concentrations of each OP was calculated (Table 1). Because honey
represents all the nectar sources collected, whether these are
treated crops or wild flowers, and represents food consumed by
adults and juvenile bees inside the colony, we focused on residues
found in honey to derive test concentrations used in our experiments.
The median and 95th centile concentrations of each OP
were determined. Stock solutions (1000 ppm) of each insecticide
were prepared in acetone according to guidelines for efficacy and
side effect testing from the European and Mediterranean Plant
Protection Organization (EPPO, 1992) and the International
Commission for Plant-Bee-Relationships (ICPBR) and then required
dilutions were made for the median and 95th centile concentrations
of each OP. Insecticides were collectively added to a
500 g L1 sucrose solution. The final concentration of acetone in
solutions was 1% (v/v). For all experiments, bees were exposed
to: (1) a mixture of the median concentrations of the four OPs;
or (2) a mixture of the 95th centile concentrations of the four
OPs. A solution of 1% acetone (vol/vol) in 500 g L1 of sucrose
was used as a control. Fresh solutions were prepared for each
bioassay replicate (i.e. repeat of the experiment).