Maryland Black Fly Survey Results for the Year 2015

February 29, 2016

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

In Maryland many residents are bothered by swarming insects locally known as “gnats” throughout the summer months. In 2013 our lab determined these gnats were the black fly species Simulium jenningsi, a common pest throughout the Mid-Atlantic. Sampling in 2014 found severe nuisance swarms of S. jenningsi adults in both Washington and Frederick Counties. The larvae of S. jenningsi, which live in large rivers, were also found at multiple sites along the Potomac River and some of its larger tributaries within these counties. For the summer of 2015 we wanted to determine the spatial and environmental factors that lead to the development of S. jenningsi nuisance swarms in some communities but not others. In addition, we began to investigate the region of the Potomac immediately downstream of the Shenandoah as a potential region of relatively high larval density. Our specific objectives included:

1. Compare the relative densities of S. jenningsi larvae at locations within the Potomac River.

2. Collect S. jenningsi adult distribution data for the formation of an interpolative model.

3. Determine the presence and severity of S. jenningsi swarms in Maryland counties reported to contain nuisance swarms.

For Objective 1, we developed a quantitative sampling protocol for black fly larvae within the Potomac River to compare colonization densities between sites. In previous years our larval sampling was conducted by hand, collecting larvae from vegetation or rocks to qualitatively determine the presence/absence of S. jenningsi. Although this sampling method allowed us to determine sites that contained S. jenningsi larvae, it did not provide us with a way to compare which sites were more productive for larvae than others. To do this we placed uniform length strips of polyethylene tape attached to bricks at locations within the river, and counted the number of larvae attached to each strip after one week. Tape strands were deployed at four locations, two above and two below the confluence of the Shenandoah River. These deployments ran continuously from late July to early September. Two sites had significantly higher larval colonization, one above and one below the confluence. Although our preliminary analysis did not indicate a difference in larval densities related to the confluence, the tape strands will be used in future sampling to compare larval densities at sites along a longer span of the Potomac.

For Objective 2, we used an aerial net to sample adult flies throughout a 750 square mile region spanning parts of Washington County, Frederick County, and adjacent areas of Virginia and West Virginia. This served as a continuation of a research objective started in 2014 (Objective 3, found here). 125 locations were each visited in June, July, and August. These locations were equally split among five habit classifications (residential, paved, agricultural, riparian, and forest). The fly counts from these locations and the locations visited in 2014, along with land use, elevation, and meteorological data, will serve as the basis for a predictive model to determine what other regions of the state may be under risk of S. jenningsi nuisance swarms. Analysis of the two years of data and formation of this model are currently in progress.

For Objective 3, we followed up on a report of black flies from a resident in Montgomery County by sampling for adult flies at twelve parks within the county (a full report found here). We collected adult S. jenningsi specimens from eight of these locations. Although the swarms encountered at these locations did not reach the numbers found at some locations in Washington and Frederick Counties, swarms at three locations did reach a level at which we expect many residents would feel discomfort. In addition to physically sampling for flies in Montgomery County, we also received reports of gnats/black flies from residents in Garrett and Cecil Counties. These reports suggest S. jenningsi has a wide distribution in Maryland, and that there may be many regions in the state that have nuisance problems which we have not received reports from.