Sampling the mighty Potomac

This month's sampling trip took us back to a few of the streams sampled early-on this summer, to a couple of new streams, and finally to the great Potomac River. We began our sampling off of Rohrersville Rd, re-sampling some of the streams where we had found black fly larvae earlier this summer. This month however, we found little or no larvae in some of  the streams we sampled, although there were adult flies buzzing around the banks. We did find many other insects besides the black flies we were searching for, including stoneflies, hellgrammites, and one large caterpillar of the imperial moth (Eacles imperialis ). We also found the banks of each of the streams littered with large webs from tetragnathid and micrathena spiders. Presumably these spiders are feasting on any black fly adults that get caught in their webs, but we didn't look closely enough to count whether any were caught.

Imperial moth caterpillar

Photo: Alan Leslie, University of Maryland

Spider web above tributary to the Potomac River

Photo: Alan Leslie, University of Maryland

Black fly larvae on leaf

Photo: Alan Leslie, University of Maryland

However, not all of the streams we visited were devoid of black fly larvae. A small, unnamed tributary running across Bent Willow Rd had several black fly larvae attached to jewelweed plants that had fallen in the stream from the banks. Another unnamed tributary flowing into the Potomac River near Taylor's Landing produced some of the highest densities of black fly larvae we have seen yet from any sampling trips.

 

Potomac River gauge at Taylor's Landing

Photo: Alan Leslie, University of Maryland

Wading for larvae in the Potomac River near Lock 38

Photo: Alan Leslie, University of Maryland

Submerged aquatic grasses in the Potomac River

Photo: Alan Leslie, University of Maryland

By far the most exciting part of this month's sampling was the chance to look for larvae in the Potomac River, which was finally low enough to allow safe wading from the banks. We sampled the Potomac at two spots, at Taylor's Landing, and at Lock 38, under the bridge leading to Shepherdstown, WV.  At both sites we found a pattern similar to the smaller streams we had visited, where black fly larvae were not attached to rocks that were covered in algae, and instead were found in great abundance on plants growing within the river. These larvae are currently being identified along with the specimens collected from previous sampling trips. We have so far identified at least three different species of larvae from the streams where we have sampled. The next step will be to wait for the completion of adult fly collections and to make comparisons between the distributions of species as larvae within the streams we have sampled, and the identities of adults that have been collected around people's homes.