Black flies (family Simuliidae) are small, humpbacked insects known globally for the nuisance their blood-feeding females cause to humans and livestock. Sometimes called buffalo gnats, these flies are a common scourge of late spring and early summer in temperate regions, particularly near water sources. Their arrival is highly predictable, dictated by specific environmental cues that govern their transition from an aquatic to a terrestrial existence. Understanding the timing of their emergence provides the best defense against their persistent swarming and painful bites.
The Seasonal Triggers for Black Fly Emergence
The timing of black fly emergence is directly linked to the warming of stream and river water following winter. The appearance of adults signals that water temperatures have reached a specific developmental threshold, though different species have varying requirements. For many species that overwinter as eggs, hatching is triggered when the water temperature climbs into the range of 40 to 50 degrees Fahrenheit.
Other species spend the winter as larvae. Their pupation—the final step before emergence—is often completed when the water reaches a slightly lower range, sometimes as low as 37 or 38 degrees Fahrenheit. This temperature dependency means the black fly season begins earlier in southerly regions or areas with milder winters. In northern states and Canada, the main emergence typically occurs from late April into May, coinciding with the spring thaw.
The Aquatic Life Stage
The immature life of the black fly is spent exclusively in clean, flowing water, which ties their emergence closely to water conditions. Female flies deposit their eggs directly into the water or on submerged vegetation in fast-moving streams and rivers. This requirement for moving water ensures the larvae receive adequate oxygen and a constant supply of food particles.
The larvae are specialized filter-feeders, using fan-like mouthparts to strain organic matter from the current. Larval development time is highly variable, lasting from a few weeks in warmer conditions to several months when water temperatures are low. After growth, they transition into a non-feeding pupal stage, encased in a silk cocoon attached to the stream bottom, before emerging as flying adults.
How Long the Biting Season Lasts
The duration of the black fly nuisance is often shorter than many people assume. The most concentrated biting activity typically lasts only three to four weeks in a given area. This primary biting season is driven by the synchronized emergence of the first and largest generation of adults following the spring thaw.
The season for one species ends when the majority of that generation has completed its short adult lifespan, generally only a few weeks. The main factor limiting the season for early-emerging species is the arrival of high summer heat, which causes water temperatures to exceed larval tolerance. However, biting activity can be extended in regions hosting multiple species, as some black flies are multivoltine, producing two or more generations throughout the warmer months.
Protecting Yourself During Peak Activity
Effective protection against black flies relies on chemical repellents, physical barriers, and adjusting outdoor activity times.
Among chemical options, products containing 20% Picaridin are recommended. Picaridin has proven effective against biting flies and lacks the distinct odor and potential to damage plastics associated with DEET. While DEET is a potent repellent, Picaridin often provides superior protection specifically against the Simuliidae family.
Physical protection is equally important because black flies are persistent and can crawl under loose clothing to bite. Wearing tightly woven, light-colored clothing, including long sleeves and pants, minimizes exposed skin. This also helps repel the insects, which are often attracted to darker colors. For intense swarming conditions, a fine-mesh head net tucked into a shirt collar offers the best defense for the head and neck.
Limiting outdoor exposure during the periods of highest activity can significantly reduce encounters. These peak times are typically mid-morning (9:00 to 11:00 AM) and again in the late afternoon (4:00 to 7:00 PM).