Black flies in Montgomery County

Early this past summer we received a report of nuisance gnat swarms from Montgomery County.  The gnat specimens sent to us from Silver Spring were indeed our black fly species of interest, Simulium jenningsi. This is not the first S. jenningsi female identified from this county. In the summer of 2014 members of the Lamp lab incidentally collected a S. jenningsi specimen in Silver Spring while searching for kudzu bugs. However, as we have learned from our sampling in Frederick and Washington counties, presence of S. jenningsi does not necessarily indicate large enough populations to form a nuisance problem to residents. So the complaint report in addition to the collected specimens provided us with new information.

This made us wonder, if S. jenningsi was causing a nuisance in Silver Spring, where else in the county could it be a problem? Over the summer we sampled for black flies at twelve locations throughout Montgomery County. We specifically targeted parks and walking trails, as these are places where residents and tourists are likely to notice nuisance flies while enjoying the outdoors. Of these twelve parks we encountered and identified S. jenningsi from eight, their locations and the location of the resident-collected specimens shown in the map below.

Known locations of adult S. jenningsi in Montgomery county, Maryland from the summer of 2015.

Known locations of adult S. jenningsi in Montgomery county, Maryland from the summer of 2015.

None of the Montgomery County sampling trips resulted in S. jenningsi numbers close to the worst we have experienced in parts of Washington County, designated as a "Level 3," or an extreme nuisance, on our 0-3 nuisance scale. However, three locations did reach "Level 2," a moderate nuisance, which is the level we would expect most people to start noticing and feeling annoyed by the swarming black flies. These locations were in Wootton's Mill Park in Rockville, Seneca Creek State Park in Gaithersburg, and Ken-Gar Palisades Park in Kensington. One of our field technicians was unfortunately more attractive to black flies than the rest of us, and found Wootton's Mill to be particularly unpleasant that day regardless of the plentiful in-season wineberries.  

Wineberries at Wootton's Mill Park in Rockville

Wineberries at Wootton's Mill Park in Rockville

Picnic area at Seneca Creek State Park in Gaithersburg

Picnic area at Seneca Creek State Park in Gaithersburg

Our standard black fly sampling equipment strewn about at Ken-Gar Palisades Park in Kensington

Our standard black fly sampling equipment strewn about at Ken-Gar Palisades Park in Kensington

Black fly numbers can be highly variable day-to-day. As such, it is very possible that the parks we sampled could have a higher nuisance level on some days, or times of day, than what we experienced. What this map does tell us is that female S. jenningsi can be found to some extent in many areas of Montgomery County. Although we did not collect any specimens from the western or eastern corners of the county, it appears very likely that S. jenningsi can travel from their aquatic breeding site to these regions. Reports from residents in these areas will provide us with a clearer indication to the accuracy of this assumption.

Regarding breeding locations, as we have learned from previous years the Potomac River is the most productive body of water for S. jenningsi larvae in Frederick and Washington counties. In the late 1950's, S. jenningsi larvae were very abundant at sites within the Potomac River bordering Montgomery County (McComb and Bickley 1959). Although we were unable to find larvae at the locations we visited in this stretch of the Potomac in 2015, we were limited in our ability to access the river for most of the summer due to high water levels. We hope to continue searching for larvae in the Montgomery-bordering regions of the Potomac in 2016 to determine if the larvae can still be found in their historic distribution.


McComb, C., and Bickley, W., 1959. Observations on Black Flies in Two Maryland Counties. Journal of Economic Entomology 52:629-632.