Learning performances of honeybees (Apis mellifera L) are differentially affected by imidacloprid according to the season

To establish the sublethal concentrations domain, acute and chronic oral tests were conducted on caged honeybee workers (Apis mellifera L) using imidacloprid and a metabolite, 5-OH- imidacloprid, under laboratory conditions. The latter showed a 48-h oral LD 50 value (153ng per bee) five times higher than that of imidacloprid (30ng per bee). Chronic feeding tests indicated that the lowest observed effect concentrations (LOEC) of imidacloprid and of 5-OH-imidacloprid on mortality of winter bees were 24 and 120 m gk g 1 respectively. Behavioural effects of imidacloprid and 5-OH- imidacloprid were studied using the olfactory conditioning of proboscis extension response at two periods of the year. Winter bees surviving chronic treatment with imidacloprid and 5-OH-imida- cloprid had reduced learning performances. The LOEC of imidacloprid was lower in summer bees (12 m gk g 1 ) than in winter bees (48 m gk g 1 ), which points to a greater sensitivity of honeybees beha- viour in summer bees, compared to winter bees. # 2003 Society of Chemical Industry