Timing matters: Nighttime pesticide application reduces honeybee exposure and acute mortality

Pollinator exposure to pesticides remains a major challenge for sustainable agriculture, yet risk mitigation measures based on application practices remain poorly tested under realistic conditions. This study evaluates whether nighttime pesticide application can reduce pesticide exposure and colony-level impacts on honeybees compared with conventional daytime spraying. Honeybee colonies were placed in semi-field tunnel conditions and exposed to the insecticide formulation Karate K® (active ingredients: lambda-cyhalothrin 5 g/L and pirimicarb 100 g/L), which was sprayed on flowering phacelia either during daytime or shortly after sunset. Colony activity and mortality were continuously monitored (3 days pre-treatment plus 3 days post-treatment) using automated video-based bee counters, while pesticide residues were quantified in foraging bees following treatments. Nocturnal colony activity represented less than 1 % of daily flights, confirming minimal nighttime foraging. Flight activity showed transient and contrasting responses between treatments, with bee activity increasing and decreasing over one day after daytime and nighttime treatments, respectively. These changes returned to baseline over the three days following application. Daytime spraying caused significant short-term increases in daily mortality levels and rates, more than doubling over one day, whereas nighttime application produced no detectable increase in mortality. Elevated mortality levels persisted for three days following daytime application. Residues of both lambda-cyhalothrin and pirimicarb declined markedly between 4 h and 20 h post-application. Residue analyses also showed significantly higher lambda-cyhalothrin levels in bees following daytime application compared with nighttime spraying, consistent with reduced direct exposure when spraying occurred outside foraging periods. Detected doses represented only a small fraction of laboratory-derived LD 50 values (less than 10 %), yet were associated with significant colony-level mortality, underscoring the ecological relevance of sublethal and field-realistic exposures. These findings provide experimental evidence that shifting pesticide applications outside bee foraging periods substantially reduces acute pesticide impacts on honeybee colonies. Adjusting application timing therefore represents a practical and immediately deployable mitigation measure to reduce honeybees’ and other diurnal pollinators’ exposure while maintaining crop protection practices, although potential risks to nocturnal pollinators and sublethal effects warrant further investigation.