Those Pesky Little Black Flies

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Those Pesky Little Black Flies

By Lee S. Yudin, Ph.D.
August 28, 1999

Due to the many phone calls I have received from island residents over the past couple of weeks concerning “Those Little Black Flies,” it is time once again to write about these little pests that can be such a big problem. Black-winged Fungus Gnat is the common name of these pesky flies. A gnat, being a true fly, has only one pair of wings.

            The majority of callers want to know how to control these gnats. Some wonder if the gnats are harmful if accidentally swallowed. Others question whether gnats can cause allergic skin reactions or even ocular infections if they get into their eyes. There are many other homeowners that have similar questions.

            Let’s take the easy question first. No, they are not harmful if you happen to swallow them or digest them with some of your food. Most people have no idea the percent of insects they eat, on a daily bases, in the fresh and processed food we all consume. There are some cultures where insects make up a very large percent of the diet.

            Can they cause allergic skin reactions? I would estimate that 99.9% of those of us who come in contact with these gnats (either in our hair or on our skin) will not have any allergic skin reactions. But there may be some individuals who in fact are more sensitive to insects and show some skin allergies if these gnats are crushed into the skin as they are being slapped.

            Compared to our thick skin, the human eye is a very sensitive organ. Any small insect, including these fungus gnats, could easily cause eye irritation and perhaps an ocular infection if the gnat is not immediately washed out of the eye. My advice to anyone who is having difficulty seeing due to these insects getting into their eye is to see a doctor as soon as possible.

            Now the most difficult question, how to control them? First you must understand that all flies go through four stages of development: egg, larva (often called maggots), pupa and adult. Adult fungus gnats are about 1/8 to 1/10 inch (2.5 mm) long, black, slender, mosquito-like, and delicate with long legs, antennae and one pair of wings. Larvae or maggots are legless, thread-like, white, shiny white and semi-transparent.

            With the small black fungus gnats, population levels are directly correlated to the increase of fungal decomposition. Rain with corresponding high humidity will increase the growth of fungi (mold) and increase the food potential for the developing larval. As adults, gnats have the ability to fly and seek out more breeding sites for their offspring. Fungus gnats are highly attracted to lights and, due to their very small size, can easily go underneath doors and through most window screens as they enter our homes.

            Because of their small size it is difficult to totally secure one’s house. A very mild dilution of beach (two tablespoons of bleach per quart of water), mixed and sprayed from a spray bottle, will kill off any flying gnat or even those gnats at rest on counter tops or resting on glass windows. I would also suggest that you use the bleach spay in any open floor drains which are inside you house. Care needs to be taken not to damage fabric with this mixture. Make sure your window screens and especially those doors that lead to the outside of your home are well sealed.

            For outside the house, I suggest that you try a couple of control measures. The first one is a non-chemical approach. Set out a jar (or jars) of wine or cider vinegar outside the house in a covered area. The gnats will be attracted to the smell, dive in for a drink and drown. You will need to replenish the wine or cider at least once a week. Most garden insecticides can also be sprayed around the perimeter of the house to help reduce the young larvae that may be developing in organic material around the house. The difficulty is that many of these adult fungus gnats are living outside the perimeter of your home and are attracted to outside house lights and take shelter underneath car ports during periodic rains. Since the developing larvae are fungus feeders, the more organic material (decaying plants and grass) that surrounds you house the greater number of these flies you yourself could be raising. One hundred percent control is just not possible.

            Fungus gnats are going to be around until the raining season is over. The dry season will reduce their breeding sites and their numbers will be greatly reduced. But you can be assured they will be back again next year when the rains return. Some say, it is the price we pay when living in the tropics.

            For more information about household pests please contact your local Cooperative Extension Service.