Pest species sensitivity tests: Necessity of pesticide content analysis of dipped leaves

Today different laboratory tests are used for analysing efficacy of pesticides against different arthropods. The IRAC susceptibility test methods propose among others the exposure of the pests to dry residues on leaf discs, depending on the pest species and the tested insecticide (www.irac-online.org). The preparation of dry residues is usually realized via spray application in a Laboratory Spray Tower or time- and cost-saving by dipping leaf discs in the desired test solution assuming to test the approved application rate. To check this latter assumption we analysed the content of active ingredients (ai) on leaf discs from eleven agri- and horticultural plant species or varieties dipped into solutions of different pesticide formulations. Ai measurements were performed with liquid or gas chromatography-mass spectrometry. Depending on the plant species/variety, the dry residues on the leaf discs showed a high variability and ai-contents were for some plant species in mean up to 200% of the field spray rate. Apart from simple calculation errors the surface structure of the leaf disc and the properties of the formulation can influence the attachment of the pesticide, especially when the dipping method is applied. Therefore, the ai content adhering to the leaf discs cannot be generally predicted. Thus, statements about the susceptibility of organisms to pesticides are only acceptable if the intended residue content is appropriately controlled in the preparation of the bioassay.