkey gnats are tiny black flies that resemble mosquitoes. They are notorious for their painful bites and can cause severe reactions in some individuals. These flies are prevalent near rivers and are particularly common during the warm months.

  • Common features: Tiny, black, resemble mosquitoes, painful bites
  • Habitat: Near rivers
  • Season: Warm months

Deer Flies

Deer flies are smaller than house flies, with a dark-colored body and patterned wings. They primarily feed on blood, and their bites can transmit diseases to animals and humans, such as river blindness. Deer flies are most active in warm weather and are attracted to moving objects and carbon dioxide.

  • Common features: Small, dark-colored, patterned wings, blood-feeders
  • Habitat: Global
  • Disease transmission: River blindness

Horse Flies

Horse flies are large, heavy-bodied flies with wings that span about 15-25 mm. They are also blood-suckers and can deliver painful bites to humans and animals. Their primary habitat is near water sources or moist environments. Horse flies are also notorious for transmitting disease-causing microorganisms.

  • Common features: Large, heavy-bodied, blood-feeders, painful bites
  • Habitat: Near water sources or moist environments
  • Disease transmission: Various diseases
Fly Type Size Color Bite Habitat Diseases Transmitted
Cluster Flies Larger Tan N/A North America, indoors in the fall None
Turkey Gnats Tiny Black Painful Near rivers, warm months None
Deer Flies Small Dark Painful Global, warm weather River blindness
Horse Flies Large Dark Painful Near water sources, moist areas Various diseases

Treatment and Remedies for Black Fly Bites

Over-the-Counter Treatments

  • Anti-itch creams: For instance, hydrocortisone cream can help reduce itching and inflammation.
  • Pain relievers: Aspirin, ibuprofen, or acetaminophen can help to manage pain from black fly bites.

Home Remedies

  • Soap and water: Gently wash the bite area with soap and water to prevent infection.
  • Apple cider vinegar: Applying diluted apple cider vinegar to the bite may help to reduce itchiness and swelling.

When to Seek Medical Attention

If you notice any of these symptoms, it is essential to consult a doctor:

  • Signs of infection: Increased redness, pus, or worsening pain may indicate an infection.
  • Severe allergic reaction: Difficulty breathing, chest pain, or swelling of the face or throat may require emergency medical attention.

Tips for Avoiding Black Fly Bites Outdoors

  • Cover up: Wear long sleeves and pants to protect your skin from bites.
  • Use insect repellent: Apply a repellent containing DEET or permethrin to your clothing and exposed skin.
Comparison table: DEET vs. Permethrin
DEET Permethrin
Pros – Applicable in skin
– Suitable for most insects
– Effective against ticks
– Can be applied to clothes
Cons – May damage some plastics – Not applicable in skin
– Only repels insects

Remember to practice these tips when in infested areas and stay safe outdoors!

Ecological Importance of Black Flies

Role in Aquatic Ecosystems

Black flies primarily inhabit streams with clean, flowing water and rely on rocks and vegetation for laying their eggs. These insects play a vital role in breaking down organic matter, such as leaves and small twigs, in aquatic ecosystems during their larval and pupal stages. Some key features of black flies in aquatic ecosystems are:

  • Inhabiting clean streams
  • Laying eggs on rocks and vegetation
  • Breaking down organic matter in larval and pupal stages

Prey for Fish and Other Animals

Many animals, including fish, birds, and bats, feed on black flies. They are a significant food source for fish species such as trout, which depend on the large number of black fly larvae and pupae in streams. For example, the compound eyes of fish species like trout enable them to easily detect and catch black fly larvae. Some aspects to consider when discussing black flies as prey include:

  • Important food source for fish, birds, and bats
  • Predominantly consumed during larval and pupal stages
  • Contributing to healthy fish populations in streams
Fish Species Black Fly Diet Impact
Trout Vital food source

Black Fly Characteristics

  • Size: Small (1.2 to 3 mm)
  • Shape: Humpbacked
  • Other Names: Buffalo gnats
  • Mouthparts: Mandibles (for biting)

Black Fly Impact on Animals

  • Feeding: Only female black flies bite, seeking blood meals
  • Health: Black fly bites can transmit diseases
  • Annoyance: Swarming around animals without biting
  • Threats: Severe blood loss can lead to death in extreme cases

Management Techniques

  • Use of repellents and protective clothing
  • Environmental control measures, such as modifying aquatic habitats
  • Chemical control for extreme infestations

Identifying and Managing Cluster Flies

Appearance and Behavior

Cluster flies are slightly larger than the common house fly, dull-gray with black markings, and have golden-yellow hairs on their thorax that can give the appearance of a golden sheen1. Some key differences between cluster flies and houseflies:

Feature Cluster Flies House Flies
Size Slightly larger1 Smaller
Color Dull-gray with black markings1 Darker, metallic color2
Hair Golden-yellow hairs1 No distinct hairs

Prevention Methods

To prevent cluster flies from entering your home, some steps can be taken:

  • Seal up cracks in siding and around windows with caulking3
  • Cover attic vents with wire screening before September 10th3

How to Get Rid of Cluster Flies

In case of cluster fly infestations, there are multiple ways to manage them:

  1. Physical Removal: Cluster flies are usually sluggish, so it’s easier to collect them manually3.
  2. DIY Pest Control: Start with common pest control products like flypapers or insecticide sprays.
  3. Specialist Help: If the infestation persists, consider calling a pest control specialist.

Remember, it is crucial to address the issue before it escalates, as cluster flies can multiply and infest your home further.


