Insects and insecticides in the lower Rhine

Two chemical accidents with insecticides in the River Rhine, i.e. the endosulfan accident in 1969 and the Sandoz accident in 1986, played an important role in public and political awareness regarding water pollution. The endosulfan accident gave cause for the development of a chemical monitoring and sanitation programme, whereas the Sandoz accident accelerated this sanitation programme and triggered a number of eco(toxico)logical studies and their implementation in water quality control. The gradual decrease in concentrations of organophosphorus insecticides since 1977 coincided with the recolonization of a number of insect species in the Lower Rhine. First some species of chironomids appeared and later the caddisflies Hydropsyche contubernalis followed by Ecnomus tenellus. Ecological research before, during and after the Sandoz accident showed that chironomids and caddisflies occurring in large numbers in the IJssel, a branch of the River Rhine, were affected by the Sandoz accident. After the Sandoz accident, the pattern of recolonization showed the same sequence. Again, various chironomid species were among the first to recolonize, whereas recovery of caddisfly populations lasted longer. It appeared that for all species no more than one generation was needed for recovery, whereas the second generation showed complete recovery. In general terms it means that species requiring stable conditions, e.g. because of their long life-cycle, will not be able to maintain self-sustaining populations in the Rhine where different stress factors are operating.