Exposure to neonicotinoids influences the motor function of adult worker honeybees

Abstract

Systemic pesticides such as neonicotinoids are commonly used on flowering crops visited by pollinators, and their use has been implicated in the decline of insect pollinator populations in Europe and North America. Several studies show that neonicotinoids affect navigation and learning in bees but few studies have examined whether these substances influence their basic motor function. Here, we investigated how prolonged exposure to sublethal doses of four neonicotinoid pesticides (imidacloprid, thiamethoxam, clothianidin, dinotefuran) and the plant toxin, nicotine, affect basic motor function and postural control in foraging-age worker honeybees. We used doses of 10 nM for each neonicotinoid: field-relevant doses that we determined to be sublethal and willingly consumed by bees. The neonicotinoids were placed in food solutions given to bees for 24 h. After the exposure period, bees were more likely to lose postural control during the motor function assay and fail to right themselves if exposed to imidacloprid, thiamethoxam, clothianidin. Bees exposed to thiamethoxam and nicotine also spent more time grooming. Other behaviours (walking, sitting and flying) were not significantly affected. Expression of changes in motor function after exposure to imidacloprid was dose-dependent and affected all measured behaviours. Our data illustrate that 24 h exposure to sublethal doses of neonicotinoid pesticides has a subtle influence on bee behaviour that is likely to affect normal function in a field setting.

Fig. 1

Identification of sublethal concentrations and consumption of solutions. a Mortality was unaffected by ad libitum consumption of 10 nM neonicotinoid solutions; of the 100 nM solutions, only thiamethoxam increased mortality. b Bees consumed slightly less of the 100 nM concentration of the neonicotinoids in 1 M sucrose solution on average. Post hoc comparisons against the control (sucrose) are indicated by letters (e.g. ‘a’); the number indicates the treatment (a ₁ = 10 nM, a ₂ = 100 nM). The asterisk indicates a significant (P < 0.05) pairwise, post hoc comparison of 10 versus. 100 nM for each neonicotinoid treatment. Note a separate sucrose control group was performed for each concentration neonicotinoid. Suc sucrose (control), Nic nicotine, Imd imidacloprid, Tmx thiamethoxam, Clo clothianidin, Din dinotefuran. N = 4 cohorts of 15 bees for each treatment group. Bars represent means ± SE

Fig. 2

Effects of 10 nM doses of neonicotinoids on behaviour. Walking behaviour (a–c), time sitting still (d – f), and flying behaviour (g – i) were not significantly different among pesticide treatments. Exposure to imidacloprid, thiamethoxam, and clothianidin influenced the time spent upside down (j). The number of bouts of upside down behaviour (k) was affected by thiamethoxam, clothianidin, and dinotefuran, whereas imidacloprid, thiamethoxam, and clothianidin influenced mean bout duration of upside down behaviour (l). The time spent grooming was affected by nicotine and thiamethoxam (m). The number of grooming bouts was greater in thimethoxam treated bees (n) and mean grooming bout duration was longest in nicotine and thiamethoxam treated bees (o). S sucrose, N nicotine, I imidacloprid, T thiamethoxam, C clothianidin, D dinotefuran. N = 15 individual bees for each treatment group. The asterisk indicates a significant (P < 0.05) pairwise, post hoc comparison for each neonicotinoid treatment to the control (sucrose). Bars represent means ± SE

Fig. 3

The effect of imidacloprid exposure on motor function is concentration-dependent. High doses of imidacloprid reduced walking behaviour (a – c), increased the time spent standing still (d – f), and completely abolished flight behaviour (g – i). Bees given intermediate concentrations (10 and 100 nM) of imidacloprid spent more time upside down (j) and had longer bouts of upside down behaviour (l). High doses of imidacloprid completely abolished grooming behaviour (m) and reduced the number of bouts of grooming (n). A 10 nM concentration increased the mean grooming bout duration (o). N _S = 16, N _10nM = 15, N _100nM = 13, N _1µM = 10. X-axis is the concentration of imidacloprid in 1 M sucrose solution. The asterisk indicates a significant (P < 0.05) pairwise, post hoc comparison for each neonicotinoid treatment to the control (sucrose). Bars represent means ± SE