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Conspecific cases as alternative grazing surfaces for larval Glossosoma intermedium (Trichoptera:Glossosomatidae)

Abstract Larval Glossosoma intermedium are dominant benthic grazers that often deplete their food resources (periphyton). We observed G. intermedium grazing periphyton from conspecific cases, a possible result of limited resources on stream cobbles, and we hypothesized that case grazing increases when periphyton resources become scarce. This hypothesis was tested by monitoring frequency of case grazing among G. intermedium in 3 streams in southwest Wisconsin for 1 y, and also in the laboratory using time-lapse video of larvae on tiles with and without periphyton. In situ periphyton biomass was higher on larval cases than on the stream cobbles to which larvae had access, irrespective of density, age, or size structure of G. intermedium. Case grazing was positively related to larval densities at 2 of the 3 streams. In laboratory experiments, larval encounter rates were similar in both tile treatments, although frequency and duration of case grazing increased on tiles lacking periphyton. These results suggest that periphyton on G. intermedium cases can provide an important resource patch for this species, especially when periphyton biomass is low in the ambient environment.