A United States appeals court ruled on Thursday that federal regulators erred in allowing an insecticide developed by Dow AgroSciences onto the market, canceling its approval and giving environmentalists a major victory.

The ruling by the United States Court of Appeals for the Ninth Circuit, in San Francisco, is significant for commercial beekeepers and others who say a decline in bee colonies needed to pollinate key food crops is tied to the widespread use of a class of insecticides known as neonicotinoids.

The lawsuit was filed in 2013 against the Environmental Protection Agency by a number of organizations representing the honey and beekeeping industries. The groups specifically challenged the E.P.A. approval of insecticides containing sulfoxaflor, saying studies have shown they are highly toxic to honeybees. Sulfoxaflor is a neonicotinoid subclass, according to the ruling.

Dow AgroSciences, a unit of Dow Chemical, first sought approval for sulfoxaflor in 2010 for use in three different products. Brand names include Transform and Closer.

“It’s a complete victory for the beekeepers we represent,” said Greg Loarie, a lawyer for the American Honey Producers Association, the American Beekeeping Federation and other plaintiffs in the case. “The E.P.A. has not been very vigilant.”

Dow said in a statement that it “respectfully disagrees” with the ruling and will “work with E.P.A. to implement the order and to promptly complete additional regulatory work to support the registration of the products.” The agency said it was reviewing the decision and would have no further comment.