This summer the Lamp lab sampled for black fly larvae in over 25 locations along the Potomac River and its larger tributaries. As we learned from last year, the problem species Simulium jenningsi is found in at least one location each in the Potomac and Antietam Creek. Our preliminary larval identifications from this summer have now increased the total of known S. jenningsi larval locations to five in the Potomac, and one each for the Shenandoah River, Monocacy River, and Antietam Creek.
When it comes to identifying black fly larvae to species, bigger is always better. The larger "mature" larvae, or those very close to pupation, have darkened gill spots. These gill spots are then dissected in our lab, put on a slide, and magnified in order to count the number of filaments, or branches, in the gills. S. jenningsi has 10 filaments per gill, while the nearly identical, but harmless, Simulium luggeri has 12 filaments. Several locations we sampled contained only small larvae without developed gill spots, which can not be identified to species with just a microscope. Therefore, it is highly likely that more of the locations we sampled in the Potomac contain S. jenningsi than we are currently aware of.
In the future we hope to clarify both the species of these smaller larvae through DNA analysis, and which spots are the "worst" by estimating the density of the larvae at each location.