Black Fly: All You Need to Know for a Bug-Free Summer

Black flies are small, bloodsucking insects that can be quite a nuisance during warm days.

Often referred to as gnats, buffalo gnats, or turkey gnats, these insects swarm around the heads of people and animals, delivering painful and itchy bites in some cases.

They measure about 1/8 inch in length and have a stout-body, hump-backed appearance [1].

Black Fly
SONY DSC

These insects thrive in flowing water, preferring non-polluted water with high dissolved oxygen levels [2].

Larval habitats can range from large rivers and icy mountain streams to trickling creeks and waterfalls, with each species favoring different habitats.

The presence of black flies can vary depending on the area, the time of year, and even annual fluctuations in population sizes.

Black Fly Basics

Identification and Habitat

Black flies, also known as buffalo gnats and turkey gnats, are small, bloodsucking insects with a hump-backed appearance.

They live as larvae in shallow, clear, fast-running water in rivers and streams.

Suitable aquatic habitats for black fly larval development vary and include large rivers, icy mountain streams, trickling creeks, and waterfalls.

Life Cycle

The life cycle of a black fly has four stages:

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

Larvae and pupae develop in flowing water, typically non-polluted water with a high level of dissolved oxygen.

Adult flies emerge in the spring and are usually present for about 3 weeks before they die.

Size and Appearance

Black flies are about 1/8 inch in length, making them much smaller than a house fly. They have a stout body and a hump-backed appearance.

These insects are known for swarming around people’s heads on warm days. They often fly into the eyes and can deliver a painful, itchy bite.

Comparison Table

FeatureBlack FlyHouse Fly
Size1/8 inchLarger than 1/8 inch
AppearanceHump-backed, stout bodyFlat, no hump-back
HabitatFast-flowing waterVarious environments
Life Cycle3-4 weeks2-4 weeks

Source: KKPCWCC BY-SA 4.0, via Wikimedia Commons

To summarize, the key features and characteristics of black flies include:

  • Small size (1/8 inch)
  • Stout, hump-backed body
  • Live in fast-flowing water habitats
  • Bite humans and animals
  • Swarms around people’s heads
  • Short life cycle (3 weeks)

Feeding Habits and Biting Behavior

Diet and Blood Meal

Black flies, also known as buffalo gnats, are small insects with a humpbacked appearance.

They feed on nectar, plant sap, and insect honeydew, while the females require a blood meal for reproduction1. Examples of animals bitten by black flies include:

  • Humans
  • Birds
  • Mammals

Effects on Humans and Animals

Black flies can have different effects on humans and animals. People who are bitten may experience:

  • Pain or itchiness at the bite site
  • Swelling
  • Headache
  • Nausea2

In some cases, reactions to black fly bites can lead to “black fly fever,” which includes swollen lymph nodes in the neck3.

As for animals, black fly bites can be lethal in extreme circumstances, causing severe blood loss4.

When it comes to avoiding black fly bites, there are a few preventive measures to consider:

  • Wearing light-colored clothing, as black flies are more attracted to dark colors5
  • Using insect repellents containing DEET6
  • Employing a fan or creating a breeze to deter the flies
  • Protecting more sensitive areas like the head, eyes, and neck
MethodsProsCons
Light colorsLess attractive to fliesLimited clothing options
DEET repellentEffective protectionCan cause allergic reactions for some people
Fan/breezeHelps to deter fliesNot always available or practical

Source: xpdaCC BY-SA 4.0, via Wikimedia Commons

Remember, taking appropriate preventive measures can help minimize the risk of encountering black flies and their painful bites.

Diseases Transmitted by Black Flies

Black Fly Fever

Black fly fever is a condition caused by the bites of female black flies. Symptoms include:

  • Nausea
  • Fever
  • Headaches

Black fly fever usually lasts for a few days and isn’t fatal. However, individuals with weakened immune systems may experience more severe symptoms.

River Blindness

Onchocerciasis or River blindness is a disease caused by a parasitic worm transmitted through the bites of infected black flies. Some effects of the disease are:

  • Skin itching and rashes
  • Nodules under the skin
  • Vision loss or blindness

River blindness is a major public health concern in some parts of Africa and Central and South America.

Other Diseases

Black flies can also transmit other diseases to humans and animals. Some common issues include:

  • Infections from bites
  • Blood loss leading to anemia or even death in severe cases
  • Suffocation in poultry and livestock due to black flies swarming and crawling into their airways

In addition to humans, black flies can attack many different domestic and wild animals, including birds, pets, and livestock, leading to significant economic and health impacts.

