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<\/p>\nMay 6, 2026, 4:58 a.m. ET<\/p>\n
Pennsylvania’s Department of Environmental Protection (DEP) runs a suppression program to control black fly populations.Black flies, also known as biting gnats, returned to the state’s waterways after the Clean Water Act improved water quality.Treatment is necessary for public health, tourism, and to prevent negative economic impacts on outdoor activities.<\/p>\n
If\u00a0you\u2019ve\u00a0ever\u00a0encountered\u00a0dozens of tiny flying bugs buzzing around your head on your early morning walk,\u00a0you\u2019ve\u00a0probably\u00a0encountered\u00a0one of Pennsylvania\u2019s biggest pests.\u00a0<\/p>\n
Black flies\u00a0have become such a nuisance over the past 40 years\u00a0in York, Adams and\u00a033 other\u00a0counties,\u00a0the Commonwealth\u2019s Department of Environmental Protection (DEP)\u00a0developed a spraying program to keep them under control.\u00a0<\/p>\n
The Pennsylvania Black Fly Suppression Program began its treatment of rivers and streams in April and will continue to do so\u00a0until Labor Day in September in an effort to keep the little pests under control.\u00a0<\/p>\n
DEP Black Fly Suppression Program Manager Doug Orr\u00a0said that a black fly is the proper name for a biting gnat, which is a huge nuisance in the Commonwealth in areas where rivers and streams are prevalent.\u00a0<\/p>\n
Biting gnats<\/p>\n
\u201cThese biting gnats require clean, fast-flowing water,\u201d Orr said. \u201cPennsylvania has\u00a085,000 miles\u00a0of stream. We have a lot of water that produces black flies in\u00a0tremendous\u00a0amounts.\u201d<\/p>\n
MORE: How this homeless woman came to live in a shuttered York Twp. Rite Aid<\/a>MORE: Cold snap could reduce York County’s pick-your-own strawberry season<\/a><\/p>\n Native black flies\u00a0made a\u00a0comeback\u00a0after the enactment of the Clean Water Act in 1972,\u00a0when many of Pennsylvania\u2019s rivers and streams were cleaned up.\u00a0<\/p>\n \u201cThey\u2019re just expanding every year,\u201d Orr said. \u201cThis year, we\u00a0actually added\u00a0another 100 miles of streams and rivers that have been cleaned up from acid rain and thermal pollution from a power plant.\u00a0It\u2019s\u00a0a\u00a0pretty big\u00a0problem here in Pennsylvania.\u201d\u00a0<\/p>\n Conewago Creek in Adams and York counties; Yellow Breeches Creek, which is on the border between York and Cumberland counties; and the main stem of the Susquehanna River\u00a0in York County\u00a0are treated during\u00a0black fly\u00a0season.\u00a0<\/p>\n A biologic product, not\u00a0a pesticide,\u00a0is\u00a0used to keep them under control, Orr said.\u00a0Application of Bacillus thuringiensis\u00a0israelensis,\u00a0more commonly known as Bti,\u00a0began on\u00a0April\u00a016\u00a0in York and Adams counties. It\u00a0is a bacterial larvicide, not a chemical, and is the only product used in Pennsylvania’s waterways for black fly suppression.\u00a0<\/p>\n \u201cIt\u2019s an amazing product,\u201d\u00a0Orr\u00a0said. \u201cIt\u2019s\u00a0amazingly effective when done correctly. The\u00a0great thing\u00a0is, this program has been running for 40 years now, and we have\u00a0very good\u00a0biologists who understand the process from A to Z.\u201d\u00a0<\/p>\n He said\u00a090 to 95% of the black fly population\u00a0is eliminated when\u00a0Bti\u00a0is applied.\u00a0<\/p>\n Black flies lay eggs in fast, flowing water and spend three-quarters of their lives in water. That’s why the waterways are treated, Orr said.\u00a0When they come out as adults, they can fly up to 20 miles.\u00a0\u00a0<\/p>\n \u201cWe have to control large sections of the\u00a0river\u00a0before they come out,\u201d he said. \u201cOnce\u00a0they\u2019re\u00a0out, there is no controlling\u00a0black flies\u00a0like we could control\u00a0mosquitoes in a neighborhood with\u00a0a nighttime\u00a0spraying.\u201d\u00a0<\/p>\n The\u00a0Bti\u00a0treatment is done every 7 to\u00a014 days, depending on the water temperature. Orr said\u00a0Bti\u00a0is an\u00a0organic\u00a0material that\u00a0only\u00a0kills black flies and mosquitoes.\u00a0<\/p>\n \u201cIt is very safe to put into the environment. It photodegrades, so\u00a0the sun destroys it once we put it in and\u00a0doesn\u2019t\u00a0accumulate in the environment,\u201d Orr said. \u201cBlack flies eat that and they die.\u201d\u00a0<\/p>\n >> Want to stay up to date on the latest news? Sign up for our newsletters.<\/a>\u00a0<\/p>\n DEP\u00a0determines\u00a0when\u00a0and where\u00a0to spray through sampling the water from streams and rivers.