[+]Enlarge Credit: Dara Stanley

Neonicotinoid pesticides have been blamed for declines in bee populations worldwide. The chemicals don’t kill bees, instead neonicotinoids impair the insects’ abilities to learn, navigate, forage for nectar, and reproduce, according to studies published over the past several years.

Now, researchers report that bees exposed to the pesticides also become less effective pollinators for crops (Nature 2015, DOI: 10.1038/nature16167).

The study is the first to demonstrate that neonicotinoids can decrease the quality of a food crop by affecting bee pollination, says Maj Rundlöf of Lund University, who was not involved in the work. “This is a strong indication that farmers, particularly those growing insect-pollinated crops, should consider both the gains and the costs of pesticide use when selecting a pest management strategy.”

About 30% of our food comes from crops, including fruits, nuts, seeds, and oils, that depend on insect pollinators, according to Dara A. Stanley of Royal Holloway, University of London, who led the new study. “Basically,” she says, “you can’t have a balanced diet without insect pollination.”

“Until now, all of the focus has been on the impact of neonicotinoids on bees themselves,” she continues. “But obviously the reason why we’re interested in bees is because they provide pollination services.”

To study the effect of the pesticides on bee pollination, Stanley and her colleagues monitored 24 bumblebee colonies that had dined on a nectarlike solution spiked with 10, 2.4, or 0 ppb thiamethoxam, a common neonicotinoid, for 13 days. They allowed each colony to visit four apple trees within an enclosure that prevented other insects from pollinating the flowers.

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Bee colonies exposed to the pesticide visited apple blossoms less frequently and carried back less pollen to their nests. When the researchers examined the trees’ fruits, they found that trees pollinated by colonies exposed to 10 ppb thiamethoxam yielded apples with 36% fewer seeds than those visited by the 2.4-ppb-exposed or unexposed bees. A lower seed count is a consequence of less pollination and is an indicator of lower quality fruit.

“This shows that exposure to pesticide can reduce the delivery of pollination services by bumblebees,” Stanley says.

Geraldine Wright of Newcastle University, who studies bees and neonicotinoids, says the experiment has one caveat: Feeding the bees the pesticide through a nectarlike solution in their nests is a slightly artificial way to expose the insects and doesn’t fully reproduce how they might encounter the chemicals in the field.

“Administrating the neonicotinoid to them in this way might expose them to a higher dose than they would get if they were just out foraging,” Wright says. “But it’s very difficult to control what the bees in the experiment are eating otherwise.”