Interactions between Nosema microspores and a neonicotinoid weaken honeybees (Apis mellifera)

Abstract

Global pollinators, like honeybees, are declining in abundance and diversity, which can adversely affect natural ecosystems and agriculture. Therefore, we tested the current hypotheses describing honeybee losses as a multifactorial syndrome, by investigating integrative effects of an infectious organism and an insecticide on honeybee health. We demonstrated that the interaction between the microsporidia Nosema and a neonicotinoid (imidacloprid) significantly weakened honeybees. In the short term, the combination of both agents caused the highest individual mortality rates and energetic stress. By quantifying the strength of immunity at both the individual and social levels, we showed that neither the haemocyte number nor the phenoloxidase activity of individuals was affected by the different treatments. However, the activity of glucose oxidase, enabling bees to sterilize colony and brood food, was significantly decreased only by the combination of both factors compared with control, Nosema or imidacloprid groups, suggesting a synergistic interaction and in the long term a higher susceptibility of the colony to pathogens. This provides the first evidences that interaction between an infectious organism and a chemical can also threaten pollinators, interactions that are widely used to eliminate insect pests in integrative pest management.

Fig. 1

Effect of Nosema infection and/or exposure to imidacloprid on bee mortality and energetic demands. A. Effect on mortality. Mortality is expressed as the percentage of cumulated number of dead bees per cage and per day (n = 270 bees). Three colonies were analysed, with three cage replicates for each colony (n = 30 bees per cage). Each letter indicates significant differences between treatments (P < 0.05). B. Effect on energetic demand. Sucrose consumption is expressed as the amount of sucrose solution (50% w/v, ad libitum delivery) consumed per day and per bee (n = 30 bees per cage) during the 10 h of treatment. The same cages as in A were analysed. Each letter indicates significant differences between treatments (P < 0.05).

Fig. 2

Level of Nosema infection in bees fed with Nosema and/or exposed to imidacloprid. Level of infection was determined at days 5 and 10 on seven to eight bees per cage for each experimental group (n = 382 bees). Three colonies were analysed, with two cage replicates for each colony. Data show mean ± SE.

Fig. 3

Effect of Nosema infection and/or exposure to imidacloprid on individual immunity. A. Total haemocyte counts at days 5 and 10 on seven to eight bees per cage for each experimental group (n = 373 bees). B. Phenoloxidase activity at days 5 and 10 in eight bees per cage for each experimental group (n = 384 bees). For each parameter, three colonies were analysed, with two cage replicates for each colony. Boxes show 1st and 3rd interquartile range with line denoting median. Whiskers encompass 90% of the individuals, beyond which each outliers are represented by circles.

Fig. 4

Effect of Nosema infection and/or exposure to imidacloprid on social immunity. A. Glucose oxidase activity at day 10 on eight bees per cage for each experimental group (n = 192 bees). Boxes show 1st and 3rd interquartile range with line denoting median. Whiskers encompass 90% of the individuals, beyond which each outliers are represented by circles. *denotes significant difference between Nosema × imidacloprid groups and the three others groups (P < 0.05). B. HPG size at day 10 in seven to eight bees per cage for each experimental group (n = 191 bees). For each parameter, three colonies were analysed, with two cage replicates for each colony. The size was indexed from 1 to 5 (see Experimental procedures). Each letter indicates significant differences between treatments (P < 0.05). Data show mean ± SE.