\u00a0\u00a0<\/p>\n \u201cWe send out our biologists to sample all these streams weekly,\u201d Orr said. \u201cThe samples are sent to a lab, our\u00a0Vector\u00a0Management\u00a0Laboratory in Harrisburg, to\u00a0identify\u00a0if the target species is there and whether\u00a0numbers\u00a0of the\u00a0target\u00a0species\u00a0are high\u00a0enough to decide to treat it.\u201d\u00a0<\/p>\n There are 53 species of black fly in Pennsylvania, but Orr said they only go after the ones that target humans.\u00a0<\/p>\n \u201cThat cuts it down to four species and one group\u00a0that comes after humans,\u201d he said.\u00a0<\/p>\n If an area\u00a0doesn\u2019t\u00a0have those target species, Orr said DEP\u00a0doesn\u2019t\u00a0treat those waters.\u00a0<\/p>\n \u201cIf we find the target species in moderate to high numbers,\u00a0we find out how many streams in the area\u00a0have the target species, then we\u2019ll put them altogether into a package and treat for a day or two\u00a0to control streams within\u00a015 miles of each other or maybe 100 miles of each\u00a0other,\u201d Orr said.\u00a0\u201cWith a helicopter, you can cover a lot of territory in a day.\u201d\u00a0<\/p>\n A smaller creek, like the\u00a0Yellow Breeches, is usually treated by hand because a large canopy of trees makes aerial spraying difficult.\u00a0<\/p>\n The species are prolific in their reproduction, Orr said.\u00a0<\/p>\n Please consider subscribing to support local journalism.<\/a>\u00a0<\/p>\n When the flies are most active<\/p>\n \u201cIt\u2019s not like we can\u00a0treat\u00a0once and walk away,\u201d\u00a0he\u00a0said. \u201cIt only takes a few females, who can up to 500 eggs. Even if we knock the\u00a0population\u00a0way back, it only takes a small\u00a0portion\u00a0of the population to regenerate and repopulate the stream.\u00a0It\u2019s\u00a0an ongoing thing.\u201d\u00a0<\/p>\n Black flies are\u00a0very sensitive\u00a0to temperature and humidity, so their activity, or when they bite you,\u00a0depends a lot on that, Orr said.\u00a0<\/p>\n \u201cIf you are ever outside on a hot summer day and there is a thunderstorm warning, you will see that black flies are super active,\u201d he said.\u00a0<\/p>\n Other than on those occasions, Orr said they are most active in the early morning and\u00a0early\u00a0evening,\u00a0and they\u00a0don\u2019t\u00a0fly after dark.\u00a0<\/p>\n \u201cOnce it gets\u00a0super-hot, they are so small the sun can\u00a0dry them up\u00a0real\u00a0quick.\u00a0It\u2019s\u00a0very harmful to them,\u201d he said.\u00a0\u201cThat\u2019s\u00a0why on a hot sunny day, you\u00a0won\u2019t\u00a0see them most of the time.\u201d<\/p>\n MORE: Finding gas below $4 per gallon is getting harder in York County<\/a>MORE: YAIAA track and field meet, tournament dates set for spring sports<\/a><\/p>\n While there are biting varieties of black fly, they\u00a0don\u2019t\u00a0bite everyone, Orr said.\u00a0<\/p>\n \u201cCertain body types are susceptible to them,\u201d he said.\u00a0<\/p>\n Orr says when they bite him, his ears will bleed and get\u00a0really\u00a0hard.\u00a0<\/p>\n \u201cThere are 3 reasons we control black flies in Pennsylvania,\u201d Orr said. \u201cFirst, a certain amount of the population is\u00a0very sensitive\u00a0to them and\u00a0are\u00a0allergic to them and get golf-ball-sized welts on them.\u00a0For most people, it is just annoying. They can bite.\u00a0They\u2019re\u00a0in their hair. When you have a swarm of about 100 around your head,\u00a0it\u2019s\u00a0very uncomfortable.\u201d\u00a0<\/p>\n The second reason\u00a0DEP\u00a0treats for\u00a0black flies\u00a0is so\u00a0visitors to Pennsylvania\u00a0are\u00a0able to\u00a0enjoy the outdoors\u00a0without being harassed by\u00a0those pests.\u00a0<\/p>\n The third reason\u00a0DEP\u00a0treats\u00a0for\u00a0black fly, Orr said,\u00a0is\u00a0an\u00a0economic\u00a0one.\u00a0\u00a0<\/p>\n \u201cIf you can\u2019t have a Little League or a soccer game or you can’t have the Little League World Series, which is on the west branch of the Susquehanna and where black flies are prolific, or you\u00a0go to a winery for an outdoor event or an outdoor wedding, there can be huge economic impacts that can be involved,\u201d he said.\u00a0<\/p>\n If you have a complaint or questions about DEP\u2019s Black Fly Spraying Program<\/a>, go to\u00a0their website\u00a0for more information.\u00a0<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/us-pa\/wp-content\/uploads\/2026\/05\/89909442007-671247257-1271502645155342-1219830386386599024-n.jpg\")
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