May 2002

Yellow Flies

            In Florida, the name “yellow fly” is used to describe about a dozen different species of yellow-bodied biting flies. “Yellow flies” readily attack man and are usually abundant in Florida from March through November with peak annoyance occurring in May and June. “Yellow flies” are in the family known as Tabanidae. All tabanids go through an egg, larva, pupa and adult stage, referred to as “complete metamorphosis,” the same development process that mosquitoes go through. Tabanids lay egg masses containing 50 to several hundred eggs. Most species deposit their eggs around ponds, streams or swamps on overhanging vegetation such as grasses or cattails. After 5 to 12 days the eggs hatch and the larvae drop into the water or mud where they feed on organic matter or prey on small aquatic organisms. Larvae can be collected from salt marshes, swamps, bogs, heavily wooded ponds, lakes and streams and are similar in appearance to housefly maggots. The larva, or growing stage of these insects lasts through the winter. Larvae grow to about a half-inch in size, migrate to drier soil and develop into the pupa. The pupa is a non-feeding, resting stage in which the organism develops into an adult fly. From egg to adult takes about one year. Adult females are ready to seek a blood meal within hours of emergence, whereas male flies do not take blood and are thought to be nectar feeders.

            “Yellow flies” congregate in shaded, humid areas on the edges of forests, rivers and creeks, avoiding large, open, sunny areas. Some species are active in the early morning while other species are more active in the late afternoon. However, depending upon conditions such as heavy tree canopy, cloudy days or when an animal is moving through an area, yellow flies can become active and bothersome throughout the day. “Yellow flies” prefer to attack man about the head, neck or shoulders, however, any exposed part of the body may be attacked. Their persistence to obtain a bloodmeal, once a host is found, can make 5 or 6 flies unbearable. Sometimes the use of the old-fashioned fly swatter will give relief by taking out the few flies in an area. A trapping method homeowners can use, which does not involve the use of pesticides, is called the “sticky black ball” trap. Basically it is a beach ball, painted black and coated with a sticky substance. These devices are hung from a tree limb using string, in a shaded area, about 4 feet above the ground. Movement of the ball by the wind attracts the flies, thinking it is an animal, they land, get stuck, and die on the ball. Several of these balls will reduce the local adult populations. In the South Walton Mosquito Control District, we will provide a ball and instructions so a resident can make more if he or she feels they are effective for their situation. Vegetation management, by removing underbrush around a residence, helps to eliminate resting sites for the flies and increases light levels making the area less attractive.

            Outdoor aerosol pesticide spraying, such as the type produced by our Ultra-Low-Volume (ULV) spray units, used for adult mosquito control, is not effective against adult “yellow flies”. Aerosol sprays are designed to stay in the air column near the ground and slowly drift through an area and contact actively flying insects. The reason the aerosol spray doesn’t work well with “yellow flies” is because they fly during the daytime but only when they see a potential host, they are not active fliers. Also, the environmental conditions, updrafts or higher winds during the day, blow the aerosol spray out of the area making it useless.

            Larval control of “yellow flies” with pesticides cannot be done. Because their larval stage is in the mud, at the bottom of aquatic systems, you would need to place the pesticide into the substrate. Such treatment would kill everything including beneficial organisms in the water above the substrate. Because of this, there are no pesticides labeled for such use.

            One method shown to be effective is the application of residual insecticides on vegetation that adult flies rest on. This is called a “barrier spray” treatment. The flies rest on the vegetation next to an open area, contact the residual pesticide that has been sprayed on the vegetation and die. Products containing permethrin, carbaryl or malathion, applied once a week, have been shown to be effective.

            Personal protective measures include the use of repellants containing DEET (diethyl metatoluamide) applied to exposed skin. Wearing physical barriers such as a head net, long sleeve shirts and long pants are effective protection if you must be out in areas where “yellow fly” populations are high. Permethrin containing products labeled for application to clothing only, can also be effective in repelling and killing “yellow flies”. Thankfully, “yellow fly” season is relatively short, one maybe two months, and then mosquitoes become our major pest.