From time to time, despite the best efforts to close U.S. borders to agricultural pests, dangerous new ones sneak into the country, spreading alarm and worry among entomologists and farmers.

The most recent of these is a mite that attacks honeybees. It is the most serious threat to honeybees since they were first brought to this country 300 years ago, according to Roger A. Morse, a Cornell University entomologist studying the mite outbreak.

The infestation has the potential to devastate the nation's $130 million honey crop and hamper the pollination of a number of agricultural crops as well, at least until control measures can be taken. Farmers depend on bees, and sometimes even bring them into an area, to assure that some crops are pollinated.

The mite attacking the bees is about the size of a pinhead. It attaches itself to the underside of the bees, hanging on with its eight legs and injecting its sharp, two-pronged tongue to suck out the bee's body fluids.

It is a parasite that can live in a beehive for a year or more with no apparent ill effects. Eventually, a mite population explosion occurs, and bees can be attacked by several mites at once, weakening or killing them.

An infestation of these Asian mites occurred in Europe several years ago, cutting honey production in half. Now, chemical treatments are keeping the infestation there under control.

Most of the chemicals the Europeans used, however, are not permitted in the United States. One called Fluvalinate was approved on a temporary basis only three weeks ago, said Richard Nowogrodzki, a Cornell entomologist.

He said it is unclear what impact the mites will have on the nation's honey production. At the least, they will cause a significant loss to beekeepers who must spend money for chemicals to control the mites.

The chief beneficiary of the infestation may be the Africanized honeybees, also called killer bees because of their aggressive behavior when disturbed.

The killer bees, now in Mexico and expected to be in Texas within a year, are highly resistant to attacks by the mites. Their colonies are also infested, but entomologists report that the killer bee colonies remain unharmed by their presence, while other honeybee colonies are wiped out.

The mites are spread from one bee colony to another by bees that become lost when returning to the hive from flowers. They commonly end up in another colony, bringing the mites with them.

Beehives transported from state to state to help pollinate crops are also vehicles for the spread of the mite.