Maryland Black Fly Survey Results for the Year 2014

March 17, 2015

Rebecca Wilson, William Lamp, and Alan Leslie
Department of Entomology
University of Maryland
College Park, MD 20742

Summary. In 2013, we identified the cause of nuisance gnats in Washington County as the black fly species, Simulium jenningsi. The source of the adults is aquatic larvae growing in the Potomac River.  Management of the fly requires Bti treatment of large rivers, as is being practiced in neighboring states. In 2014, the known range of the S. jenningsi nuisance has been expanded to include communities in western Frederick County, while the range of the species itself includes Montgomery and Prince George’s counties. S. jenningsi larvae are found at multiple sites within the Potomac River bordering both Washington and Frederick counties. However, the range of both the adult flies and larvae are likely to cover a much larger than we were able to sample. In the late 1950’s, S. jenningsi was a major nuisance problem in Montgomery and Prince George’s counties, with larvae found in large numbers below Great Falls (McComb and Bickley 1959). A side effect of improving water conditions in large rivers appears to be the expansion of S. jenningsi populations in new regions, as seen in New Jersey (Carle 2010). We believe there is potential for future range expansion or increased populations of S. jenningsi in Maryland as well.

The black fly Simulium jenningsi is a nuisance pest found in multiple states in the Mid-Atlantic region. In Pennsylvania and West Virginia, nuisance populations of S. jenningsi are routinely suppressed by applying a bacterial based pesticide at the larval breeding sites within large rivers. In 2013, we found that S. jenningsi was the species responsible for complaints of swarming gnats from residents in Washington County, Maryland. We also found evidence to suggest the Potomac River was the primary source of the flies. However, we still had questions regarding the specific breeding locations and extent of the nuisance flies. For the summer of 2014, we expanded our research to cover the following objectives:

1.       Locate larval breeding sites within the Potomac River and its larger tributaries.

2.       Find the geographic range of the nuisance problem.

3.       Determine the relationship between fly distributions and land use.

For Objective 1, we collected back fly larvae and pupae from 11 sites in the Potomac River, 2 sites in Antietam Creek, 2 sites in the Monocacy River, and 1 site in the Shenandoah River. Larvae were picked directly from rocks and submerged vegetation at each site during a 15 minute sampling time period. Samples were preserved using 80% ethanol. Preliminary identification was conducted using the morphology of mature larvae and pupae. Immature larvae of S. jenningsi are indistinguishable morphologically from other larvae within their species group. Between the combined sampling of 2013 and 2014, S. jenningsi has now been identified at 6 locations within the Potomac River, and 1 location each for Antietam Creek, the Monocacy River, and the Shenandoah River. The sites with identified S. jenningsi range from Sharpsburg to Point of Rocks, Maryland. However, DNA based identification to be conducted within the next year is likely to reveal that S. jenningsi is found at additional locations.

For Objective 2, a shortened version of our 2013 online survey was posted to One purpose of this survey was again to determine the general locations of nuisance problems within Maryland. Responses in 2014 came from residents in both Washington and Frederick counties, as opposed to only Washington County in 2013. In addition, collections kits were distributed to residents in both counties. Of the 1087 identifiable flies collected by residents in 2014, all were found to be S. jenningsi. While conducting unrelated research in Montgomery and Prince George’s counties, we collected specimens of S. jenningsi from locations that we had not heard from residents. However, these flies were found at very low numbers. Although S. jenningsi can be found in these counties, the primary nuisance regions appear to be within Washington and Frederick counties.

For Objective 3, we collected adult flies throughout an area spanning eastern Washington County, western Frederick County, and adjacent areas of Virginia and West Virginia. Flies were collected using an aerial new swung over the head of a researcher for a total of 18 sweeps per location. Sampling sites were evenly split by 5 habitat classifications (residential, paved, agricultural, riparian, and forest). Preliminary analysis has shown that paved locations throughout the sampling region were less likely to contain black flies. In addition, flies were uncommon in the areas near Frederick and Hagerstown, the two most populated towns in the region. Moderate to high nuisance levels were encountered near the smaller communities of Rohrersville, Middletown, Thurmont, and Brunswick. Future analysis may show a relationship between large scale land use and nuisance patterns.


References cited

Carle, D. M. 2010. The Black Flies (Diptera: Simuliidae) of New Jersey, U.S.A. Entomol. News. 121: 6–22.

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