LORENZO, Texas—West Texas cotton producer Mark Schoepf never dreamed the price of farm diesel would skyrocket so.

But now that it has climbed above $4.15 a gallon, Schoepf decided he needed to protect his investment. The Lorenzo resident recently bought 10 padlocks to afix to tanks that fuel the diesel motors irrigating his fields.

Many of those tanks are visible from a highway bordering some of his fields, making them easy targets.

“Before, we’ve never kept them locked,” said Schoepf, who is able to store up to 5,000 gallons to fuel his farming operation east of Lubbock. “With diesel prices that high, somebody’s going to try to get it.”

Schoepf isn’t alone. As more motorists and truckers buy locking gas caps for cars and other vehicles, farmers and ranchers across the country are realizing diesel fuel is a target thieves are increasingly eyeing.

The cost of farm diesel is less than the diesel used by truckers and the general public and thieves either sell it or use it themselves.

Thieves use syphon hoses and pumps, the latter sometimes built into vans that roam wide-open fields at night to steal diesel.

“If they’re out there in the middle of the night, they’re up to no good,” said Steve Riley, who works at Crosby County Fuel Association. “It’s going to get worse.”

Sheldon Wilder, who owns a cotton gin 30 miles east of Memphis, Tenn., has endured worse already. Twice in two weeks, he had diesel drained from saddlebag tanks on a truck at his gin. The second time the thieves left the hoses loose and what diesel they didn’t steal drained out onto the ground.

“You get irritated, but that’s life,” he said of the combined 300 gallons taken in the thefts. “It’s just people who want some money.”

Wilder said there is a quasi black market for diesel that tempts some looking to save a buck or two.

“You’d probably buy it because it’d be a good deal,” he said. “A lot of people are hurting. Their purchasing power has been eaten up by the price of gas and food.”

Some farmers have had their entire tanks stolen and later found them elsewhere — empty.

Agricultural users pay about 60 cents per gallon less.

Red-dye diesel can be used in agricultural equipment, such as tractors or combines, and on farms or ranches, which includes feedlots, dairy, poultry and timber operations, and commercial orchards and nurseries. A home garden does not qualify.

Diesel designated for non-highway purposes contains a red dye that distinguishes it from regular diesel fuel. Using it for unauthorized purposes is a federal crime.

The IRS does periodic and random checks for illegal use of the red-dyed diesel at weigh stations across the country, IRS spokesman Clay Sanford said.

So has the higher price spurred more enforcement?

“I can’t confirm or deny we’ve stepped up (checks), but I would definitely hope that everyone is complying with the law,” he said.

The federal penalty for using the off-highway diesel on public roadways is the greater of $1,000 or $10 per gallon of the fuel involved. After the first violation, the $1,000 portion of the penalty, which is additional to any tax imposed on the fuel, increases depending on the number of violations.

In Texas, enforcement is handled by the state comptroller’s office, which also does random checks at weigh stations. If caught, violators face Class C misdemeanor charges, comptroller spokesman Allen Spelce said. From Sept. 1 through May, the state collected about $41,000 in fines, he said.