Chronic Effects of an Insecticide, Fenobucarb, on the Larvae of Two Mayflies, Epeorus latifolium and Baetis thermicus, in Model Streams

Abstract

Effects of the insecticide fenobucarb on 2 mayfly species, Epeorus latifolium and Baetis thermicus, were examined in indoor model streams. Aqueous concentrations of fenobucarb residues in the model streams were 69–71% of its nominal level until 24 h, except at low concentrations (less than 2 μg l−1 in the water) in the water. Chronic effects of fenobucarb on the growth and emergence of Ephemeropteran larvae were examined over 2 months. For concentrations of 1 and 2 μg l−1, the numbers of individuals of the third larval growth stage (average head width ≥3.0 mm) gradually decreased up to 20 days after the applications; the number of emergent individuals also remained 20 to 25 by the end of the experiment. The number of individuals to emerge at 1 to 2 μg l−1 fenobucarb was restricted in the long-term experiment. These results suggest that fenobucarb affects the emergence of this species through disruption of the endocrine system. On the other hand, the number of emergent individuals of B. thermicus at low concentrations of fenobucarb was not significantly less than that in the control.