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 biting insects is 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 they, too, have a place in our gardens.
Black flies are true flies (order Diptera) in the family Simuliidae and are closely 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 species bite humans. The remaining species in Pennsylvania target other warm-blooded animals, mostly 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 needlelike mouthparts, the structures in a black fly’s mouthparts enable it to slice through the skin and lap up the blood that flows from the wound. Only the females 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 a fast-flowing river or stream, where they hatch and develop into larvae.
In early spring, look closely in a riffle 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 then the adults emerge 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 miles and use multiple senses to hone in on their targets. First, they fly perpendicular to the wind direction until they detect a stream of carbon dioxide emitted in the breath and through the pores of a warm-bodied animal. They follow it upwind directly to their potential meal. They are close enough now to detect the heat emitted by their warm-blooded target, and 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 make 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 these insects will repopulate the area immediately. Black flies are also not attracted to lights, so the insect control lights are ineffective.
The best option is to keep black flies from biting you. Chemical repellents, natural or synthetic, only prevent black flies from landing on and biting you. Repellents do not kill black flies nor stop their incessant buzzing. The commonly recommended synthetic repellents contain the chemical DEET, which effectively keeps black flies from landing and biting. You may try other repellents, natural or synthetic, including citronella candles and sweet-scented geranium (Pelargonium graveolens), also known as the mosquito plant. However, no solid evidence shows these are effective in repelling black flies.
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. Black flies are very small and are not strong enough to fly in strong wind.
The Pennsylvania Department of Environmental Protection sprays 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. Butler County does not participate in this program.
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. If you have questions about black flies, call the Penn State Extension Master Gardeners of Butler County’s Garden Hotline at 724-287 4761, Ext. 7, or email the Master Gardeners at butlermg@psu.edu.
Carol Chmielewski is a Penn State Extension Butler County Master Gardener.
