Aggregation of Potamophylax cingulatus (Trichoptera : Limnephilidae) larvae in response to possible food limitation in a southern French stream

Larvae of Potamophylax cingulatus (Trichoptera: Limnephilidae) were observe d to be living at high densities in a low-order stream in a beechwood in so uthern France. The hypothesis that this population was food-limited was inv estigated by adding supplementary food sources in the form of dried leaf li tter, contained within mesh bags. Caddis larvae rapidly colonised alder lea ves, even while naturally occurring beech litter was still relatively abund ant. After a major storm, which removed natural leaf aggregations but left the benthic population of P. cingulatus intact, colonisation of alder leave s remained rapid; beech leaves added at the same time, however, were coloni sed much more slowly. Mass loss of alder leaves was significantly more rapi d than that of beech leaves. population estimates of P. cingulatus demonstr ated continuous recolonisation of the benthos, with rapid aggregation onto alder leaf packs both before and after the storm, suggesting that this move ment was related to food availability. Relative lack of consumption in beec h leaves, however, when natural food supplies were apparently severely redu ced, demonstrated that total food limitation was not occurring. It is specu lated that P. cingulatus, in its immediate pre-pupal instar, has specific n utritional requirements that alder litter can satisfy but beech litter cann ot.