Neonicotinoids and honeybee health : the effect of the neonicotinoid clothianidin, applied as a seed dressing in Brassica napus, on pathogen and parasite prevalence and quantities in free-foraging adult honeybees (Apis mellifera)

Abstract: Sub-lethal doses of neonicotinoids have been shown to negatively impact the health of honeybees. However, most studies to date have exposed bees only artificially to these pesticides under laboratory conditions. There have been just a few well designed and replicated studies of the impacts of realistic neonicotinoid exposure on honeybees foraging under field conditions. In order to close this knowledge gap, and to test the influence of the neonicotinoid clothianidin on honeybees, we used a study system of 16 paired, spatially separated (>4 km) spring oilseed rape fields in the south of Sweden. The fields were paired according to land use, the surrounding landscape and geographical proximity, using GIS. Eight of the fields were randomly assigned to be sown with clothianidin dressed Brassica napus (oilseed rape) seeds and their corresponding pairs with undressed B. napus seeds, as controls. Six equally sized Apis mellifera colonies, with known queen origin, were placed at each field resulting in a total of 96 colonies. Samples of bees, pollen and nectar taken from the colonies showed that the honeybee colonies at the treated fields were exposed to several orders of magnitude higher clothianidin concentrations than the colonies at the control sites. To determine the effect of this neonicotinoid on pathogen and parasite prevalence and quantities in honeybee colonies samples of adult bees were taken from each colony both before and after the flowering period in the paired fields. The parasites studied included the ectoparasitic mite Varroa destructor and the microsporidian gut parasite Nosema. The pathogens studied included eight different honeybee viruses (BQCV, SBV, DWV, KBV, SBPV, CBPV, ABPV, and IAPV). Both the prevalence (proportion of positive colonies) and the amount of parasites/pathogens in each colony (infestation rate/titres) were analysed. The infestation with V. destructor was relatively low and the exposure to clothianidin had no significant impact on the V. destructor prevalence and infestation rate of the colonies. A seasonal effect was found where more mites were detected and more colonies were infested after the experiment than before. The exposure to clothianidin had no significant influence on the Nosema spp. prevalence or the amount of Nosema spores in infested colonies. However, the Nosema spp. infestation rate was significantly higher before than after the rape flowering season. Furthermore the proportion of the two Nosema species changed over time. Before the experiment, 33 % of the samples were not infested with Nosema spp., 6 % were infested with pure N. apis, 25 % with pure N. ceranae and 36 % with mixed infestation. In the mixed infestations, on average, one third of the DNA originated from N. apis and the rest from N. ceranae. After the experiment only N. ceranae was present in the colonies. Three out of the eight viruses studied were detected: DWV, SBV and BQCV. Both BQCV and SBV were detected in practically all colonies, both before and after the experiment, with consequently no difference in prevalence due to clothianidin exposure or season. There was also no difference in BQCV and SBV titres due to clothianidin exposure, although for BQCV there was a significant reduction in titre as the season went along. SBV titres remained constant throughout the season. The DWV prevalence was relatively low; 4% and 36% of colonies infected, before and after the experiment respectively, resulting in a significant seasonal difference in DWV prevalence in contrast to the DWV titres in positive samples which showed no seasonal effect. The clothianidin exposure had no effect on the DWV prevalence or on the titres in DWV positive samples. The higher prevalence of DWV in the control group compared to the treated group can be explained by the different initial conditions. It can be concluded that in this experiment, clothianidin exposure had no effect on the prevalence or the amount of the studied pathogens and parasites in honeybee colonies.