Preventing and Controlling Black Fly Infestations

Protective Clothing and Repellents

When venturing outdoors in infested areas, wearing protective clothing can help minimize bites from black flies. Some examples include:

  • Long-sleeved shirts
  • Long pants
  • Hats with a head net

Additionally, applying repellents with DEET or picaridin can effectively deter black flies. Reapply repellents as needed, and always follow the product’s instructions.

Environmental and Organic Control Methods

In order to control black fly infestations, it is essential to reduce their preferred breeding sites.

  • Remove organic matter from streams and rivers.
  • Encourage natural predators such as fish.

Pro tip: Install netting or use fans to create a barrier against black flies.

Comparison Table of Control Methods

MethodProsCons
Protective ClothingNon-toxic; reusableMay be hot; limits skin exposure
RepellentsEffective; portableRequires reapplication; may cause itching
Environmental ControlsLong-term; eco-friendlyRequires effort; depends on nature
Organic Controls (essential oils)Non-toxic; eco-friendlyLess effective; requires reapplication

Source: Flickr, Username: Jean and Fred Hort

Keep in mind, black fly bites can cause swollen lymph nodes and itching. If bitten, wash the area with soap and water, and avoid scratching.

Remember, black flies can also transmit diseases to both humans and wildlife, so it is essential to take preventative measures when spending time outdoors in their presence.

Conclusion

In conclusion, the Black Fly, an aquatic insect, is a significant concern during the summer months.

Found in running waters, these flies can deliver bites that, while painless initially, result in itchy, swollen areas that may bleed.

Severe attacks can lead to symptoms like headaches, fever, and swollen neck glands, a condition sometimes termed “black-fly fever.”

Although they play a role in the ecosystem, understanding their behavior and habitat is crucial for a bug-free summer.

Footnotes

  1. Buffalo gnats and how to avoid being bitten – Illinois Extension

  2. Black flies | UMN Extension

  3. Black Flies | Public Health and Medical Entomology | Purdue | Biology …

  4. Black flies | UMN Extension

  5. Biting Flies – 5.582 – Extension

  6. Black flies | UMN Extension

Reader Emails

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

Letter 1 – Black Fly Larvae from Australia

Subject:  Bug in freshwater fountain
Geographic location of the bug:  Victoria Australia
Date: 02/07/2018
Time: 05:00 PM EDT
Your letter to the bugman:  Hello,
We have a fresh water pond with running water along a pot. This is at the top of the pot and these hanging in the water. Fixed by their back end, hanging down, and with two antennae like.


They are 6-8mm in length and in a group of about 20!
They seem the wrong shape for the mosquito larvae I have seen, but not sure if they are!
Thanks for your help
How you want your letter signed:  Julien

Black Fly Larvae

Dear Julian,
Your query has us quite intrigued.  We concur that these are NOT Mosquito Larvae.  Mosquito Larvae breathe through a siphon and they congregate at the surface of generally stagnant water.  We suspect these are larvae and that they are members of the Fly order Diptera. 

We also suspect that in their natural environment, they affix themselves to rocks in flowing streams.  We located this image on SlideShare of some aquatic Dipteran larvae and several resemble your individuals. 

At this point, we suspect this might be a member of the Black Fly family Simulidae, and according to BugGuide:  “Larva: brown, gray, or black with light brown head; body cylindrical, somewhat club-shaped; head with prominent pair of mouth brushes used for filtering food from the water” and “larvae develop in running water of all types, from the smallest seepages and streams to the largest rivers and waterfalls; they attach themselves to underwater rocks and other objects by means of small hooklets in a sucker-like disc at the tip of the abdomen.” 

Bug Eric has some very similar looking images.  Though they are not Mosquitoes, female Black Flies are blood suckers.  According to BugGuide:  “Black flies attack most severely about sunrise and at sunset — either massively and viciously or in such small numbers that they are scarcely noticeable. They bite painlessly so that you may not be aware of having been attacked until small droplets of blood start oozing from your skin.

Black flies often crawl into your hairline or through openings in your clothes before they bite you. Therefore, the bites are usually behind your ears, around your neck and beltline, and on the lower parts of your legs. A typical bite consists of a round, pink, itchy swollen area, with a droplet of fresh or dried blood at the center.

