Metabolism of radiolabeled insecticides in insects and related arthropods: a critical study of various techniques

The metabolism of radiolabeled insecticides in insects and acarina is studied largely by coupling radiotracer techniques with analytical methods, such as TLC, paper and column chromatography, gel-permeation chromatography, and enzymatic assays. These techniques in various combinations yield both the identification and quantification of the metabolites. Other analytical methods such as gas chromatography or IR spectrometry may also be used to obtain additional support for identification of metabolites. In the absence of authentic chromatographic standards, however, NMR and mass spectrometry are necessary in the identification of the unknown compound. The quantity of the radiolabeled insecticide administered should be within the toxicological range of the insect. Therefore, the dosage-mortality response of the insect using unlabeled material should be determined. A dose should be selected that keeps insect mortality to a minimum in order to avoid complications in the computation of the balance data. The radiolabeled insecticide is usually applied topically to the insect. Alternately, the material may be administered by dipping in a solution containing the radiolabeled compound or by exposure to filter paper impregnated with radiolabeled material. Administration of the radiolabeled material by the oral route presents several problems. Sterile rearing conditions are mandatory to avoid contamination of treated diet with microorganisms. Some knowledge of the insect's feeding rhythm is desirable so that the labeled diet is given at peak feeding time. Synthetic diets should be adjusted to pH 7.0. These precautions minimize degradation of the insecticide in the diet prior to consumption by the insect. Precise doses of radiolabeled materials may be administered by injection. The technique is mainly useful in metabolism studies of intermediate materials resulting from the biotransformation of the parent compound.