Impaired olfactory associative behavior of honeybee workers due to contamination of imidacloprid in the larval stage

Abstract

The residue of imidacloprid in the nectar and pollens of the plants is toxic not only to adult honeybees but also the larvae. Our understanding of the risk of imidacloprid to larvae of the honeybees is still in a very early stage. In this study, the capped-brood, pupation and eclosion rates of the honeybee larvae were recorded after treating them directly in the hive with different dosages of imidacloprid. The brood-capped rates of the larvae decreased significantly when the dosages increased from 24 to 8000 ng/larva. However, there were no significant effects of DMSO or 0.4 ng of imidacloprid per larva on the brood-capped, pupation and eclosion rates. Although the sublethal dosage of imidacloprid had no effect on the eclosion rate, we found that the olfactory associative behavior of the adult bees was impaired if they had been treated with 0.04 ng/larva imidacloprid in the larval stage. These results demonstrate that a sublethal dosage of imidacloprid given to the larvae affects the subsequent associative ability of the adult honeybee workers. Thus, a low dose of imidacloprid may affect the survival condition of the entire colony, even though the larvae survive to adulthood.

Figure 1. Lethal effect of imidacloprid on the honeybee larvae.

The subfigures show the effects of imidacloprid on the capped-brood rate (A), pupation rate (B) and the eclosion rate (C) of honeybee larvae under field conditions (Tested larvae were obtained from 4 colonies, N _{0.1% DMSO} = 40+40+35+40, N _{1% DMSO} = 30+40+40+30, N _{0.4 ng} = 40+40+35+40, N _{24 ng} = 30+30+40+30, N _{200 ng} = 30+40+40+30, N _{2000 ng} = 40+40+30+30, N _{4000 ng} = 40+40+30+30, N _{6000 ng} = 40+40+30+30, N _{8000 ng} = 30+40+30+30 larvae). The doses of imidacloprid treatments are 0.4, 24, 200, 2000, 4000, 6000 and 8000 ng/larva. C1 and C2 are the control groups (0.1 and 1% DMSO). In each subfigure, any two effects of the experimental treatments without any same letter above the columns are significantly different (two-tailed Kruskal-Wallis H test, P<0.001, compared by two-tailed Mann-Whitney U tests with Dunn-Šidák correction at the 95% confidence level).

Figure 2. Impaired olfactory associative behavior in adulthood caused by the larval contamination of imidacloprid.

The subfigures show the effect of DMSO (A) and imidacloprid (B) on the olfactory associative behavior of the honeybee (Tested honeybees were obtained from 3 colonies, N _{1% DMSO} = 35+25+30, N _Control = 33+30+30, N _DDW = 30+30+28, N _{0.0004 ng} = 30+30+30, N _{0.004 ng} = 32+30+30, N _{0.04 ng} = 30+30+35, N _{0.4 ng} = 26+29+32 bees). T1, T2, T3 and T4 are conditioning trials 1–4, respectively. In figure 2B, any two effects of the experimental treatments among T2, T3 or T4 without any same letter beside the data points are significantly different (two-tailed Kruskal-Wallis H test, P≤0.001, compared by two-tailed Mann-Whitney U tests with Dunn-Šidák correction at the 95% confidence level).