Influence of cypermethrin and fenvalerate on natural soil algal populations

Two synthetic pyrethroids, cypermethrin and fenvalerate, were assessed for their effects on soil algae. Single or two repeated applications of cypermethrin or a single application of fenvalerate to soil at 0.5 to 1.0 kg ha-1 had no inhibitory effect on algal flora. Two applications of fenvalerate, at concentrations of 0.75 to 5 kg ha-1, resulted in a significant increase in populations of algae. No change in the population size was observed even when either monocrotophos or quinalphos was added between two applications of a pyrethroid, all at the 1.0 kg ha-1 level.