Effects of the pyrethroid esfenvalerate on life-cycle traits and population dynamics of Chironomus riparius--importance of exposure scenario.

Field populations of aquatic organisms, particularly those inhabiting flowing waters, likely are exposed to short pulses of pesticides following periods of spray drift, surface runoff, or drain flow. In the present study, we investigated the effect of pulse exposures of a pyrethroid insecticide, esfenvalerate, on survival, development, and reproduction of the midge, Chironomus riparius. The first experiment was designed to simulate the exposure of settling larvae to a waterborne pulse of esfenvalerate. A second experiment simulated exposure of larvae entering a sediment that had been exposed previously to a waterborne pulse of esfenvalerate. A third experiment simulated exposure of sediment-inhabiting larval populations to a waterborne pulse of esfenvalerate and examined whether population density influenced the response of C. riparius. Our results indicate that even very brief (i.e., 1-h) exposures to environmentally realistic concentrations of esfenvalerate during early larval life can have measurable effects on larval survival and developmental rates. For those individuals that survived a pulse exposure early in life, however, there appeared to be no lasting effects of esfenvalerate on their subsequent egg laying or egg viability. Thus, brief exposure to environmentally realistic concentrations of pesticides can have measurable effects on freshwater invertebrate populations, but whether such effects occur depends on the details of the exposure regime. In addition, population density may influence the responses of life-cycle traits and, thereby, population-level impacts of pesticides.