Black Flies in Our Gardens

One of the most enjoyable benefits of gardening is spending time outdoors. Braving inclement weather, getting covered in mud, and working up a sweat are all part and parcel of the job. However, most gardeners would agree that biting insects are one aspect of the great outdoors they could do without. Black flies, some of the most voracious biters, are a common pest in Pennsylvania. Perhaps if we know a little more about them and how to avoid them, we could appreciate that they, too, have a place in our gardens.

Black flies are true flies (Order Diptera) in the family Simuliidae. They are related to mosquitoes, deer flies, and house flies. They are small, slightly less than one-quarter inch long, with a characteristic humped back. There are over 2,300 species worldwide, with 53 species found in Pennsylvania. However, only four of these bite humans. The remaining species in Pennsylvania target other warm-blooded animals, primarily birds. These black flies prefer young birds in the nest, which are naked and defenseless. In tropical regions, black flies can transmit a serious disease called river blindness to humans. However, in Pennsylvania, black flies do not transmit diseases to humans. Any black fly bite is usually just an annoyance.

Unlike mosquitoes that pierce the skin with needle-like mouth parts, the structures in a black fly's mouth parts enable it to slice through the skin and lap up the blood that flows from the wound. Only the female black flies bite as they require the protein in the blood meal for egg development. Both males and females eat nectar as a carbohydrate source, although they are not significant pollinators. The eggs are laid directly into flowing water, where they hatch and develop into larvae. In early spring, look closely at a riffle in a river or stream where the water is moving quickly, and you may find half-inch-long larvae with one end attached to rocks and the other waving in the current. They have two tiny fans on their heads that filter food particles from the water. Eventually, they pupate and emerge as adults from the stream, riding an air bubble to the surface. The peak time for emergence is mid-May to mid-June. During this time, thousands of black flies emerge per square yard of stream surface.

With so many black flies emerging, they cannot be avoided during these months. They can fly for miles and use multiple senses to home in on their victims. First, they fly perpendicular to the wind direction until the black flies detect a stream of carbon dioxide emitted in the breath and through the pores of a warm-bodied animal. They follow the carbon dioxide plume upwind directly to their potential meal. Now close enough to detect the heat emitted by their warm-blooded victim, the large eyes of the black fly enable them to find the exact location of the best place to bite. They are attracted to dark colors, especially the borders between dark and light colors. For example, if you wear a dark jacket, black flies are attracted to the lighter-colored skin on your neck or wrists. Thinner skin in these areas and around the eyes, nose, and ears makes it easier for black flies to slice through the skin.

To enjoy the outdoors at this time of year, you can do very little to rid an area of black flies. There are just too many black flies, and they are too efficient at finding their next meal. Avoid yard fogging or pest control agents that claim to eliminate black flies from an area, as the biting insects will re-populate the area immediately. These broad-spectrum insecticides negatively impact our essential pollinators. Black flies are not attracted to lights, so the insect control lamps are ineffective.

Black flies are active biters during the day. They do not bite at night. Black flies are more active at certain times of the day, depending on the weather. Biting activity peaks around 9:00 to 11:00 a.m. and again from 4:00 to 7:00 p.m. or until the sun falls below the horizon. Black flies are usually most active on humid, cloudy days and just before storms. If possible, avoid outdoor activities during those peak times. The best times to work outside are early, midday, and late evening.

The best option is to keep black flies from biting you. Chemical repellents, natural or synthetic, can prevent black flies from landing on and biting you. Repellents do not kill black flies nor stop their incessant buzzing. The most recommended synthetic repellents contain DEET (N, N-diethyl-meta-toluamide), keeping black flies from landing and biting. While some essential plant oils found in lotions or candles may act as black fly repellents, the plants themselves do not repel black flies. The strength of the scent in the living plant is not concentrated enough to be effective as a repellent.

Physical methods to avoid being bitten by black flies include wearing light-colored clothing that covers as much skin as possible. Clothing or head coverings made of insect netting are helpful if one must spend time outdoors. Netting on window screens and tents is useful if the mesh is small enough to exclude black flies. Another method to keep black flies out of a small outdoor area, such as a patio, is to place a fan to blow them away. The wind from a fan works in two ways. Black flies are very small and are not strong fliers against an airstream. This breeze also helps blow away the carbon dioxide produced as we breathe.

The Pennsylvania Department of Environmental Protection conducts an aerial spray program that applies the naturally occurring soil bacterium Bacillus thuringiensis israelensis (Bti) on 1,800 miles of streams and rivers to reduce the number of emerging black flies. Bti is non-toxic to humans, mammals, birds, fish, and most invertebrates. Of Pennsylvania's 67 counties, 37 participate in this program. Because black fly larvae move within a stream and adults fly miles to obtain a blood meal, treating a portion of a nearby stream is ineffective.

As annoying as black flies may be, they are an integral part of our ecosystem, with their aquatic and terrestrial life stages providing food for birds, bats, fish, amphibians, and other invertebrates.