Enhanced toxicity of GABA-gated chloride chB36tors against resistant Drosophila melanogaster

The insect gamma-amino butyric acid (GABA) receptor is a ligand-gated chloride ion channel found in the central nervous system of insects, where it plays a crucial role in various physiological processes. Insecticides that target GABA receptors trigger hyperexcitation of neurons, leading to uncontrolled nerve impulses that initially cause excitation, followed by paralysis and, ultimately, death. The GABA receptor has been, and remains, a focal point for multiple classes of chemistry categorized under two groups of the Insecticide Resistance Action Committee (IRAC) modes of action, specifically Groups 2 and 30. Group 30 includes more recently developed compounds, such as meta-diamides (e.g., broflanalide) and isoxazolines (e.g., fluralaner and isocycloseram). In a resistant strain of Drosophila melanogaster, a point mutation in the GABA receptor (A301S) confers resistance to molecules in Group 2 but not those in Group 30. Our studies also indicate that Group 30 insecticides do not show cross-resistance with the A301S mutations. However, we demonstrate, using surface contact toxicity bioassays, that isocycloseram shows a 4-fold increase in toxicity in resistant flies carrying the A301S mutation. This phenomenon is further characterized through various toxicological bioassays and electrophysiological methods. These investigations suggest that isocycloseram preferentially binds to the GABA receptor in the presence of the A301S mutation. Such findings have the potential to effectively target both resistant and susceptible populations, thereby informing integrated resistance management control programs and may also prove essential in developing new chemistry that favors mutated resistant targets.