Syngenta, in a statement last month, called on the European Commission to retract its proposal. The food safety authority’s study, Syngenta said, was “fundamentally flawed” because it was based “on unrealistic and excessive seed planting rates between two and four times higher than would be used under modern agricultural practice.”

Whatever might be killing off the bees, there is no doubt about the lethality for insects of neonicotinoids, which are also used in forestry and tree nurseries — even in flea collars for pets. Once plants or seeds are treated with the chemical, it permeates their tissues; it then attacks the nervous system of any bug feeding on a leaf or root, or a bee that collects nectar or pollen.

Even if a bee does not receive enough of the poison to kill it, there is a fear that “sublethal” doses it carries back to the hive may be weakening already-stressed colonies. Two studies reported last year in the journal Science suggested that low levels of neonicotinoids from a common pesticide can have significant effects on bee colonies.

But pesticides are just one of several threats to bees, along with mites and viruses, environmental changes and poor nutrition. Those factors may work individually or in combination to weaken colonies and kill bees.

Paul de Zylva, an environmental campaigner in London with Friends of the Earth, said that while the pesticide makers “adamantly deny” that their products are affecting bee health, the range of the testing they had carried out was too narrow to be conclusive. Those studies, he said, looked only at possible effects on honeybees and not the effects that “sublethal” doses might be having in the hive, including on larvae.

He noted that in its January report, the European Food Safety Authority said it was unable to reach any conclusions in a number of areas “due to shortcomings in the available data.”

“How can they know if it’s safe?” Mr. de Zylva asked.

Noa Simon, a veterinary scientist who acts as a technical adviser to the European Beekeeping Coordination, an industry group, argues that the chemicals should be banned for at least five years to provide time for further studies.