Comparative toxicity of two neonicotinoids and a pyrethroid to forager honeybees (Apis mellifera L., 1758) (Hymenoptera: Apidae) by different exposure methods

Honeybees are exposed to insecticides by direct contact with spray droplets or residues on plant, or through ingestion of contaminated pollen or nectar. Direct contact with foliar spray might be the most common exposure route and contact bioassays are preferred as they better simulate field situation. Bioassays were conducted during 2018 at Sultan Qaboos University, Oman. The acute contact and oral toxicity of commercial formulations of deltamethrin 2.5 EC, thiamethoxam 25 WG and acetamiprid 20 SL to Apis mellifera subsp. lamarckii Cockerell 1906 (Hymenoptera: Apidae) foragers were measured by three exposure methods (contact by a 1-µL droplet on thorax, contact by Potter spray tower and oral ingestion). Potter tower exposure gave significantly higher mortality at lower concentration of deltamethrin than contact exposure by single droplet on thorax. Thiamethoxam showed significantly higher mortality through oral exposure at all concentrations. HQoral values were also calculated. Acetamiprid did not give more than 50% mortality even with the highest concentration. Potter tower produced fine droplets (0.286±0.071 µm) and a total of 0.829 µL was deposited on a single honeybee. Forager honeybees are more likely be exposed to the very fine droplets in field and toxicological results obtained by Potter tower or similar devices will be more realistic than a single droplet on thorax.