The effects of two successive annual treatments of imidacloprid and fipronil on dragonfly nymph communities, which are one of the best-known bioindicators in Japanese agroecosystems, were monitored in experimental paddies. The abundance of dragonfly nymphs was lower in both insecticides-treated fields than it was in the controls, particularly following fipronil treatments. Residues of both insecticides were found in the soil throughout the two years, and imidacloprid persisted in water up to three months following each treatment. A Principal Response Curve analysis (PRC) showed that the second annual treatments caused greater structural changes in dragonfly nymph communities than the initial treatments caused, particularly for fipronil. The community structures continued to change even after the insecticides dissipated from the water. This suggests that ecological impacts, and therefore risks, of imidacloprid and fipronil on dragonfly nymph communities depend more on soil residues than they do on waterborne residues. As expected, susceptibility of dragonfly nymphs to these two insecticides differed among species.

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The guideline of pesticide registration in Japan requires vast array of the toxicological studies. The driving principle is to present evidences for apparent toxicity, relevant dose–response, and the threshold as the real dosage in all studies to be written. For carcinogenesis and teratogenesis, when positive, mechanistic studies are mandatory. Otherwise, no mechanistic study would be necessary, because NOAEL should be obtained in all the studies. This is not always real; therefore, additional studies on the mode of action may help establishing the NOAEL. These studies are especially useful in cases with wide species differences in the NOAELs. Examples include (1) PPO inhibitors, (2) strobilurins, (3) simeconazole, (4) macrolide lactones, (5) 4HPPD inhibitors and (6) RyR activators, details of which will appear in the supplement. © Pesticide Science Society of Japan

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