Combined toxicity of pyrethroid insecticides and azole fungicides in honey bee larvae

Pyrethroid insecticides and azole fungicides are frequently employed in agriculture, and honey bees are commonly exposed to these pesticides during foraging. Nevertheless, most studies on honey bee larvae have focused on single compound exposure, and studies on mixture toxicity in honey bee larvae are limited. Herein, we investigated the individual and combined toxic effects of pyrethroid insecticides (cypermethrin and deltamethrin) and azole fungicides (propiconazole and tebuconazole). Deltamethrin had the highest toxicity to honey bee larvae with 0.04 μg/larva LD₅₀, followed by cypermethrin with 0.13 μg/larva LD₅₀ value. Contrastingly, relatively low toxicities were found in azole fungicides, propiconazole and tebuconazole with their LD₅₀ values of 49.91 and 21.90 μg/larva, respectively. Except for the cypermethrin + tebuconazole combination, all the binary and ternary mixtures showed synergistic effects. Additionally, exposure to pesticide mixtures changed the distribution of mortality symptoms. Particularly, early death and stretched death emerged as the predominant symptoms, with proportions varying among the pesticide combinations. Thus, co-exposure to pyrethroid insecticides and azole fungicides can enhance the toxicity in honey bee larvae.