Fipronil was poorly mobile in soil, and at the end of the cultivation period it was mostly concentrated in the soil layer close to the seed (3240 μg kg−1 soil). Starting from the seed-coating, a fipronil concentration gradient was measured in the soil, up to a distance of 11 cm from the seed.
Degradation in the soil occurred at a moderate rate, probably due to the fact that water solubilization of the solid active ingredient present in the seed coating was rate limiting. Indeed, after 6 months of cultivation, only 51%of the fipronil seed-coating was found in the soil, about 7%having been absorbed by the sunflower plant, and 42%remaining in the seed coat. The predominant metabolites produced in the soil were sulfone-fipronil, sulfide-fipronil and amide-fipronil, which were produced at average rates of 5 μg kg−1 soil day−1, 3 μg kg−1 soil day−1, and 0.4 μg kg−1 soil day−1, respectively. In contrast, the photoproduct, desulfinyl-fipronil, was barely detected.
All phenylpyrazole compounds were poorly mobile, except for the amide derivative, which is devoid of insecticidal activity in marked contrast to the other metabolites. Furthermore, detectable soil contamination was limited to a zone of about 11 cm around the seed.