SAN DIEGO – They fly low and slow over the border, their wings painted black and motors humming faintly under moonlit skies. The pilots, some armed in the open cockpits, steer the horizontal control bar with one hand and pull a latch with the other, releasing 250-pound payloads that land with a thud, leaving only craters as evidence of another successful smuggling run.

Mexican organized crime groups, increasingly stymied by stepped-up enforcement on land, have dug tunnels and captained boats to get drugs across the U.S.-Mexico border. Now they are taking to the skies, using ultralight aircraft that resemble motorized hang gliders to drop marijuana bundles in agricultural fields and desert scrub across the Southwest border.

What began with a few flights in Arizona in 2008 is now common from Texas to California’s Imperial Valley and, mostly recently, San Diego, where at least two ultralights suspected of carrying drugs have been detected flying over Interstate 8, according to U.S. border authorities.

The number of incursions by ultralights reached 228 in the last federal fiscal year ending Sept. 30, almost double from the previous year. Seventy-one have been detected in this fiscal year through April, according to border authorities.

Flying at night with lights out, and zipping back across the border in minutes, ultralight aircraft sightings are rare, but often dramatic. At least two have been chased out of Arizona skies by Customs and Border Protection Black Hawk helicopters and F-16 jet fighters. Last month, a pair of visiting British helicopter pilots almost crashed into an ultralight during training exercises over California’s Imperial Valley.

The smuggling work is fraught with danger. High winds can flip the light aircraft. Moonlight provides illumination, but some pilots wear night-vision goggles. Others fly over major roads to orient themselves. Drop zones are illuminated by ground crews using strobe lights or glow sticks. There is little room for error.

At least one pilot has been paralyzed; another died in a crash.

In Calexico, Calif., Detective Mario Salinas was walking to his car one morning last year when he heard something buzzing over the Police Department. “I hear this weird noise, like a lawn mower. I look up and I see this small plane,” said Salinas, who pursued the aircraft before it eluded him as it flew over the desert.

The ultralight activity is seen as strong evidence that smugglers are having an increasingly difficult time getting marijuana over land crossings. Authorities noticed a surge in flights in Imperial County after newly erected fencing along California’s southeast corner blocked smugglers from crossing desert dunes in all-terrain vehicles.

U.S. Border Patrol agents, accustomed to scouring for footprints and tracks in the sand, have had to adapt. They are now instructed to turn off their engines and roll down their windows so they can listen for incursions by air.

“We’re trained to look down and at the fence. Now we have to look up for tell-tale signs of ultralight traffic,” said Roy D. Villarreal, deputy chief patrol agent of the El Centro sector in the Imperial Valley.

Although the new trend poses serious challenges, authorities point out that ultralights are a decidedly inefficient way of getting drugs across the border. Traffickers who once moved thousands of pounds of drugs across the border now appear to be packing their loads by the pound, not the ton, authorities say.

The ultralights — lightweight planes typically used as recreational aircraft — are customized for smuggling purposes. All-terrain wheels are added for bumpy landings. Second seats are ripped out to add fuel capacity. Drugs are loaded onto metal baskets affixed to the bottom of the framing. From 150 to 250 pounds of marijuana are generally carried, depending on the weight of the pilot. Some ultralights are shrouded in black paint, with even the plastic tarp covers for the marijuana blackened for stealth entries.

Radar operators at Riverside County’s Air and Marine Operations Center, where general aviation air traffic across the country is monitored, have trouble detecting the aircraft.

Flying as low as 500 feet, their small frames are hard to distinguish from trucks. Many appear, then disappear from radar screens. Others never appear at all, and the ultralight trend has prompted border authorities to develop new radar technologies specifically designed to detect the aircraft.

“There are indications of larger amounts of activity,” said Tony Crowder, director of the Air and Marine Operations Center, which is housed at March Air Reserve Base.

The close cooperation among radar operators, helicopter pilots and agents on the ground has resulted in some successes.

Ultralight pilots no longer land on U.S. soil after authorities began responding quickly to offloading sites. The Mexican Army has seized four ultralights around Baja California in recent weeks after being tipped off by U.S. authorities.

In Arizona, where the vast majority of the flights occur, authorities have arrested 36 people in connection with ultralight smuggling, most of them ground crew members who load the dropped marijuana into cars.

The trend has grown so rapidly in sparsely populated areas of Arizona that Rep. Gabrielle Giffords, D-Ariz., introduced a bill last year to stiffen prison terms for ultralight smugglers.

Recruiting pilots is more difficult than finding truck operators, drivers or backpackers to ferry drugs across the border. Some of the pilots are believed to come from Mexican coastal resorts where they work day jobs taking tourists on lessons or adventure flights.

A pilot who was paralyzed in 2008 after he clipped power lines near Tucson and crashed ended up being deported to Mexico in a wheelchair. Another pilot died after only one side of his load released, causing the plane to destabilize and spiral into a lettuce field near Yuma, Ariz. In June, another trafficker encountered a hovering Black Hawk helicopter that raced to intercept him over the Tohono O’odom Indian reservation in Arizona.

The pilot, Jose Alberto Figueroa Valenzuela, 37, got a 41/2 year prison term after making a rough landing, said Rodney Irby, assistant special agent in charge of Immigration and Customs Enforcement in Arizona.

“He took all these risks, got chased by a Black Hawk helicopter and ended up crashing for 2,000 bucks that he never got,” Irby said.