Synergy between prochloraz and esfenvalerate in Daphnia magna from acute and subchronic exposures in the laboratory and microcosms

Azole fungicides can enhance the toxicity of pyrethroid insecticides on aquatic species. It is, however, unknown to what extend the synergy found under laboratory conditions for strongly sorbing compounds (Azole log Kow: 3–4, pyrethroid log Kow: 6–7) will also take place in the field. We therefore investigated the synergising potential of the fungicide prochloraz on the pyrethroid esfenvalerate on Daphnia magna in the laboratory and in situ in cages placed in outdoor aquatic microcosms treated with 0.17, 0.33 and 0.83 μg L−1 esfenvalerate with and without 90 μg L−1 prochloraz. We found that the 8–14 fold synergy observed after 2 and 7 days of exposure to mixtures in the microcosms was equivalent to or greater than the 3–7 fold synergy found in 2-day laboratory tests. Incubating new neonates in situ 7 and 14 days after pesticide addition gave EC50 values of 0.012 ± 0.001 and <0.005 μg esfenvalerate L−1 in the mixture treatments, based on measured water column concentrations. The detection limit is more than ten-fold lower than the lowest esfenvalerate concentration observed to cause ecologically significant effects across seven long term mesocosms studies, hence, even on a longer time scale prochloraz apparently synergises the effect of esfenvalerate under field-like conditions in the microcosms. The results show that synergy found in the laboratory also takes place under field like conditions at quantitatively similar levels, and that it lasts for several weeks. More knowledge on the identification of potential synergists, their true bioavailability and the concentrations and time span within which they can cause synergy needs further study, before an overall evaluation of the occurrence and severity of synergy under field conditions can take place.

► The ability of the azole fungicide prochloraz to synergise the effect of the pyrethroid insecticide esfenvalerate on Daphnia magna was investigated.
► Survival under standard laboratory conditions was compare to survival in cages situated in treated microcosms.
► The 8–14 fold synergy observed after 2 and 7 days in the microcosms were comparable to the 3–7 fold synergy found in 2 days laboratory tests.
► Incubating new neonates in situ 7 and 14 days after pesticide addition gave mixture EC50 values of 0.012 ± 0.001 and <0.005 μg esfenvalerate L−1.
► These low values question the use of water concentration as a measure of bioavailability for lipophillic compounds.