A bumblebee queen (Bombus terrestris) on the flowers of small-leaved lime (Tilia cordata). Credit: Ivar Leidus/ CC BY-SA 4.0 DEED.
The current European legislation on pesticides in agriculture demands that each compound is tested individually in field trials before approval, to measure its toxicity for pollinator insects. But in the real world, pollinators are exposed to a plethora of pesticides, and the effects of these mixtures are not well known.
The largest study to date on the subject1 shows that most bumblebees are exposed to more than one pesticide in European fields, and that the higher the exposure the worse the decline of colonies’ health. Scientists monitored more than 300 colonies of Bombus terrestris, a key wild and commercial pollinator, placed in nearly a hundred different sites across eight European countries. The study is the result of the PoshBee project, funded by the European Union.
“The strength of this study is its geographical scale, which includes all three of the European climate zones and thus makes its results robust and with relevant policy implications,” says Cecilia Costa, at the Council for Agricultural Research and Agricultural Economy Analysis (CREA), who is among the study’s authors.
The scientists placed ‘sentinel’ bumblebee colonies at agricultural sites growing apple and oilseed rape. The landscapes around the sites had a proportion of cropland varying from 3% to 98%, to assess how land-use modulates the impact of pesticides. Sixteen sites were in Italy, eight apple orchards in Trentino-Alto Adige and eight oilseed rape fields in Piedmont.
The health of insect colonies was assessed before, during and after crop bloom, when all colonies were retrieved and brought to laboratories to count the number of individuals. Pollen samples from all the colonies were tested for 268 different compounds used as pesticides. The researchers defined ‘pesticide risk’ as the concentration of each toxic compound weighted by its median lethal dose, which is the dose that on average kills 50% of an insect population.
“This procedure allows to make meaningful comparisons, for example between a pollen sample containing a high concentration of a fairly harmless pesticide and one containing a lower concentration of a much dangerous substance,” says Piotr Medrzycki, a researcher at CREA who participated in the study.
On average, the authors found 8 pesticides per colony, but in some cases, they detected upto 27 distinct compounds. The samples with the higher pesticide risk were those that contained relatively high concentrations of only nine pesticides, and a higher pesticide risk was associated with a smaller population after bloom and a smaller weight gain. The authors also observed that the impact of pesticides was mitigated when colonies were placed in areas with less intensive land-use.
“More heterogeneous blooms in the surrounding environment mean a more heterogeneous food supply in terms of nutrients,” explains Gennaro Di Prisco, researcher at the National Research Council who monitored the Italian sites.
Medrzycki says the study highlights the importance of post-approval monitoring, because considering these effects in the approval phase is difficult.
