Yan Wengui, left top, a geneticist, and Zhang Weiqiang, a rice breeder, have been accused of giving exclusive seeds to crop researchers from China. Seedlings being studied in Iowa.

KANSAS CITY, Missouri — The case of the missing corn seeds first broke in May 2011 when a manager at a DuPont research farm in Iowa no-ticed a man digging up the field. The man, Mo Hailong, who was with his colleague Wang Lei, told the manag-er that he worked for the University of Iowa . When the manager paused to answer his cellphone, the two men sped off in a car, federal au-thorities said.

What ensued was about a year of F.B.I. surveil-lance of Mr. Mo and his associates, all but one of whom worked for the Bei-jing Dabeinong Technolo-gy 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 research.

China has long been implicated in economic espionage efforts involv-ing aviation technology, paint formu-las and financial data.

But the case of Mr. Mo — who was arraigned recently in Des Moines, pleaded not guilty and remains in custody — and a separate one in Kansas last year suggest that the ag-riculture sector is becoming a great-er target, something that industry analysts fear could hurt the compet-itive advantage of farmers and big agriculture alike.

“Agriculture is an emerging trend that we’re seeing,” said Rob-ert Anderson Jr., assistant director of counterintelligence at the F.B.I., adding that the trend has developed internationally in the last two years.

The defendants in the Mo case visited seed testing fields in Iowa and Illinois that were used by the big agriculture companies Pioneer, Monsanto and LG Seeds, the authorities said. They bought a test plot of their own in Illinois, according to the complaint, and concealed stolen seeds in, among other things, microwave popcorn boxes and napkins from Subway restaurants.

The seeds that they were after are called inbreds, meaning they come from self-pollinating corn plants. Inbreds are eventually crossed with other inbreds to create hybrid seeds that are then sold to farmers, and they are bred to be durable in the face of drought and pests.

One inbred line takes five to eight years of research and can cost $30 million to $40 million to develop, fed-eral prosecutors said.

A company or farmer can replant a stolen inbred seed and use the new seeds to cross with a separate inbred to produce a hybrid — a shortcut that avoids years of costly research.

Mr. Mo’s lawyer denies that his client did anything wrong.

In the other seed case, Zhang Weiqiang, of Manhattan, Kansas, a rice breeder for Ventria Bioscience, a Colorado-based biopharmaceutical company, and Yan Wengui, of Stutt-gart, Arkansas , a research geneticist for the United States Agriculture De-partment, are accused of giving pro-prietary rice seeds to researchers in their native China.

Mr. Zhang and Mr. Yan were ar-rested in December.

Foreign vegetable seeds make up 80 percent of the Chinese market, said Guo Ming, a consultant spe-cializing in corn breeds for a Bei-jing-based agribusiness firm.

The Chinese have not developed a major corn hybrid since 2001 .

Analysts say one problem is the fragmented seed industry in China. That has fostered theft within the market, Ms. Guo said.

“Some seed trading companies just went to breeding bases to steal the seeds,” she said. “Some breed-ing companies would outsource breeding to farmers, but when the seeds were harvested, the farmers wouldn’t sell back to the breeding company because seed trading com-panies pay more.”

Those trading companies would then sell the seeds at a premium, Ms. Guo said , making a profit on a prod-uct that cost them nothing to develop.

She said, “That’s the ethos here.”