VIII SIMPOSIO INTERNAZIONALE ICPBR

HAZARDS OF PESTICIDES TO BEES

Bologna, 4-6 Settembre 2002

Effetti dell’imidacloprid sulle api

Plant metabolism of imidacloprid

Robin Sur

Bayer AG, Bayer CropScience, Metabolism/Environmental Fate, Alfred-Nobel-Str. 50, 40789 Monheim, Germany. E-mail: robin.sur@bayercropscience.com

Imidacloprid is a broad spectrum, systemic insecticide and the first representative of a new chemical class, the neonicotinyles. The fate of imidacloprid in plants was investigated with radiolabelled [pyridinyl-14C-methyl]imidacloprid in 13 different plant species using three different application forms. Despite of the great spectrum of plant species, application forms and application rates, a rather consistent picture of uptake, translocation and metabolism of imidacloprid in plants was observed.

The uptake of imidacloprid from the soil after granular application or seed treatment was very low in most species, e.g. eggplant, potatoes, rice, cotton, ranging from 1.8 to 4.9 % (up to 20% in corn) of the applied radioactivity in the vegetative part of mature plants.

The translocation into plants obviously occurs by acropetal transport via the xylem, mainly from the roots to the leaves. After soil application, the main part of the radioactivity was found in the foliage, while only minute amounts were detected in reproductive organs (fruits or seeds). A study with spray application in potatoes showed that the basipetal transport of imidacloprid was negligible. A dominant acropetal translocation was also demonstrated in special translocation experiments in apples and tomatoes. 14 days after application of imidacloprid to leaves radioactivity in fruits was negligible.

After translocation imidacloprid was degraded in nearly all plant parts mainly via three different routes:

1. Hydroxylation of the imidazolidine ring leading to the mono- and dihydroxylated compounds and subsequent removal of water to form the olefin metabolite.

2. Reduction of the nitro group to form the nitrosimine compound and loss of this group with formation of the guanidine metabolites.

3. Oxidative cleavage of the methylene bridge to form 6-chloropicolyl alcohol (and conjugates) and further oxidation to 6-chloronicotinic acid.

Amounts of unchanged parent compound depended on the application form. After spray application, the penetration through the epidermis into leaves occurred relatively slowly. Consequently, the metabolic degradation of imidacloprid was slow and unchanged parent compound, at up to 88 %, was found as the major component of the TRR. Uptake via roots after soil application resulted in most cases in more intensive metabolization and smaller amounts of unchanged imidacloprid at study termination.