When the blood is rubbed away, a minute subcutaneous hemorrhage is visible. This hemorrhage and the surrounding pink area become diffuse and larger, and then disappear within a few days. Itching may continue intermittently for weeks, whenever the bitten area is rubbed. Scratching may cause severe secondary skin infections.

Toxins injected during an extended severe attack can cause a general illness sometimes called black-fly fever, characterized by headache, fever, nausea, and swollen, painful neck glands. Attacks occur throughout late spring and early summer (sometimes throughout the summer).

(Fredeen 1973)  often ranked third worldwide among arthropods in importance as disease vectors, but only ~10-20% of the world’s spp. are pests of humans/livestock.” According to Atlas of Living Australia

“Most black flies gain nourishment by feeding on the blood of mammals, including humans, although the males feed mainly on nectar.”

Black Fly Larvae

Dear Daniel,
Thank you so much for looking into that!! Your suggestion and pictures are very convincing. We do have these flies around as well. I have taken pictures again and I think that we can see the one in their cocoon on the bottom and the other one on top!
Your site is a great help!
Best wishes
Julien

Black Fly larvae and pupae

Letter 2 – Yellow & Black large fly

Hi – We seem to be attracting a type of fly into our house that I’ve never seen before; It’s about 2cm long. Has a yellow & black striped abdomen approx 1cm & quite fat.

A brown beetle like back between its head and abdomen 6 Brown legs 2 very big browny transparent wings large eyes with a central yellow stripe between thema pointy chin with a little feeler hanging centrally under it.

Does it sound familiar…??? Hope you can help.
Regards,
Anthony

Dear Anthony,
There are some types of flies which mimic bees, and are colored as you describe. They belong
to the Family Syrphidae and are called Flower Flies. They are beneficial to gardeners.

Letter 3 – Black Scavenger Flies

Curious Girl does Spring in Europe (New species for WTB — I think)
Location:  Porto, Portugal
March 20, 2014
Hope you will forgive me Daniel for sending you the pictures this way. You can open just one mail and then pick & choose, and it’s much easier for me as I would not probably get to it if going through the web form. I do not know what most of them are and would like confirmation on those I think I might know. :~)
Seems the slow season is over!
Anyway…


I have some decent pictures of some insects both in Germany (Hanau this time) and Portugal (Porto though some from Tras os Montes soon) before I left (currently in Istanbul) so I will share a few with you, including a few I thought were just flying ants but turned out to be something far more interesting and even quite nice pictures so I am glad I bothered. Plus I do not think they are represented on the WTB site.


They are Ensign or Black Scavenger Flies of the family Sepsidae. 🙂 And like the big Crane Flies seem to have vestigal wings. Apparently all females too.
They are Ensign or Black Scavenger Flies of the family Sepsidae. 🙂
And thus concludes part one of CG’s Spring 2014 Euro tour. :~)
Thanks & Happy Nowruz Daniel! 🙂

Black Scavenger Fly
Black Scavenger Fly

Dear Curious Girl,
Thank you for your multiple attempts to get this plethora of new imagery to us.  You are correct that we do not have Black Scavenger Flies in the family Sepsidae represented on our site, so we are posting several of those images and creating a new category. 

According to BugGuide:  “Small, shining blackish flies, sometimes with a reddish tinge; spherical head; abdomen narrowed at the base. Many species have a dark spot along the costal margin of the wing near the tip.” BugGuide also notes:  “Larvae live in excrement and various types of decaying matter. Adults found near material that the larvae feed on.” 

It is somewhat difficult for us to create multiple postings from a single email, so for the time being, the only posting we are creating with these images is the Black Scavenger Fly posting.  When time permits, you can try resubmitting other images using our standard submission form, limiting each submission to a single species, and ensuring that location information is correct for that species.

Black Scavenger Fly
Black Scavenger Fly

Letter 4 – Unknown Winged Insect found Submerged in a Stream is Black Fly

Subject: What’s that Bug (Aquatic Fly)?
Location: South Carolina
June 28, 2016 8:50 am
We found these while doing a stream walk along an urban stream channel in the upper sand hills/lower piedmont of South Carolina. They were found clinging to the underside of the submerged cobble in the riffle sections.

Any idea of what they might be? Sorry for the low quality images, but were taken with our phone.
Signature: Thanks!

Midge, we believe
Unknown Insect

We have been having a difficult time understanding why a winged insect would be submerged under a rock as this insect looks like it would not be aquatic as an adult. 

It looks very much like some of the winged insects in this posting from our archives that we believe may be Midges with a Fungus Infection, also found near a stream. 

