Wildflowers growing near fields sown with pesticide-treated seeds can be reservoirs of bee-harming neonicotinoid compounds, according to new research. The study suggests bees get most of their exposure to these pesticides from wildflowers, rather than from the crops the pesticides are designed to protect (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b03459).
Seeds for large-scale agricultural crops such as canola, also known as oilseed rape, and wheat often are treated with neonicotinoid pesticides before planting as a prophylactic measure to guard against a variety of pests. These compounds are incorporated into the plants’ tissues via water, reaching stems, nectar, and pollen. Neonicotinoids have been suspected in the decline of bees. Earlier studies, for instance, have shown reduced growth and a dramatic reduction in the production of queen bees in hives exposed to neonicotinoids (Science 2012, DOI: 10.1126/science.1215025). Now Cristina Botías of the University of Sussex and her colleagues have taken a closer look at where neonicotinoids might be coming from in the field.
Over 10 months, the team sampled soils and collected pollen and nectar by hand from crops of canola, as well as from strips of wildflowers growing at the edges of fields on five farms in the U.K. They also placed a honey bee hive on each farm with a special trap that captures the bees’ pollen as they enter the hive. The team could then measure how much pollen the bees brought home and observe the shape of the pollen grains under a microscope to determine which plants they came from. They used ultra-high-performance liquid chromatography with tandem mass spectrometry to measure neonicotinoid levels in the pollen, nectar, and soil.
To the team’s surprise, during the peak flowering period in June, which is also a busy foraging and growing period for bees, only 3% of the neonicotinoid residues that the bees brought home in pollen came from canola; the rest came from wildflowers. The researchers calculated that bees brought more than 70 ng of neonicotinoid residues to their hives per day in June, and about 11 ng per day in August, when fewer flowers are blooming and bees’ growth is slower. These levels are lower than doses known to be lethal, but above levels that have shown harm over long-term exposure in previous research.
If wildflowers are a significant source of neonicotinoids, bees might get greater exposure than expected: canola plants flower for four or five weeks while wildflowers of one type or another bloom all summer long. That means that bees would be in contact with neonicotinoids for the entire six-month foraging season, not for just a few weeks, Botías says.
All the team’s soil samples contained neonicotinoids, though wildflowers’ soils had much lower concentrations than the field crop soils. This suggests, Botías says, that the water-soluble neonicotinoids travel from agricultural fields to wildflower strips. Meant to be clean refuges for pollinators, wildflower buffers instead become sources of pesticides.
Christian H. Krupke of Purdue University, who, with his colleagues, has found similar results in research now under review, notes that this is challenging, costly, time-consuming work. So while this study represents only five hives, it’s an important contribution to understanding how bees are exposed to neonicotinoids that should be replicated on a broader scale. Further work is also necessary to show how the pesticides might travel through soils, he says.
But Peter Campbell of Syngenta, a company that makes neonicotinoid pesticides, says the report has “inconsistencies in the analytical data and residues found that are unusual and unexplained,” noting that the residues in wildflowers are much higher than in the treated crop and in the soil in which they are growing. He also cites the low levels detected in the nectar, where the researchers should detect similar levels to pollen or related levels of breakdown products.
Botías says that nectar is a challenge to collect and that the compounds could degrade faster in nectar than pollen. She wants to further study nectar as well as the variability observed in different plants—and perhaps even bee preferences for canola versus wildflowers. She and her colleagues would also like to look at whether moving wildflower buffers farther from fields would help. “We have found high levels in some of these wildflowers, and these flowers are really attractive to bees,” Botías says. “We may be putting bees at risk.”