Group size of feeding stream case-bearing caddisfly grazers and resource abundance

Several herbivorous insects are known to benefit from feeding in groups; however, little is known about (1) the resource conditions under which herbivorous insects perform group feeding and (2) the optimum population size to get any benefits by group feeding, for example, in terms of growth performance. To test the hypotheses that the benefits from group feeding change with resource level and population size, we performed field investigations and an enclosure experiment using the grazer caddisfly larva Micrasema quadriloba. The field investigations revealed aggregated distributions of larvae (indicator of aggregation, 1(delta) = 4.1 +/- 1.55, aggregated density: 12.7 +/- 5.3 individuals per 3.1 x 3.1 cm(2) (mean +/- 1 SD) when periphyton was abundant on stream cobbles and random distributions (I-delta = 1.0 +/- 0.11) when periphyton was scarce. In the enclosure experiment, the relative-growth rate (RGR) of the larvae at each population size showed different tendencies at high and low periphyton abundance Levels; RGR with abundant periphyton had a convex curve with a peak at intermediate population size, whereas RGR with scarce periphyton decreased linearly with increasing population size. The benefits from group feeding thus changed with resource level; larvae obtained high growth performance by group feeding behavior only when the resource was sufficiently abundant. The present study revealed not only that the optimum group size of larvae increased their growth performance, but also that this optimum group size occurred frequently in the field. We also discuss the mechanisms and benefits of group feeding by case-bearing caddisfly grazers. (C) 2006 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.