Malathion impairs sensory perception, learning, and memory, and increases oxidative stress in the western honeybee,...

The western honeybee, Apis mellifera, is a crucial pollinator in agricultural ecosystems, contributing to approximately 70-80% of insect-pollinated crops globally. However, their populations are declining worldwide, with annual losses of managed colonies often reaching 30-40% in many regions. Among the multiple stressors, insecticide toxicity is a major contributor. This study investigates the toxic effects of malathion, a widely used organophosphate insecticide, on the survival, sensory function, learning and memory, and physiology of A. mellifera. An acute oral median lethal dose (LD₅₀) of 0.39 μg/bee was estimated, and two sublethal doses, respectively 0.19 μg/bee (LD₅₀/2) and 0.039 μg/bee (LD₅₀/10), were used for behavioral assays. Bees exposed to the dose of 0.19 μg/bee showed reduced gustatory responsiveness to sucrose and water, along with selective impairments in associative learning and memory for the excitatory conditioned stimulus (CS+). In contrast, performance for the inhibitory conditioned stimulus (CS-) remained unaffected. Administration of the same sublethal dose even after conditioning also selectively impaired the CS+ memory, and repeated exposure to the lower sublethal dose of 0.039 μg/bee over 3 days also produced similar effects. Biochemical assays further revealed significantly elevated catalase and superoxide dismutase activities in the malathion-treated bees, indicating increased oxidative stress. These findings demonstrate that malathion, even at sublethal doses, impairs honeybee cognition and physiological homeostasis.