Natural history of a retreat-building midge, Pagastia partica , in a regulated reach of the upper Colorado River

The retreat-building midge Pagastia prob. partica (Roback) is an abundant and conspicuous member of the benthic community of the upper Colorado River. We investigated the ecology and behavior of P. partica in this regulated stream using both qualitative observations and quantitative surveys. Our studies revealed that P. partica foraged primarily by grazing algae from their retreats and adjacent surfaces and exhibited territorial behavior toward other P. partica larvae. Pagastia partica were widely distributed among streambed surfaces but preferred those with higher biomasses of periphyton and swifter currents. The retreats of P. partica were composed largely of silk and colonized algae and were important structural habitat features for other benthic fauna. We found numerous small benthic invertebrates cohabiting retreats, while larger insects were observed to have distinct affinities for or aversions to retreat-structured surfaces. Our results indicate that P. partica is not only an abundant member of this benthic community, but also a habitat engineer whose retreats represent an important component of streambed habitat for upper Colorado River benthos.