Abstract
Effects of dietary fenthion (hereafter MPP), an organophosphorus insecticide, on mortality of damselfly larvae, Ischnura senegalensis, and freshwater shrimp, Macrobrachium nipponense, were assessed using an insecticide-tolerant strain of Chironomus yoshimatsui (Chironomid) as a prey organism. Midge larvae had been exposed to MPP of 125, 250, 500 and 1000 μg/L, respectively, for 24-hrs, and resultant, accumulated MPP at 0.9, 2.1, 11.8 and 68 ng/g (wet wt.). In addition, fenthion sulfoxide (FSO) was detected in the midge larvae at higher levels rather than MPP itself. Individual shrimp and damselfly larva (each, ten replicates) were daily fed on MPP accumulated midge larvae (3 or 4 individuals/d) in a flowthrough aquarium. All damselfly larvae died within 3 days by ingesting the midge larvae accumulated 2.1 (2.8) or 11.8 (17.7) ng/g of MPP (FSO in parentheses). While, mortality of the shrimp fed on the midge larvae accumulated 0.9 or 2.1 ng/g MPP increased slowly to 40 and 80%, respectively, until 24 days after start of the experiment. Both predators continued to eat the MPP-accumulated midge larvae without showing hesitation, suggesting that they lack ability to keep away from toxic prey. We estimated the exposure levels of MPP and FSO in the live midge larvae that brought 50% mortality of predators used for the tests.
