Neonicotinoids

Neonicotinoids are extremely ecotoxic pesticides. By infiltrating all environmental compartments, they indiscriminately kill both vertebrates and invertebrates. Despite their ban in 2018, traces of these substances persist in the environment at lethal concentrations.

In 2013, the European Union restricted the use of three standard neonicotinoids in outdoor crops and, in 2018, abolished their use completely. These measures were taken in particular to protect populations of pollinating bees from the toxic effects of these nicotinoids, which contribute to colony collapse syndrome. However, recent studies have shown that despite their ban, traces of neonicotinoids persist in crops at lethal concentrations and still pose a significant danger to bees.

Since 2013, a moratorium of the European Union (EU) has restricted the application of three neonicotinoids on bee-attractive crops because of their harmful effects on these insects.

Researchers from the CNRS, INRA and the Institut de l’Abeille (ITSAP) have recently demonstrated that residues of these insecticides — and in particular imidacloprid — can still be detected in rapeseed nectar in 48% of the field plots studied, with highly variable concentrations from year to year.

A risk assessment for bees, based on models and parameters used by health agencies, revealed that at least 12% of the fields were sufficiently contaminated during two out of five years to kill 50% of the honey bees and bumblebees foraging there. The role of neonicotinoids in bee decline led to the 2013 moratorium limiting the use of three insecticides — clothianidin, imidacloprid and thiamethoxam — on crops attractive to pollinating bees.

Neonicotinoids are agonists of acetylcholine (ACh). The pesticide molecules bind to nicotinic ACh receptors and thus prevent acetylcholine from fulfilling its role as a neurotransmitter. Bees therefore suffer severe neurological disorders before dying.

In September 2018, their application was banned on all outdoor crops in France. Nevertheless, neonicotinoids are frequently detected on wildflowers and untreated crops, suggesting their dispersion in the environment following agricultural use.

Persistent traces of neonicotinoids in rapeseed crops

To explore this issue further, scientists searched for and quantified neonicotinoid residues in the nectar of 291 winter rapeseed plots (536 samples) during the five years following the adoption of the moratorium, from 2014 to 2018. The results were published in the journal Science of the Total Environment. Their first observation was that all three neonicotinoids concerned could be found in the samples.

Imidacloprid, in particular, was detected every year in 43% of the samples analysed (corresponding to 48% of the fields), with no downward trend over time but with large variations in concentration. In 2016, more than 90% of the sampled plots tested positive, compared with 5% in 2015.

Residue levels depend on soil type and are higher when precipitation is greater, but they do not appear to be directly linked to the spatial or temporal proximity of potentially treated crops. Although 92% of positive samples contained only 0.1 to 1 ng/mL of imidacloprid, maximum concentrations in some cases exceeded those reported for treated plots, reaching up to 70 ng/mL.

A still worrying mortality risk for bees

Using these data, mortality assessments based on health-agency models and parameters suggest a non-negligible risk for pollinating bees. For honey bees, the risk peaked in 2014 and 2016, when around 50% of pollinators were likely to die from imidacloprid exposure in 12% of the plots studied. During these years, 10–20% of plots showed contamination levels associated with a similar mortality risk for bumblebees and solitary bees.

These results indicate that the persistent use of neonicotinoids for certain outdoor crops threatens bees and pollinators visiting other untreated crops. The researchers confirm that imidacloprid residues remain in the environment and spread, and can even be found in rapeseed nectar, even though neonicotinoids have not been applied to rapeseed crops since 2013. They also justify strengthening pesticide controls through the total ban on the use of neonicotinoids for all outdoor crops in France, adopted in September 2018.