Long-term effects of intensive pesticide applications on the aquatic community in orchard drainage ditches near Hamburg, Germany.

The results of an ecological investigation carried out to determine the structure of an aquatic community in orchard drainage ditches were compared with the results of a similar study conducted about 25 years earlier, before the full impact of modern pesticides had been felt. Comparisons of the community structures and abundances of the various species indicate that many species have become completely resistant to the agricultural chemicals to which they are exposed, while others have been completely eliminated from the habitat. Certain species have apparently benefited from the disappearance of predators and competitors and are now present in great abundance. Herbicides have had little, if any, effect on the floral species diversity. Insecticides have taken the greatest toll on predatory species, while acaricidal compounds have completely eliminated all species of water mite. Only one predatory eugamasid mite species was found in the ditches among a large population of collembolans on the water's surface. The individual substances now used in the largest amounts to protect the orchards are discussed, and their properties are listed. The simultaneous use of several toxic substances seems to make the development of resistance more difficult because the probability that one individual will be naturally insensitive to two toxic substances is much less than that it will be resistant to one. The sequential application of different pesticides, on the other hand, allows species to develop populations resistant to each, in turn.