TUCSON, Ariz. (AP) _ The holidays are prime time for the nation's retailers, but they're also high season for the storeowners' nemesis, the shoplifter, industry officials say.

''Thirty-five percent of store losses happen from Thanksgiving to Jan. 15,'' said Glenn Ricker, a former Sears, Roebuck and Co. loss prevention security manager and now a district manager of investigations for Pinkerton, the nation's oldest private investigative and security agency.

Shoplifting is a $40 billion a year scourge for retailers - and it is becoming increasingly lucrative for criminals such as burglars, said Lt. James Hughes, head of the Tucson Police Department's major offenders unit and burglary task force.

''It's newer merchandise, easier than breaking into homes and it has better resale value when the burglars take it to their fence,'' Hughes said.

Uniform crime report statistics compiled by the FBI, based on information from 12,019 law enforcement agencies reflecting a population of 211.6 million Americans, showed 982,552 shoplifting offenses in 1988, 3.9 percent more than the 945,671 reported in 1988. The average value of stolen property was $104.

There was a 35 percent upswing in shoplifting between 1984 and 1988, the FBI said.

According to Hughes, for every shoplifting incident reported, another three to four go unreported.

The National Retail Merchants Association said a study shows ''stock shortages'' - largely a combination of shoplifting and employee theft - account for nearly 2 percent of net retail sales in department stores.

The spectrum of shoplifters cuts across age, class and gender lines, and their methods vary widely, said Ricker.

''I've caught 5-year-olds and I've caught 75-year-old ladies, I've caught guys in wheelchairs,'' he said. ''You don't find too many 70- or 80-year-olds breaking into houses or cars, but trust me, you find 70-and 80-year-olds shoplifting.''

Among shoplifters' schemes:

-Working in teams, allowing one person to distract a clerk while the other steals merchandise.

-Sneaking empty store bags inside the store and filling them with goods.

-Detaching electronic alarm tags from merchandise.

-Loading items into false-bottom bags, coats or loose-fitting clothing.

-Razor-cutting the top off a cereal box, emptying the contents and filling the box with more expensive items, such as cigarettes, then retaping the top and going through the checkout line.

An item's size doesn't deter shoplifters. Ricker said he caught one man who tried to steal a chain-saw.

Ricker estimated that 75 to 80 percent of all such crime is drug-related.

Attempts to combat the problem are almost like ''trying to dam a river with sand,'' he said.

Many retailers are turning increasingly to security firms for help.

''We're getting more and more calls from retail sales'' asking both for uniform security and for undercover investigative services, said Ronald Janick, Pinkerton senior vice president of investigations in suburban Philadelphia.

Said Ricker: ''A lot of corporations now are going to contract loss prevention agents rather than their own in-house (staff) because it's a lot more cost-effective.''

Agencies such as Pinkerton take on responsibility for hiring, screening and training agents, insurance, turnover and paperwork.

Pinkerton also supplies undercover operatives who blend in with regular shoppers and the shoplifters.

One such is Dianne Ruhl, a private investigator who previously worked as a security agent in department stores in Tucson.

Ms. Ruhl said suspicious or unusual behavior is a tipoff to a trained observer.

Frequently, those caught shoplifting ask if they can just pay for the items. But, increasingly, merchants are prosecuting.

Ms. Ruhl said she recently caught a 25-year-old unemployed man who had stuffed two packs of Camels between a liner and the outside part of his coat.

''And so he's got a record now for $2.70,'' she said. ''Isn't that stupid?''