Comparative acute toxicity of eight insecticides against worker Honeybees (Apis mellifera L.)

Abstract

Honeybees (Apis mellifera L.) are vital pollinators in natural and agricultural ecosystems, yet pesticide exposure remains a primary factor contributing to their decline. The studies were conducted to figure out the acute oral and contact toxicity of eight commonly used insecticides including acetamiprid, imidacloprid, diafenthiuron, abamectin, chlorpyrifos, spinosad, methoxyfenozide, and cypermethrin to worker honeybees under controlled laboratory conditions. Bees were exposed to 0.5, 1.0, 2.0, 4.0 and 5.0 µg/ml concentration of each insecticide, and mortality was recorded after 12, 24, 48, and 72 hours’ exposure periods. Median lethal concentrations (LC₅₀, LC₉₀) and doses (LD₅₀, LD₉₀) were estimated using probit analysis (POLO-PC). Toxicity varied significantly among insecticides and exposure routes. After 72 hours of oral exposure, abamectin (LC₅₀ = 0.26 µg mL⁻¹) and imidacloprid (LC₅₀ = 0.27 µg mL⁻¹) were the most toxic, while chlorpyrifos exhibited the lowest toxicity (LC₅₀ = 5.77 µg mL⁻¹). In contact bioassays, abamectin also demonstrated the greatest toxicity (LD₅₀ = 0.64 µg mL⁻¹), followed by imidacloprid and spinosad, whereas; cypermethrin and chlorpyrifos were found less harmful. The steeper slope values for abamectin and imidacloprid indicate uniform susceptibility among exposed bees. These results underscore substantial differences in the hazard profiles of widely used insecticides and emphasize the importance of considering both exposure routes when assessing risk. The findings offer a quantitative foundation for assessing pollinator risks and guiding pesticide regulation, thereby facilitating the selection of insecticides with reduced toxicity to bees within integrated pest management (IPM) programs