How Can the Fipronil Insecticide Access Phloem

Seeds of sunflower plants coated with the fipronil 14C-insecticide were allowed to grow in the greenhouse. The distribution of the 14C-compounds was studied in each part of the plant after three months. After 83 days of culture small amounts of 14C-compounds were found in the inflorescence (0.6‰ of the seed deposit) which were fipronil itself or its lipophilic or hydrophilic metabolites. The 14C-compounds were found in each part of the inflorescence (bracts, ray and disk florets containing pollen, akenes). The 14C-concentration in the xylem sap evaluated at this stage was much too low to explain the accumulated amount in the inflorescence. Under controlled conditions in a culture chamber, it was then demonstrated that a net phloem transfer of 14C-fipronil occurred from developed leaves to growing organs. This allowed us to suppose that a similar 14C-fipronil phloem transfer could occur toward the inflorescence during its formation. A quantitative evaluation suggests that most of the labeled compounds at t...