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

Abstract

Larvae of Potamophylax cingulatus (Trichoptera: Limnephilidae) were observed to be living at high densities in a low-order stream in a beechwood in southern France. The hypothesis that this population was food-limited was investigated by adding supplementary food sources in the form of dried leaf litter, contained within mesh bags. Caddis larvae rapidly colonised alder leaves, even while naturally occurring beech litter was still relatively abundant. After a major storm, which removed natural leaf aggregations but left the benthic population of P. cingulatus intact, colonisation of alder leaves remained rapid; beech leaves added at the same time, however, were colonised much more slowly. Mass loss of alder leaves was significantly more rapid than that of beech leaves. Population estimates of P. cingulatus demonstrated continuous recolonisation of the benthos, with rapid aggregation onto alder leaf packs both before and after the storm, suggesting that this movement was related to food availability. Relative lack of consumption in beech leaves, however, when natural food supplies were apparently severely reduced, demonstrated that total food limitation was not occurring. It is speculated that P. cingulatus, in its immediate pre-pupal instar, has specific nutritional requirements that alder litter can satisfy but beech litter cannot.