Neonicotinoids and honey bee health - The effect of the neonicotinoid
clothianidin applied as a seed dressing in Brassica napus on pathogen and parasite
prevalence and loads in free-foraging adult honeybees (Apis mellifera)

Julia Goss, Maj Rundlöf, Joachim de Miranda, Riccardo Bommarco, Thorsten R. Pedersen,
Henrik G. Smith, Ingemar Fries
Weinbergweg 15, 97702 Burghausen, Deutschland

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 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 in 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)7. Both the impact of clothianidin exposure on 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. Furthermore
the exposure to clothianidin had no significant influence on the Nosema spp. prevalence or the
amount of Nosema spores in infested 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. The DWV prevalence was relatively low; 4% and 36% of colonies infected,
before and after the experiment respectively. 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.

Hazards of pesticides to bees - 12th International Symposium of the ICP-PR Bee Protection Group, Ghent (Belgium), September 15-17, 2014

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