  1. Penn State Extension 2 3 4

  2. Home & Garden Information Center

  3. Extension 2 3

Reader Emails

Over the years, our website, has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Male Black Horse Fly


Black Western Horsefly – Male?
Hi Bugman,
I know you are really busy with all the letters you are receiving and hope you will be able to respond to my inquiry. My husband saw a huge black fly in our backyard about a week ago. We discussed what it could be. My guess was a horse fly, but both of us have only seen horse flies that are metallic green. Fortunately, I found either the same fly or his family member hanging out on our front porch and captured a couple of pictures. Is it a horse fly? I’m guessing a Black Western Horsefly…male perhaps. (Yes, I’m making my assumption off what I’ve learned from your site. Which by the way helped me identify a pestering group of P. Carolina Wasps.) Thanks for your assistance and keep up the good work! Your site is GREAT!
Valerie Kacmarcik
Found in Euless, TX (DFW Area) 9/11/06. Taken with Digital Zoom…sorry it isn’t better.

Hi Valerie,
Eliminate the word Western from your identification and you are absolutely correct. This is a male Black Horse Fly, Tabanus atratus. You can tell it is a male as its eyes meet in the center of the head. Only female horse flies suck blood. Males drink nectar.

Letter 2 – Male Black Horse Fly


Subject: Darth Vader bug
Location: north Illinois WI border
July 7, 2013 5:57 pm
This black insect died next to me . The insect is all black one inch long with a wingspan of 1 1/4 Inch. The eyes are the most interesting
Signature: jack Speer

Male Black Horse Fly
Male Black Horse Fly

Hi Jack,
This is a male Black Horse Fly,
Tabanus atratus, and this submission from 2010 also described it as looking like Darth Vader.  The eyes of male Horse Flies have no spacing between them, while the eyes of female Horse Flies have a noticeable space.  Only female Horse Flies suck the blood from warm blooded creatures, and they will bite humans if there is no livestock upon which to feed.  According to BugGuide:  “Females feed on mammalian blood, while males, which lack mandibles, feed on nectar and plant juices (Long 2001). This species especially prone to attack cattle and other livestock.”

Letter 3 – Male Black Horse Fly


Subject: Silver & Black Insect
Location: Maryville, TN
July 22, 2013 11:29 am
This particular insect was found by a co-worker of mine today (07/22/2013) at approximately 1:30 p.m. in Maryville, Tennessee. To me, it seems to have many similarities to a horse fly but I have never seen one quite like this. Could you please help me indentify this insect? Thank you kindly.
Signature: Cody Moyers

Black Horse Fly
Male Black Horse Fly

Hi Cody,
This is a Horse Fly, and we believe it is a Black Horse Fly, Tabanus atratus.  This photo from BugGuide shows a similar coloration and we believe the eyes are reflecting back light, making them appear silvery.  The close-set eyes indicates this is a male.

Letter 4 – Male Black Horse Fly


Subject: strange fly
Location: Chesapeake va
August 7, 2014 4:19 pm
Just curious what kind of fly this is..never seen anything like it. It landed on me and scared me half to
Signature: Christina

Male Black Horse Fly
Male Black Horse Fly

Dear Christina,
You can tell by the close set eyes that this Black Horse Fly,
Tabanus atratus, is a male.  You can compare your image to this image on BugGuide.  Only female Horse Flies bite and suck blood.  Males take nourishment from fruit.  According to BugGuide:  “Females feed on mammalian blood, while males, which lack mandibles, feed on nectar and plant juices (Long 2001). This species especially prone to attack cattle and other livestock.”

Letter 5 – Male Black Horse Fly


Subject: Fly on the Natchez Trace
Location: Franklin, Tennessee
July 4, 2016 7:37 pm
Hi bug guys,
I came across this fellow on a hike this weekend along the Natchez Trace outside of Nashville, TN. It was buzzing loudly and attempting to fly but clearly has some wing damage and is approximately 1-2in in length. The picture doesn’t capture the pretty blue coloring of it’s eyes. I have lived here for about 8yrs and have never seen this species before.
What is it?
Signature: Drew

Male Black Horse Fly
Male Black Horse Fly

Dear Drew,
This is a male Black Horse Fly,
Tabanus atratus.  While female Horse Flies will bite and suck blood, males of the species feed on nectar.

Letter 6 – Male Black Horse Fly


Geographic location of the bug:  S.E. PA.
Date: 06/02/2021
Time: 02:31 PM EDT
How you want your letter signed:  HAL SLACKWAY

Male Black Horse Fly

Dear Hal,
You are absolutely correct that this is a Horse Fly.  More specifically it is a Black Horse Fly,
Tabanus atratus, and the large close set eyes indicate it is a male.  Despite its size, only female Horse Flies bite and feed on blood.  This male is perfectly harmless.

Letter 7 – Male Black Horse Fly


Subject:  huge black fly on awning to be out of the rain
Geographic location of the bug:  smyrna delaware 19977
Date: 08/09/2021
Time: 08:53 PM EDT
Your letter to the bugman:  Exactly 1.25 inches long. I have never seen a fly this large, ID??
How you want your letter signed:  moliken

Male Black Horse Fly

Dear Moliken,
This is a male Black Horse Fly,
Tabanus atratus.  Observe how close together the eyes are spaces.  That is a sign it is a male.  Female Horse Flies have a noticeable space between the eyes.


  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

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  • Piyushi Dhir

    Piyushi is a nature lover, blogger and traveler at heart. She lives in beautiful Canada with her family. Piyushi is an animal lover and loves to write about all creatures.

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