Development of a Visualization Method for Imidacloprid in Drosophila melanogaster via Imaging Mass Spectrometry

Imidacloprid is widely used for exterminating harmful insects; however, information regarding its distribution in insects is limited. Herein, we developed a visualization method for imidacloprid in Drosophila melanogaster, by using matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). IMS requires sample cryosections; however, certain challenges prevail in retaining fly morphology in sections owing to their small size and heterogeneous components. Therefore, the section preparation method was optimized first, followed by imidacloprid distribution visualized using MALDI-IMS. Using 10% gelatin as an embedding material and 70% ethanol for pretreatment, the gaps between embedding material and D. melanogaster body surface were reduced. The tight adhesion between embedding media and D. melanogaster retains fly morphology in sections. Furthermore, the imidacloprid standard was analyzed separately via MALDI and electrospray ionization (ESI), and imidacloprid was converted to guanidine-imidacloprid via laser irradiation. Consequently, the imidacloprid distribution in D. melanogaster was successfully visualized using guanidine-imidacloprid as the target peak.