Maryland Black Fly Survey Results for the Year 2013

December 12, 2013

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

Certain species of black flies in the family Simuliidae cause nuisance when they swarm and bite humans, making outdoor activities difficult or even impossible.  Reports that we received in spring, 2013, suggested an unusual amount of black fly problems in southern Washington County over the last several years.  Our preliminary research was designed to understand the black fly situation in order to help us determine the best course of action to relieve the problem.  Specifically, we sought to:

  1. Determine the species (one or more) which cause the nuisance.
  2. Locate larval habitat that is the source of the black flies.
  3. Find out the extent of the problem in Washington County and, more broadly, in Maryland.

For Objective 1, we requested that residents who experience black fly problems collect specimens.  A “Black Fly Adult Collecting Kit” was provided to individuals to help us determine the species of flies that are causing the problem.  Sites where adult flies were collected are indicated by yellow circles on the map found here.  These adults (all females) were identified in our lab as belonging to the Simulium jenningsi species group. Specimens were sent to Dr. Peter Adler at Clemson University, a black fly expert, who further identified the adults as the species Simulium jenningsi.  S. jenningsi is well-known as a nuisance pest, developing in large rivers.

For Objective 2, we collected larvae of black flies from 19 different reaches of running waters in the southern part of Washington County (see map for locations and results).  Insects in flowing waters were sampled using a D-net, and samples preserved using Carnoy’s solution or ethanol.  Larvae were processed for species identification and sent for confirmation from Dr. Adler, and 8 larval species or species groups were identified.  Only two streams contained larvae of S. jenningsi  to match the adult collections: the Potomac River and Antietam Creek. Sampling of the Potomac River was limited through much of the summer due to high water levels, however S. jenningsi was the only species group collected from this site.  By contrast, Antietam Creek contained a lower relative abundance of S. jenningsi larvae, and also contained larvae of Simulium tuberosum and Simulium vittatum species groups.

For Objective 3, we developed a website through which residents were able to learn more about the problem and how they can help.  In addition, a survey was posted on the website to gauge geographical range and activity levels of black flies in Washington County.  Locations of survey respondents are illustrated on the map found here.  A complete report of the survey can be found here.  We discovered that 96% of respondents observed adult black flies at their place of residence and various recreational sites.  Peak black fly activity as experienced by residents was between May and August, typically during conditions with little to no wind/rain and warm temperatures.  All respondents noted that the black flies were irritating to very irritating.  Most noted that black flies had caused them to avoid some to all outdoor activities.

After receiving confirmation of our species identifications, our results suggest that the black fly species causing nuisance in southern Washington County is Simulium jenningsi .  Our larval collections suggest that the Potomac River is the primary source of the adult flies, however high waters prevented us from discovering the exact location of the source.  Experience by others with black fly problems suggests that larval densities are especially high either just below dams or at the confluence of major rivers (such as the Shenandoah River).  Additional research is needed to determine the location of the larval populations, as well as the time of year that the larvae are developing. Any method of suppressing larval numbers, such as through applications of Bti, would require knowledge of the location and time of larval development.