Udo Reckmann

Whitefly‐active metabolites of imidacloprid: biological efficacy and translocation in cotton plants

The chloronicotinyl insecticide imidacloprid is widely used in soil application, seed treatment and as a foliar spray. Its systemic properties are well-known. It is more or less completely metabolised, depending on the method of application, plant species and time. Some of the metabolites that arise are active against different aphid species, as shown earlier, with the imidazoline derivative (olefine metabolite) more active in oral ingestion bioassays than the parent compound itself. In the present work, we demonstrate that the olefine metabolite and two hydroxy metabolites of imidacloprid are also active against the cotton whitefly, Bemisia tabaci, in oral ingestion bioassays (sachet test). The 4-hydroxy metabolite is as active as imidacloprid and the olefine compound c10 times more active. The two hydroxy metabolites were also active against biotypes from Almeria, Spain and a B-type strain from California. The physicochemical properties of the metabolites were determined and compared with those of imidacloprid. Lower log P values were found for all metabolites investigated. Interestingly, the olefine metabolite was the only compound with an acidic proton (pKa 7.2) under neutral pH conditions, suggesting that it could be more phloem-mobile. In order to investigate this hypothesis we compared the translocation of the radiolabelled olefine metabolite after foliar application with the movement of radiolabelled imidacloprid in cotton plants, but no major differences were found in acropetal or basipetal translocation.

Insecticidal heterolignans—Tubuline polymerization inhibitors with activity against chewing pests

Starting from natural product podophyllotoxin 1 substituted heterolignans were identified with promising insecticidal in vivo activity. The impact of substitution in each segment of the core structure was investigated in a detailed SAR study, and variation of substituents in both aromatic moieties afforded derivatives 5 and 43 with broad insecticidal activity against lepidopteran and coleopteran species. In vitro measurements supported by modeling studies indicate that heterolignans 3-134 act as tubuline polymerization inhibitors interacting with the colchicine-binding site. Insect specific structure-activity effects were observed showing that the insecticidal SAR described herein differs from reported cytotoxicity studies.