KANSAS CITY, Mo. — The case of the missing corn seeds first broke in May 2011 when a manager at a DuPont research farm in east-central Iowa noticed a man on his knees, digging up the field. When confronted, the man, Mo Hailong, who was with his colleague Wang Lei, appeared flushed. Mr. Mo told the manager that he worked for the University of Iowa and was traveling to a conference nearby. When the manager paused to answered his cellphone, the two men sped off in a car, racing through a ditch to get away, federal authorities said.

What ensued was about a year of F.B.I. surveillance of Mr. Mo and his associates, all but one of whom worked for the Beijing Dabeinong Technology Group or its subsidiary Kings Nower Seed. It resulted in the arrest of Mr. Mo last December and the indictment of five other Chinese citizens on charges of stealing trade secrets in what the authorities and agriculture experts have called an unusual and brazen scheme to undercut expensive, time-consuming research.

China has long been implicated in economic espionage efforts involving aviation technology, paint formulas and financial data. Chinese knockoffs of fashion accessories have long held a place in the mainstream. But the case of Mr. Mo — who was arraigned last week in Des Moines, pleaded not guilty and remains in custody — and a separate one in Kansas last year suggest that the agriculture sector is becoming a greater target, something that industry analysts fear could hurt the competitive advantage of farmers and big agriculture alike.

“Agriculture is an emerging trend that we’re seeing,” said Robert Anderson Jr., assistant director of counterintelligence at the F.B.I., adding that the trend has developed internationally in the last two years. “It’s pretty clear cut. Before then, the majority of the countries and hostile intelligence services within those countries were stealing the other stuff.”