Field experiments on the relationship between drift and henthic densities of aquatic insects in tropical streams (Ivory Coast). III Trichoptera

Abstract

SUMMARY. 1. Based on in situ gutter trials we related the drift of caddis flies to their benthic densities and to various abiotic factors in streams in the Ivory Coast (West Africa). Members of the families Hydropsychidae, Philopotamidae. Hydroptilidae and Leptoceridae were considered in detail.2. The drift of larvae peaked at night in both early and late larval instars.3. Drift of a larval group (a certain instar, species or higher taxon) was more often related to the benthic density of other larval groups than to its own benthic density.4. Self‐regulation of an upper benthic density of a larval group by emigration through drift was not statistically evident.5. There was no straightforward relationship between drift and abiotic factors.6. Drift rates differed between taxa as well as between larval instars (size groups) within a taxon. Newly hatched larvae had very high drift rates, whereas the last larval instar usually had the lowest drift rate.7. We related these results to the violently fluctuating discharge of the streams in the study area and the consequent variability of space for lotic insects.8. Drift estimates, made at the same time as a monitoring programme on possible side‐effects of insecticides (Onchocerciasis Control Programme), failed to reflect benthic densities except in the night drift of Hydropsychidae.