The question is not whether neonicotinoids can be toxic to bees. These chemicals are marketed for their insect-killing power, and bees are, after all, insects. What has been harder to pin down is whether the exposure bees experience in the wild is harmful. Tests of neonicotinoids on bees in the lab show that the compounds are dangerous to them, but field studies have largely failed to implicate the pesticides.
To resolve the controversy, two teams conducted large-scale field and lab studies designed to mimic natural conditions across four countries. Researchers in Europe placed bees in neonicotinoid-treated canola fields and monitored their health. Researchers in Canada monitored bees in apiaries within 550 yards of treated corn fields. They sampled pollen and honey for pesticides over five months and attempted to replicate that exposure under controlled conditions. Both studies showed negative but different effects on bees, and demonstrated that bees are picking up neonicotinoids from surprising sources.
In the European study, three types of bees were placed in 33 different canola fields across three countries. The differences between bees in treated or untreated fields were largely insignificant, and many of the bees in both groups died before they could be counted. This demonstrates how tough large-scale field studies are to conduct, said Richard Pywell, one of the study authors.
Of the differences that were statistically significant, results varied by country. In Hungary, honeybee egg production decreased and fewer worker bees survived the winter. In Britain, honeybee worker numbers decreased, and so did drones for a certain species of wild bee. In Germany, drone numbers for that same wild bee species and honeybee egg production actually increased.
Several important factors might help explain these regional differences. Nearly half the pollen picked up by bees in Hungary and Britain came from canola. In Germany, that number is closer to 10 percent. That means German bees had a wider variety of flowers to choose from, possibly diluting their exposure to pesticides.
Disease rates were also lower in Germany. In Britain, many of the bees fell prey to the dreaded Varroa destructor mite — a hairy, fanged parasite that many bee experts blame for the majority of losses that occur in commercial honeybee operations. In Hungary, a fungal infestation devastated the bees in the study. That bees in Britain and Hungary, but not Germany, were adversely affected by neonicotinoid exposure suggests that the pesticides may exacerbate deadlier pressures bees face, such as parasites.
However, the small number of significant effects “makes it difficult to draw any reliable conclusions,” said Norman Carreck, science director of the International Bee Research Association, who was not part of either study.
Christopher Cutler, who studies insect toxicology at Nova Scotia's Dalhousie University, echoed Carreck’s concerns, pointing out that “when many different analyses are conducted” (42 in this case), “a small number of statistically significant effects are bound to emerge by chance.”
In another complication, when the researchers in Europe sampled the nests of bumble bees and solitary bees for pesticide residues, they found contamination with one type of neonicotinoid not used in the study. This was surprising, because there is a moratorium on the use of the pesticides to treat seeds of flowering plants in the European Union. This supports previously proposed hypotheses that neonicotinoids, which are water soluble, might persist in the soil, and that the bees were picking up chemicals applied before the ban.
In the Canadian study, scientists found that bees were exposed to 26 different pesticides. Neonicotinoids were present far below dangerous levels, but the exposure continued for months. Surprisingly, the bulk of contaminated pollen didn’t come from crops treated with the pesticide but from wildflowers nearby. That adds to the evidence that neonicotinoids might move through the soil and get picked up by other plants.
Researchers fed the bees “pollen patties” laced with neonicotinoids at levels reflective of what they measured in the field.
Bees’ health suffered in several ways. Worker bees exposed as larva died younger, and treated colonies were more likely to be without a queen. Queen loss is normal during midsummer, but the exposed colonies were much less likely to rear a replacement queen.
Neonicotinoids also interfered with a behavior known as “social immunity.” Usually, bees remove their dead compatriots from the hive, but bee corpses stuck around longer in exposed colonies. Finally, neonicotinoids, when mixed with a certain fungicide, were more toxic than either alone.
Cutler, who was not part of either study, called these results “interesting” and “useful” but regretted that the researchers did not collect any “colony-level data,” such as fertility, honey production or winter survival rates, which might better reflect the resiliency of bee colonies.
Bee exposure to neonicotinoids has been a puzzle. The pesticides are chemically similar to nicotine, which evolved in plants as a natural defense against insect predators. Neonicotinoids are commonly used to treat seeds or soil before planting. They move through the plant providing allover defense throughout development. They are popular with farmers because they don’t have to be sprayed throughout the season. Theoretically, that should mean less exposure for bees than to a sprayed pesticide.
In practice, recent concerns have focused on the capacity for neonicotinoids to be kicked up in dust clouds during planting. Canada has mandated the use of agents that help keep neonicotinoids out of the air.
The new studies suggesting that neonicotinoids seem to persist in the soil may help explain how bees, which do not forage on wind-pollinated corn, could still be exposed near fields of treated corn seeds.
In terms of how neonicotinoids effect bees in the field, many questions remain. How far is far enough for a wildflower to be from a treated field without taking up neonicotinoids? And as Pywell wonders, “If the bees are healthier, will they be more resilient to stresses such as neonicotinoids?” Together, the two research groups looked at a handful of bee species. What of the 20,000 others that inhabit the planet? And perhaps the most difficult question, what should we do about it?
An approach popular with activists would be to ban neonicotinoids altogether, but many experts worry this would cause farmers to turn to older and potentially more harmful methods of pest control. “Things are better for honey bees since neonics replaced more harmful insecticides,” said beekeeper and science blogger Randy Oliver.
Since the E.U. moratorium went into effect in 2014, farmers in England have struggled with increased pest pressure, Carreck said. Many have turned to pyrethroid pesticides, which have unknown consequences on bees and other beneficial insects. Pywell emphasized that if the E.U. continues the moratorium, we need to investigate what alternative pesticides are doing to bees.
I asked Dave Walton, a family farmer in Iowa, what he would do if neonicotinoids were banned. “We would have to use other products. … Lorsban comes to mind.” Lorsban is a brand name for an equally controversial organophosphate insecticide, part of a class that is far more toxic to humans than neonicotinoids. They are also not more bee-friendly.
Amro Zayed, who was involved with the Canadian study, recommends restricting the use of insecticides until after a pest problem occurs, instead of preemptively applying them to every seed we plant. Oliver agreed. “Neonics are still vastly overused as risk management, rather than as needed treatments for actual pest control,” he said.
When I asked Walton whether this was a workable compromise, he seemed less optimistic. “We use it to combat insects that feed on the seed and seedlings,” he explained. “By the time we find the insects, the damage is already done.”
The buzzword in farming these days is what’s known as “integrative pest management.” In IPM systems, conventional pesticides are used responsibly, and pest control relies heavily on natural predators. One potential strategy Pywell discussed is limiting exposure by supplying other flowers near fields that could also serve as a refuge for pest predators. Walton explained that they employ a similar strategy on his farm, planting wildflowers and other native plants in ditches and field margins. However, given the evidence that neonicotinoids are more mobile and persistent than previously imagined, it may be important to monitor whether flowers in refuges are picking up neonicotinoids.
Wildflower refuges could also address an even greater threat to bees — habitat loss. Endless rows of wind-pollinated corn leave bees of all types scrambling for food. Likewise, urban sprawl and grassy lawns make for homeless, hungry pollinators. Many bee experts emphasize the importance of considering how changes to all landscapes could help or harm the pollinators we depend on.