Yellow Flies

Facts, Identification & Control

Scientific Name

Diachlorus ferrugatus


What Does a Horse Fly Look Like?
Yellow flies undergo complete metamorphosis, meaning their life cycle involves four stages: the egg, larva, pupa and adult stage. The adult is a primarily yellow fly about 3/8 inches long and similar in appearance to deer flies. While their body is yellow, their front legs are mostly black and the other two pairs of legs are yellow. The yellow flies’ wings are clear, but have black and yellow wing veins with prominent brown patches toward the middle and rear of the wings. Their large eyes are blue-green with two purple bands. Yellow fly eggs are small, about 1 ½ millimeters. They are white when laid by the female, but turn black several hours after being deposited. Egg masses are commonly mistaken for feces or specks of tar. Yellow fly larvae grow to about ½ inch long, are aquatic or semi-aquatic, slender and are covered with very fine yellowish-brown short hairs on their otherwise whitish-colored body.

Yellow Fly
Yellow Fly image licensed under CC

Behavior, Diet & Habit

As larvae, yellow flies feed on decaying organic substances and molt about 10 times before pupating. In Florida, field researchers have found yellow fly larvae in shaded areas in root mats of cypress, shingle oak and other woody plants, always beneath the water surface. Once mature, larvae will migrate to drier soil conditions and develop into non-feeding pupae.

The yellow fly adult female is a fierce biter and can deliver a bite that is painful and causes itchiness and swelling around the bite. Like mosquitoes, the female consumes blood while the males do not bite and are mainly pollen and nectar feeders. Females are most actively biting during the late afternoon before twilight and on cloudy days. Yellow flies are strong fliers, and females may travel long distances in search of blood meals. Generally, yellow flies stay around the larval breeding site, though.

The general signs and symptoms of yellow fly bites are:

  • Localized swelling and an itchy red area around the bite.
  • Itching and scratching of bite wounds which can lead to secondary bacterial infections if the site of the bite is not kept clean and disinfected.
  • Since yellow flies inject anticoagulant-containing saliva during blood feeding, some life-threatening reactions may occur in people that are highly allergic to the anticoagulant compounds.

Yellow flies are commonly found in and around wooded areas, but breed prolifically when near large bodies of water.
Interestingly, yellow flies are one of the few species of the Tabanidae family that will assault hosts indoors. Their peak activity season in Florida and much of its southern distribution is April through June, but they may become problematic anytime from early spring through late fall.


Mating takes place soon after adults emerge from the pupal stage. After mating, females produce egg masses, which they hide on plants, rocks, sticks, and other materials found near water. After 5 – 12 days, the eggs hatch and the young larvae drop into the water or mud and begin feeding. Depending on the species and environmental conditions, there are only one or sometimes two generations per year.

Signs Of A Yellow Fly Infestation

The most obvious sign of a yellow fly infestation is their bothersome and painful bite and the symptoms associated with it.


Diachlorus ferrugatus are found in the southeastern U.S. from New Jersey to Texas, as well as in the Bahamas and from Mexico to Costa Rica. The only species found in the U.S., Diachlorus ferrugatus is thought to have traveled to the states from Mexico.

More Information

Unfortunately, there are no effective, practical broad-scale control methods for larvae and adult with the exception of using traps to help reduce the number of adults in small areas such as yards, recreational sites and around swimming pools. While insect repellents are moderately effective, one of the best techniques for bite prevention is to wear clothing that covers exposed skin and may include using gloves and head nets. It is prudent to contact your pest management professional for advice and options that may be useful to mitigate the problems created by yellow flies.

Yellow flies are members of the Tabanidae family that also includes horse flies and deer flies.