A horrified Agriculture Department said yesterday that mailing mangoes from Hawaii to the mainland is illegal, regardless of what a U.S. Postal Service advertisement suggested in the July issue of Reader's Digest.

Mangoes, as well as other fruit, could spread insect pests, including the devastating Mediterranean fruit fly, officials said.

Advised of the department's objection, a spokesman for the Postal Service said the advertisements would cease. There was only one, but it was enough to trigger official USDA objections.

The Digest ad was headed, "How to Pick a Package Deal," and was illustrated with a postal card displaying the message: "Hawaii great! Did you get the 200 mangoes?"

Assistant Agriculture Secretary C.W. McMillan, who oversees insect control for the department, said it is illegal to carry or mail mangoes or many other kinds of fruit to the mainland from Hawaii because of the danger of transporting unwanted insects.

"We're concerned that the impression created in this advertisement could result in costly infestations of fruit flies or other harmful agricultural pests," McMillan said.

"Eradicating the Mediterranean fruit fly from California in the early 1980s cost American taxpayers more than $100 million. We believed this outbreak originated from infested fruit brought in or mailed from Hawaii."