Synergy in microcosms with environmentally realistic concentrations of prochloraz and esfenvalerate

Laboratory experiments have shown that azole fungicides enhance the toxic effect of pyrethroid insecticides towards the aquatic crustacean Daphnia magna. Due to their sorptive properties the pesticides may, however, be less bioavailable in natural environments, possibly rendering them less toxic to aquatic organisms. In the present study, the synergistic potential of azoles on pyrethroids in natural environments was assessed by treating 18 outdoor aquatic microcosms with concentrations of the pyrethroid esfenvalerate at 0.167, 0.333, or 0.833 μg/L either alone or in combination with 90 μg/L of the azole prochloraz. Pesticide concentrations and the zooplankton and phytoplankton communities were assessed prior to pesticide application and at days 0, 1, 2, 4, 7, 14, 21, and 28 after pesticide application. DT50-values for disappearance of the pesticides from the water of 4.7 days and 30 h were observed for prochloraz and esfenvalerate, respectively. The monitored communities showed larger decreases in abundance of cladoceran, copepods, and chironomids in treatments with esfenvalerate in combination with prochloraz compared to treatments with esfenvalerate alone. No systematic effects were observed in populations of Ostracoda. Adverse effects on populations of cladocerans and copepods occurred between day 2 and day 7 and, though copepods in general were less sensitive than cladocerans to both esfenvalerate alone and in combination with prochloraz, the potentiation factors for the two taxa were similar. Thus, comparison of EC20-values estimated on the basis of concentration–response curves for days 2, 4, and 7 showed that prochloraz enhanced the toxicity of esfenvalerate four to sixfold for copepods and three to sevenfold for cladocerans. Rotifers were not significantly affected by any of the treatments, though there was a tendency of a population increase when cladoceran and copepod populations decreased. In all invertebrate populations that showed response to the pesticide treatments, indications of stabilisation or the beginning of recovery occurred between day 7 and day 14 and full recovery was observed in some of the less affected populations of cladocerans, copepods, and chironomids after 28 days. The occurrence of the synergistic interactions between prochloraz and esfenvalerate in the microcosms and at environmentally realistic concentrations implies that the synergistic interactions may also take place in invertebrate communities in natural ponds and ditches being exposed to azoles and pyrethroids via for example runoff or drift. The question of how to deal with synergy between chemicals in the environment from a regulatory perspective is briefly discussed.