Larval sampling in large rivers

Looking out at the Potomac River before larval sampling. Photo: Claire Weber

Looking out at the Potomac River before larval sampling.
Photo: Claire Weber

Throughout this summer, the Lamp lab has been searching for larvae in the large rivers and creeks throughout Washington county, Frederick county, and a small corner of West Virginia. As of early August, the lab has found black fly larvae 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. However, the presence of black fly larvae does not necessarily mean the presence of Simulium jenningsi, our problem gnat. As we learned from our sampling last summer, there are at least 9 species of black flies in the region, and only S. jenningsi was found to bother people. We will know for sure which of these sites harbor the nuisance larvae when we identify them to species this fall. 

The lack of rain this summer was a bonus for our project. Water levels in the Potomac dropped earlier this year, allowing us to sample extensively during July, including at some sites that were mostly inaccessible last summer. Larvae were particularly abundant on plants and rocks in the shallow, fast flowing regions of the rivers. Deeper, slower moving areas of water, including near the Maryland side of the Appalachian Trail bridge, we were unable to find larvae at.

Appalachian Trail across the Potomac Photo: Claire Weber

Appalachian Trail across the Potomac
Photo: Claire Weber

Antietam Creek Photo: Alison Post

Antietam Creek
Photo: Alison Post

Monocacy River Photo: Alison Post

Monocacy River
Photo: Alison Post

As the summer wraps up, we will complete our 2014 larval survey of the region by checking back at some of the relatively smaller creeks, including Israel Creek, that may still contain S. jenningsi. In addition, we will be revisiting some of the most productive sites we found this summer to see if any noticeable changes have occurred to their larval populations.