IF THEY can get the "bugs" out of their new inspection device, the U.S. Department of I Agriculture will soon be able to put the squeeze on contaminated fruit being brought illegally into this country by tourists. USDA's still-unnamed and unperfected weapon was recently revealed by the department as part of an effort to make international travelers more aware of the serious problem. And if the following scenario hits you even remotely close to home, the facts may save you embarrassment, time, and a possible fine.

The traveler had no clear intention of breaking the law--the mangoes in the market looked so tempting, he just decided they would brighten his breakfast table and provide a few tasty memories of warm Mexico after his return to chilly Washington.

So he bought three or four of the harmless-looking tropical fruit (more delectable than the variety he'd usually found in U.S. supermarkets), packed them carefully in his suitcase, and flew home from vacation to Dulles Airport. Perhaps he did not read the Customs declaration instructions carefully enough. Or perhaps he didn't think it would make much difference if he fudged a bit on the form. In any case, he failed again to admit he had the mangoes when given the opportunity to declare orally to Customs anything he had neglected to write down. Thus the Agriculture Dept. inspector was not alerted that he was carrying forbidden fruit.

After all, the traveler knew he had not exceeded his duty-free allowance on merchandise acquired abroad, and certainly was not trying to smuggle in anything of value (the mangoes cost only a handful of devalued pesos). Maybe he'd be allowed to pass through the gates without a Customs officer examining his baggage. Then he could enjoy those ripe mangoes . . .

The USDA's Animal and Plant Health Inspection Service (APHIS) has nothing against fruit lovers, but it considers thoughtless tourists like that one to be dangerous. So the agency is trying to perfect a device it believes will facilitate its job of keeping foreign plant and animal pests that can wreak havoc out of the United States, while at the same time helping speed up Customs processing. Within a year it hopes to field test its new detector at major international airports including Dulles and Baltimore/Washington.

The key to the detector is carbon dioxide. Fruits keep releasing CO2 long after they are picked, so when unusual amounts of the gas surround a suitcase, the sensitive device warns USDA inspectors there's potential trouble inside.

The instrument, which has been described as being about the size and shape of an old-fashioned breadbox, "gave the alarm on 25 suitcases in a recent test, and subsequent inspection showed that 24 contained fruit," said Charles Havens, staff officer at APHIS. There were enough oranges, bananas, grapefruit and -- yes -- mangoes "to start a surge of plant pest plagues . . . if they had been infested with foreign insects," USDA notes.

Such fruit can, of course, be commercially shipped into this country, but importers must follow strict rules. Bananas, incidentally, may be brought in legally by hungry tourists from anywhere in the Western hemisphere, though it is still necessary to declare the fruit at your port of entry because Customs is authorized to enforce USDA regulations to protect U.S. crops against pests. And even a banana might, because of the way it looks, cause a Customs agent to send for a plant inspector just to be on the safe side.

Prohibited fruits accounted for roughly two-thirds of the more than 700,000 plant products seized at U.S. international airports in fiscal 1981. Government figures show that quarantine officials sifted from that contraband some 133,000 insects, nematodes, snails and identified assorted plant diseases. Unfortunately, illegal material still slips by inspectors, and USDA warns that even one piece of prohibited fruit could be "a pest or disease time bomb."

The Mediterranean fruit fly which recently invaded California was proof of just how expensive plant pest infestation can be. When finally declared eradicated last month, the Medfly had cost federal and state governments nearly $100 million.

When the USDA's fruit detector has been reduced to a small hand-held unit and is ready for use, the threat of infestation "should drop sharply," officials predict, but they admit the instrument still presents a problem. Specifically, there's the matter of false alerts. It seems that other things associated with suitcases release high levels of carbon dioxide. Like dirty clothes.

International travelers should not risk a possible delay in Customs processing and the confiscation of their prohibited fruit, plant or animal product purchases by an inspector -- or take the chance of causing a costly infestation. Fines can be, and have been, levied under Customs law for "failure to declare" such items, though USDA says that civil penalties are usually levied only when there has been "a pretty blatant attempt" to deny and conceal. Some travelers "make a game of it," so USDA hopes the Senate will act favorably on pending legislation to give Agriculture authority to levy its own fines.

When in doubt (sometimes only an inspector knows whether an item can be legally taken home with you), don't pack plants or fruit.