* Government says desertions damaging rebels ranks
* Rebels deny exodus as peace talks continue
* Deserters risk retaliation from ex-comrades
By Luis Jaime Acosta
BOGOTA, Nov 18 (Reuters) - Alfredo Usuga put up with 18 years of hunger, insomnia and fear before deserting Colombia's Marxist FARC rebels this year.
Known by his war alias as Marlon, he is one of hundreds of rebels who abandoned the country's two leftist insurgent groups in recent years, an exodus that may have encouraged rebel leaders to seek a negotiated peace with the government.
Some 1,140 combatants from the FARC, or Revolutionary Armed Forces of Colombia, and the smaller National Liberation Army, known as the ELN, demobilized in 2012, the government says, three times the number killed in combat.
Peace talks - which turn 1 year old on Tuesday - are a huge risk for President Juan Manuel Santos and his legacy but could end 50 years of conflict that has claimed more than 200,000 lives, mostly civilian.
"Every noise from an airplane makes you run," said Usuga, a heavy-set 31-year-old with short cropped hair.
"You feel surrounded. It's scary, very scary. That's why I left the rebels," he said in an interview with Reuters, organized by the government at a secret location.
Deserting is a big risk. Those suspected of planning an escape are often killed by their comrades.
Defense Minister Juan Carlos Pinzon believes desertions will climb as combatants cease to see a future in insurgency, eroding rebel ranks and potentially hastening the end of the conflict.
"Demobilization is growing this year," said Pinzon during a recent interview with Reuters. "It's a humanitarian attitude. It saves lives. They can repent."
The FARC says desertion talk is part of the government's psychological warfare. Rebel leaders deny the ranks have dwindled and say desertion is part of every war.
"In every armed conflict, especially one as long as Colombia's there are always things like this," rebel negotiator Andres Paris told Reuters in Havana. "But there are also recruits who compensate for this phenomenon.
"What is being presented is a staged film, made to portray a non-existent collapse in the armed struggle."
FAREWELL TO ARMS
Usuga's tale is familiar within the FARC. Recruited by force from an impoverished rural area when he was 12 years old, he spent two-thirds of his life fighting the government in the jungle.
By the time he fled, Usuga controlled operations along the porous Panama border, an area the government says is used to smuggle cocaine to finance the FARC's operations.
Encouraging rebels like Usuga to demobilize is part of a government strategy to demoralize the guerrillas, even as it maintains military pressure on the FARC.
A reinsertion program offers job training, a monthly stipend and amnesty for minor crimes like firearms possession.
"It damages the morale of those who stay with the group and spurs other demobilizations," said Alejandro Eder, director of the Colombian Agency for Reintegration, the government agency known as ACR.
At its height in the 1990s, the FARC had about 20,000 fighters and roamed almost freely over 50 percent of Colombia's territory. But U.S.-backed military action during the last decade and the killing of leaders have cut the fighting force to under 8,000 and shrunk its territorial presence to around 9 percent of the country.
Some 20,797 rebels have fled in the last decade, more than 80 percent from the FARC. The ELN has about 1,500 fighters, according to government figures.
The FARC makes up for some desertions through recruitment - forced or voluntary - of youths from rural areas, where 46 percent of the population lives in poverty, according to the government.
PRICE OF PEACE
The demobilization program receives about $95 million a year, including funds for radio ads which penetrate even the dense jungle and reach hidden guerrilla camps.
Reintegration of one rebel costs about $21,000 over the average seven years that they typically stay on the program.
"Jailing a prisoner costs more than double reintegration," said the ACR's Eder. "And the recidivism rate is much higher."
It was a drug deal that finally convinced Medardo Maturana, alias Tomas, to leave the FARC after 23 years. He objected to sharing a trafficking route with a drug cartel and was afraid his insubordination would get him shot.
Maturana contacted the nearest military unit by cell phone and then he walked 12 hours through the jungle with his rebel girlfriend, who rode a mule, he told Reuters at another interview organized by the government.
"I'd been planning it for a month," said Maturana, 53, who spoke to a reporter in the presence of two agents who form part of his 24-hour state security detail.
Details of the deserters' accounts of their experiences could not be independently confirmed by Reuters.
If leaving the rebels is difficult, returning to civilian life is harder. After a life in the jungle, many live in fear of retaliation by their ex-comrades.
In spite of all the challenges and risks, Usuga thinks many more of his ex-comrades will abandon the ranks and start anew.
"We will not be the last. There will be many more," he said.
"This is the best way out, there they will have no future." (Reporting by Luis Jaime Acosta, Additional reporting by Nelson Acosta in Havana, Writing by Julia Symmes Cobb; Editing by Peter Murphy, Helen Murphy and Cynthia Osterman)