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Abstract
Toxic effect concentrations of insecticides are generally determined using the technical
grade or pure active ingredient. Commercial insecticide formulations, however, contain
a significant proportion (>90%) of so-called inert ingredients, which may alter the
toxicity of the active ingredient(s). This study compares the sublethal toxicity of
two insecticides, the pyrethroid bifenthrin, and the phenylpyrazole fipronil, to their
commercial formulations, Talstar and Termidor. Both insecticides are used for landscape
treatment and structural pest control, and can be transported into surface water bodies
via stormwater and irrigation runoff. We used larval fathead minnow (Pimephales promelas),
to determine effects on growth and swimming performance after short-term (24h) exposure
to sublethal concentrations of pure insecticides and the respective formulations.
Significantly enhanced 7d growth was observed at 10% of the 24h LC(10) (53 microg
L(-)(1)) fipronil. Swimming performance was significantly impaired at 20% of the 24h
LC(10) (0.14 microg L(-)(1)) of bifenthrin and 10% of the 24h LC(10) of Talstar (0.03
microgL(-)(1)). Fipronil and Termidor led to a significant impairment of swimming
performance at 142 microgL(-)(1) and 148 microgL(-)(1) respectively, with more pronounced
effects for the formulation. Our data shows that based on dissolved concentrations
both formulations were more toxic than the pure active ingredients, suggesting that
increased toxicity due to inert ingredients should be considered in risk assessments
and regulation of insecticides.