Can bees become addicted to pesticides?

Two studies signal that neonicotinoids, with repeated exposure, can be detrimental to bees who can't stop themselves from going back for more.

Flitting from blossom to blossom, bees represent an ecological lifeline from one generation of plants to the next – paid in nectar and pollen to keep the reproductive ball rolling on farms, in woods, and in backyard gardens.

But since 2006, concerns have grown over a decline in bee colonies worldwide. One of several potential suspects researchers have identified: a class of pesticides known as neonicotinoids. Formulated to be less harmful to mammals than its predecessors, neonicotinoids spread throughout a plant to boost its resistance to insects.

But the pesticide also finds its way into nectar and pollen, and therein lies the rub. A growing body of research has identified several ways neonicotinoids can harm wild and domesticated varieties of bees. But the results sometimes have encountered criticism, especially from pesticide makers, for whom neonicotinoids, by some estimates, represent more than 20 percent of their market.

Now, two teams, working independently, have published results designed to fill in two of the gaps that critics have identified in previous studies: that lab studies have relied on doses higher than would occur in the field and that few field studies focused on the compound's potential impact on wild bees.

Taken together, the two studies point to a compound that, with repeated exposure, can be detrimental to bees who can't stop themselves from going back for more.

With each visit, "they're getting their little buzz, as it were," said Geraldine Wright, a researcher at Newcastle University's Institute of Neuroscience in Britain who led a team performing one of the two studies.

Specifically, the findings indicate that:

• Key bee species apparently cannot "taste" neonicotinoids, at least at low concentrations found in crops, and so avoid them. Yet the bees clearly prefer nectar-like solutions containing the compounds, even though they eat less when they ingest them. Researchers posit that for bees, neonicotinoids may be addictive in the same way nicotine is addictive to humans.

• At concentrations found in crops, neonicotinoids have a more significant effect on wild bees than on honey bees. The density of wild bees – bumble bees and solitary bees – was about 50 percent lower on fields with treated crops compared with untreated. Fields with treated crops saw lower nesting rates for solitary bees. And bumble-bee reproduction and colony growth were lower than in treated fields. 

• Treated crops seemed to have no effect on honey-bee colony levels, at least to within the measurement limits of the experiments.

This find regarding honey bees dovetails with the results of a three-year study published in March by scientists from the University of Maryland, the US Environmental Protection Agency, and the US Department of Agriculture.

Noting that neonicotinoids have been identified as a key suspect behind the persistent loss of honey-bee colonies and wild pollinators globally during the past nine years, the team set out to gauge the impact of an insecticide known as imidacloprid, a neonicotinoid, on honey-bee colonies.

They found that relatively large doses could reduce the health in colonies and undercut a colony's ability to rejuvenate itself after winter ends. But in smaller doses, such as those applied to seeds in advance of planting, the team found negligible effects on colonies.

When that apparent lack of impact on honey bees is weighed against the effects on wild bees, however, the contrast is disturbing, says Maj Rundlöf, an ecologist at Lund University in Lund, Sweden, and the lead author of the second of the two latest studies.

One reason? "Honey bees are used as model organisms when we want to test the environmental impact of pesticides," she pointed out during a briefing on the studies held Tuesday. The studies were published in today's issue of the journal Nature.

A model organism isn't much of a model if it fails to predict the effects on the more diverse pollinator population that people are trying to protect, she said.

Indeed, while honey bees commonly are drafted to pollinate crops, recent research suggests that wild bees play a more important role in that process than previously thought, notes Aimee Code, pesticide-program coordinator for the Xerces Society, an invertebrate-conservation group based in Portland, Ore.

For instance, researchers at the University of Wisconsin have found that for alfalfa, wild bees – particularly solitary species – trigger pollination in some 80 percent of the tiny flowering stalks per stalk cluster, or raceme, visited compared with 25 percent for honey bees or leaf-cutter bees usually used to pollinate crops.

The first of the two new studies aimed to tackle one of the criticisms of past experiments implicating neonicotinoids to declines in bee colonies: doses of the compound were unrealistically high compared with those found down on the farm or even in the backyard.

Led by Dr. Wright, the team set up feeding boxes in which honey bees and buff-tailed bumble bees could pick from one of two food sources: a sugary liquid and a sugary liquid laced with three of the most widely used neonicotinoid insecticides. Importantly, the concentrations included levels reported from field studies of the insecticides' presence in plants.

Both species developed a preference for the same two of the three insecticide types, imidacloprid and thiamethoxam, even at field concentrations. And studies of nerves related to the bees' feeding apparatus showed that they do not respond to the insecticides at field levels. This indicates that the bees do not sense the compound as a poison, Wright explained.

Bees are adept at returning to favored feeding spots, even when they can't see or smell their surroundings, Wright noted. Even in the absence of these cues, the bees found their way to the food that gave them a buzz. Her team's results suggest that neonicotinoids trigger responses in the bees' brains that signal a reward.

"They didn't want to eat as much, but they still found it rewarding, probably because they couldn't taste it," Wright said.

The second team, led by Dr. Rundlöf, aimed to address a relative lack of field studies on the effects of neonicontinoids on bees in the wild.

The researchers selected 16 fields in various locations in southern Sweden. In eight fields, farmers grew oilseed rape that had germinated from seeds coated with Clothianidin, another neonicotinoid pesticide. These were paired with eight other fields where farmers grew their rape from untreated seeds. The scientists then tracked bee densities and colony activity for each field. The researchers who visited the sites to gather data weren't told which fields hosted the treated rape and which contained the untreated variety.

Once the data were gathered and analyzed, the team found that colony activity among European honey bees in the treated fields was essentially the same as in the untreated fields. But significant gaps appeared between wild bees – bumble bees and solitary bees – in treated and untreated fields when they were compared.

For instance, one gauge of how reproductively busy bumble bees is the changing weight of a colony. In the untreated fields, the colonies followed a typical pattern over a 40-day period of starting out light, getting heavier, then lightening up again. In the fields with oilseed rape that germinated from pesticide-coated seeds, the weight of bumble-bee colonies remained essentially flat throughout the same 40-day period.

As for an apparent lack of effect on honey bees, their colonies have thousands to tens of thousands of worker bees versus tens to hundreds of workers among bumble bees. For honey bees, it's easier to hide any losses, Rundlöf suggests.

The studies come at a time when the European Union has placed a moratorium on using neonicotinoids on seeds for crops that attract bees. The EU is set to review the moratorium by December, taking into account research that has been undertaken since the moratorium was adopted in 2013.

In the US, the Environmental Protection Agency also is looking more closely at the  issue. Earlier this month the agency announced that it was unlikely to approve new outdoor uses of neonicotinoids until a new round of bee-safety analyses are completed.

In January, the agency approved the use of a new replacement for neonicotinoids.

Ms. Code and others say that neonicotinoids have their place, but their use needs to be restricted to crops that truly need the kind of protection neonicotinoids may have to offer.

The debate over neonicotinoids highlights a need for more diverse pest-management strategies, note Nigel Raine and Richard Gill, ecologists who published a commentary on the two new studies in Nature magazine. 

The challenge with new substitutes is that they may "prove to be even more harmful to insect pollinators and the essential ecosystem services that they provide," they write.