Homology of evolution of the benthic communities subject to antiblackfly insecticide spraying
The weekly utilization of insecticides sprayed by the Onchocerciasis Control Programme to control Simulium damnosum larvae modifies the abundance of the non target benthic populations. These insecticides (B.t., Abate and chlorphoxim) have neither the same toxicity nor the same selectivity for the principal taxa. In consequence, the long-term variations of the abundance of the populations are neither of the same importance nor of the same sign (some of them remain the same or decrease white others increase), according to the product and the taxa. In long term, these variations being mainly the consequence of direct toxicity but also take into account the duration of the life cycles as well as spatial and trophic competitions. Iin this study, only the saxicolous fauna is taken into account. If has been collected on rocks with the Surber sampler during the low water period (December to March). Among the ten Savannah rivers treated with antiblackfly insecticides, only few are the object of a non-target fauna monitoring. The abject of this work is to land out if it is possible to generalize to all the rivers, the variations in population abundance observed for an insecticide on a control station. In the reference station, the variation of population abundance for each of the insecticides is calculated by means of the ratio R/P = log APTi/log AVT, where log APTi is the logarithm of the average for the period treated with the i insecticide, and where log AVT is the logarithm of the average before treatment. For each taxa as well as for the total fauna, Abate, chlorphoxim and B.t. ratio were defined. The values of these ratio obtained for the Entomokro station on the Marahoué river in the Ivory Coast are then applied to the data collected before treatment on three rivers or stations (Amou-Oblo on the Amou river in Togo, Asubende on the Pru river in Toge and Danangoro on the Marahoué river in the Ivory coast). The calculated values are then compared to the values collected after treatment in the river monitoring programme. In the reference station the ratio of the populations collected before and after treatment are sometimes greater, sometimes lower than unit (when the ratio is greater than unit, it means that there is an increase of the abundance of populations during the processing period). The ratios are very close to the one for the B.t. which is the less toxic insecticide against the non target fauna, the greatest for the chlorphoxim which is the more toxic product and intermediate for Abate which has an intermediate toxicity. It appears that the more insecticides were estimated to be toxic in gutter tests, the more population abundance differ tram the reference ones. For the total fauna, differences between the values calculated and observed were very low (r 0,98). For the taxa, the prediction of this model is excellent for Caenidae and Hydropsychidae, good for Baetidae, Chironomini, Tricorythidae and Orthocladiinae, but mediocre for Tanypodinae and Tanylarsini. The authors submit some hypotheses to explain the bad adequation of the model for these two taxa; hypotheses based on taxonomic problems and niche releasing.