When the black flies are bad, people take steps from wearing special hats with netting to simply refusing to go outside.
"Black flies, often referred to as gnats, are bothersome blood-feeding insects," according to William Andrus, Pennsylvania Department of Environmental Protection water pollution biologist.
They make nuisances of themselves by hanging around people's heads and biting.
"They are attracted to the carbon dioxide that we exhale and often fly into the eyes, mouth or nose of people or animals," Andrus said.
According to Farmers' Almanac, "Black flies do not spread disease among humans, though their bites are painful, itchy, and slow to heal. They can cause severe allergic reactions in some, up to and including death. Because black flies tend to swarm their prey, a single victim can receive numerous bites in a short period of time, increasing the likelihood of a dangerous reaction."
This is a bad year for the pests.
"Black flies seem to be more numerous this year than most primarily due to the frequent storms and showers which cause several critical spray events to be delayed or canceled," Andrus said.
When it can be done, spraying over black fly spawning grounds is considered an effective method of reducing the population. People who come in contact with the spray are advised to wash with soap and water, but it is not, according to Andrus, environmentally hazardous.
"The active ingredient in the spray is a specific bacterium that, when ingested by black fly larvae, destroys the digestive system leading to death of the larvae," he said. "Ongoing studies over many years have demonstrated that spray activities have no effect on other aquatic organisms."
According to Farmers' Almanac, "Unlike mosquitoes, which breed in stagnant water, black flies lay their eggs in clean fast-running water, such as rivers and streams."
In Warren County, the Allegheny River and the Conewango and Brokenstraw creeks are the most commonly sprayed areas, according to Andrus.
Wearing long sleeves and long pants, along with hats with netting, is a good deterrent.
When more clothing isn't enough, Farmers' Almanac recommends natural repellants including vanilla extract, lavender, and the extract from pine branches - "just break open a young branch and rub the moisture from inside on your skin."
Commercial insect repellants containing DEET may or may not be effective against black flies, according to Farmers' Almanac. "Repellants containing Permethrin are more likely to be effective, but they are also more toxic that DEET sprays and can be harmful to fish, cats, and beneficial insects such as honey bees."