"I made some calls and the indication was that there still is no specific threat," he said.
"But the FBI apparently ordered this and they just want to make sure that everyone in the agriculture aviation industry is keeping their eyes and ears open."
FAA spokesman Scott Brenner said: "The intelligence community came to us and encouraged us to shut down the crop dusters."
Defence Secretary Donald Rumsfeld outlined the potential threat on CBC's Face The Nation programme, saying that countries sponsoring terrorism had "very active chemical and biological warfare programmes".
"We know that they are in close contact with terrorist networks around the world," he said.
Crop dusters were previously grounded on September 16, and for the past week have been barred from flying over metropolitan areas, with some exceptions.
They had been grounded along with all civilian aircraft immediately after the attacks, with flights resuming on September 14.
Mr Callan said the US had about 3500 agricultural aviators, and this was a crucial time of the year for aerial spraying of crops.
Time magazine has reported that a grounding order was first issued after a manual on cropdusting was found among the belongings of Zacarias Moussaoui, one of the suspects in the attacks.
Moussaoui, a French Moroccan, went frequently to Afghanistan. French intelligence believes he trained there in camps run by bin Laden.
He lived in London for several years until he left for the US early this year to take flying lessons in Oklahoma and Minnesota.
But his instructors grew suspicious and tipped off the authorities.
Moussaoui was detained on August 17.
Map: Opposing forces in the war against terror
Afghanistan facts and links
Full coverage: Terror in America