Influence of time of application of an insecticide on recovery patterns of a zooplankton community in experimental ponds

Abstract

A Zooplankton community was established in out-door concrete ponds to which an insecticide, carbaryl (0.5 mg/L final concentration), was applied at different times relative to the population trend. The chemical application markedly reduced the cladoceran and copepod populations, but not rotifer population. After the treatments,Bosmina fatalis recovered earlier thanDaphnia spp. and was predominant until recovery of theDaphnia. The reappearance ofDaphnia was gradually delayed when the treatment was carried out at later times. Thus, the treatment induced the predominance ofBosmina, and the period whenBosmina predominated was extended when the carbaryl applications were delayed. The recovery ofDaphnia was probably retarded by the decline in water temperature, which decreased steadily during most of the experimental period. When carbaryl was applied during the increasing phase of theKeratella valga population, the population increased still further in density. When the population was exposed to the chemical during its decreasing phase, it did not recover even when competitors disappeared. Thus, applications of the insecticide at different times induced different recovery patterns of the Zooplankton community in the ponds.