We are posting your image and requesting assistance from Eric Eaton and our readership.  We are further confused because your insect appears to have two pairs of wings, and a Midge would have only one set of wings.

Update:  June 30, 2016
Eric Eaton is also unable to provide an identification, but he does agree that it appears to have two pairs of wings, which would eliminate it being a Midge.

Entomologist Julian Donahue responds to our request:
I’m stumped. Body shape looks like a primitive fly (with that small head and short abdomen), but the venation looks more like that of a primitive moth.
I agree that it appears to have two pairs of wings.

It doesn’t look like any aquatic insect I know, nor does the venation agree with any aquatic insect I know.
It may be a teneral (freshly-emerged) specimen, and there’s always a chance that it’s a terrestrial insect that got washed into the stream.
Let me know what you find out about this one.
Julian

Update:  July 1, 2016
In addition to the above possible classifications, we are reminded of some Sawflies regarding the general appearance of this critter.  There are many Sawfly images on BugGuide, but Sawflies are NOT aquatic.

Julian Donahue provides tentative identification
On second thought, I’m going to go with my initial gut reaction: a black fly (Simuliidae). Habitat is perfect, body shape fits, and they have broad wings that can fold over giving the appearance of two pairs.

It still seems to have too many veins in the wings, but this could be the result of some sort of duplication in the pupa or upon emerging. I still think it’s a teneral specimen, which may account for the odd appearance of the wings.
Julian

Ed. Note:  Thanks to Julian Donahue’s identification, we are linking to the Black Fly page on BugGuide where they are described as:  “black to various shades of gray or yellow; thorax shiny, strongly convex, giving a humpbacked, gnat-like appearance; wings clear, broad, without hairs or scales; heavy veins near anterior wing margin, weak veins posteriorly; small head with large round eyes and short 11-segmented antennae; ocelli lacking.” 

The habitat is listed as:  “larvae develop in running water of all types, from the smallest seepages and streams to the largest rivers and waterfalls; they attach themselves to underwater rocks and other objects by means of small hooklets in a sucker-like disc at the tip of the abdomen.” 

This BugGuide remark also may be of interest to our readers:  “Black flies attack most severely about sunrise and at sunset — either massively and viciously or in such small numbers that they are scarcely noticeable. They bite painlessly so that you may not be aware of having been attacked until small droplets of blood start oozing from your skin.

Black flies often crawl into your hairline or through openings in your clothes before they bite you. Therefore, the bites are usually behind your ears, around your neck and beltline, and on the lower parts of your legs. A typical bite consists of a round, pink, itchy swollen area, with a droplet of fresh or dried blood at the center. When the blood is rubbed away, a minute subcutaneous hemorrhage is visible.

This hemorrhage and the surrounding pink area become diffuse and larger, and then disappear within a few days. Itching may continue intermittently for weeks, whenever the bitten area is rubbed. Scratching may cause severe secondary skin infections. Toxins injected during an extended severe attack can cause a general illness sometimes called black-fly fever, characterized by headache, fever, nausea, and swollen, painful neck glands.

Attacks occur throughout late spring and early summer (sometimes throughout the summer). (Fredeen 1973)(5)  often ranked third worldwide among arthropods in importance as disease vectors, but only ~10-20% of the world’s spp. are pests of humans/livestock.”

Authors

    by
  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. whatsthatbug.com 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.

  • 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.

3 thoughts on “Black Fly: All You Need to Know for a Bug-Free Summer”

  1. Oh, did not get a notice these were posted.

    Sorry Daniel. Learned my lesson. Seemed it would be easier (but probably not for you).

    Do you want me to re-submit the other pics (or just let it go)?

    Anyway, seems this particular fly that was posted (with no black spot on the wings) is, Nemopoda nitidula. Several sites say that the Sepsidae fly is often mistaken for flying ants which is of course what I thought they were. Glad I took the pictures anyway.

    Reply
    • That is up to you if you want to go through the trouble. The format you sent was very difficult to reformat for posting.

      Reply
  2. Oh, did not get a notice these were posted.

    Sorry Daniel. Learned my lesson. Seemed it would be easier (but probably not for you).

    Do you want me to re-submit the other pics (or just let it go)?

    Anyway, seems this particular fly that was posted (with no black spot on the wings) is, Nemopoda nitidula. Several sites say that the Sepsidae fly is often mistaken for flying ants which is of course what I thought they were. Glad I took the pictures anyway.

    Reply

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