A new pesticide may be as harmful to bees as the old one

Bees around the globe are in trouble, and many scientists worry that a common pesticide bears some blame. Now, scientists have shown that a chemical hailed as a potential replacement has similar harmful effects, reducing the number of new queens and males in bumble bee colonies. The finding could force farmers to seek alternative solutions to reduce crop damage from insect pests.

So-called neonicotinoid pesticides protect crops against pests such as aphids by blocking receptors in the insects’ brains—paralyzing and killing them. In small doses, the pesticides aren’t lethal to larger creatures, including mammals, birds, and even bees. But they can wreak havoc on bees’ abilities to navigate, find food, reproduce, and form new colonies. That kind of data convinced the European Union to ban the outdoor use of five neonicotinoid pesticides in April; Canada is phasing them out starting today, but they’re still used widely in the United States.

Some insects have developed resistance to neonicotinoids in recent years. In the search for an alternative, scientists hit on sulfoximine, a group of neonicotinoid-related chemicals that act on the same class of receptors in the insect brain but can dodge the enzymes that offer insects some resistance. But sulfoximine is starting to court the same controversy as its predecessor: Despite being approved for use in China, Canada, and Australia, a French court last year suspended licensing for two sulfoximine-containing products, citing environmental concerns including potential toxic effects on bees.

To investigate sulfoximine’s effects on bumble bee colonies, Harry Siviter, a graduate student in behavioral ecology at Royal Holloway University of London, and colleagues fed bumble bees sugar laced with sulfoxaflor, the first sulfoximine-based pesticide on the market. Dosing—one of the most controversial aspects of pesticide studies—was determined from U.S. Environmental Protection Agency data that measured the concentration of the pesticide in nectar collected by bees from cotton flowers sprayed with the chemical.

After 2 weeks, the researchers released their colonies of bees into the field. Between 2 and 3 weeks after exposure—the time it takes for bumble bee larvae to reach adulthood—colonies fed the pesticide produced fewer worker bees than control colonies that received only sugar. And after 9 weeks, exposed colonies produced 54% fewer new queens and males (the only bees that reproduce), they report today in Nature . That suggests, they write, that sulfoximine could have a significant impact on the reproductive success of bumble bee colonies.

“The body of evidence demonstrating [neonicotinoids’ negative effects] on pollinators … is now overwhelming,” says Edward Mitchell, an ecologist at the University of Neuchâtel in Switzerland, who was not involved in the research. “This study shows that we can expect the same for sulfoxaflor.”

But questions remain, such as how the exact timing of bee exposure and the kind of application—spraying crops versus coating their seeds—could alter the impact of the pesticide. Mitchell says he thinks the new results are a “conservative estimate” of the actual harm from the pesticide.

Policymakers will need more data before they can come to the same conclusion. But the new results could be included as part of licensing risk assessments in the European Union, which must take into account the risk to wild pollinators. “In terms of policy, we can’t know what the implications will be yet,” says social insect biologist Elli Leadbeater, a co-author at the Royal Holloway University of London. Mitchell says the new pesticide “should be treated like just another neonicotinoid, unless a strong case can be made that it does not pose the same environmental problems.”

But invertebrate ecologist Richard Gill of Imperial College London questions whether outright bans are the best solution. He suggests that improved scientific understanding of the risks could help develop strategies to minimize negative effects on pollinators, rather than forcing farmers to move to understudied replacement compounds.