The gypsy moth is back, as impudent and gluttonous as ever.

And hot on its trail is the Utah Department of Agriculture and Food, which plans to wipe out the insects by spraying two Salt Lake neighborhoods with a bacteria known as "Bacillus thuringiensis var. kurstaki."And that's enough to make your average paranoid reporter nervous.

You know the kind of reporter we mean: one who grew up on Rachel Carson's "Silent Spring" and this year was blown away by "Living Downstream: An Ecologist Looks at Cancer and the Environment"; the kind of reporter who hears "aerial spraying" and thinks of Agent Orange, who hears "microbial pesticide" and thinks of Saddam Hussein.

Of course the UDAF has sprayed for gypsy moths before in the Salt Lake Valley. But is Bacillus thuringiensis var. kurstaki really safe for humans, the paranoid reporter wonders.

"It's quite innocuous," says Michael Heumann, an epidemiologist with the Oregon State Health Division. Heumann is an author of one of the few scientific studies to look at the effect of Bacillus thuringiensis on public health.

Heumann and his colleagues conducted a study of the bacteria in Lane County, Ore., during the spraying seasons of 1985 and 1986. As part of the study, the four largest clinical laboratories in the area agreed to test all routine culture specimens to see if they contained bacillus.

Their findings: 55 people tested positive for Bacillus thuringiensis; in 52 of these there was no evidence that the bacillus was the cause of the disease that prompted the doctor to take a culture. The three remaining cases were a little more suspicious, but even in these, pathologists could not say for certain that the bacillus actually caused the disease in question.

Still, bacillus thuringiensis is an "opportunist," Heumann and his colleagues noted in a 1990 article in the American Journal of Public Health. It's possible, they wrote, that the bacteria may have exacerbated the existing disease. It's also possible, they cautioned, "that b.t.-caused disease may have been missed."

And, they added, "these microorganisms may have potential for causing disease in immuno-compromised persons " - people with AIDS, those on certain chemotherapeutic drugs, and people undergoing transplants. There have been no documented cases involving such people, says Heumann, but "we think prudent avoidance is the smart way to go."

Heumann recommends "prudent avoidance" even for people with healthy immune systems. Stay inside during spraying. If you happen to get caught outside, Heumann recommends taking a shower "just as a general safety precaution." But, he adds, "you're not at greater risk if you don't wash."

Unlike many insecticides, Bacillus thuringiensis is not a contact poison. Although it has been known to linger for days or even weeks on human bodies - in, say, the warmth and safety of a nasal passage - "it doesn't seem to cause any problems," says Heumann.

To be effective, Bacillus thuringiensis must be ingested and then broken down in the gut, explains Phyllis Martin, a research microbiologist with the U.S. Department of Agriculture's insect biocontrol lab in Beltsville, Md. In a human gut - which is acidic - that won't happen. But gypsy moths have alkaline guts and also have enzymes that further break down the bacillus into a toxin.

Are there any long-term effects of exposure to bacillus thuringiensis? "I'm going to say probably not," says Martin.

The only proven harm from the bacterial spraying is minor skin rashes, says Heumann. "But it's much less significant than the rash you can get from the caterpillar itself when it's molting."

Occasionally, people who do the spraying develop allergies, but it's hard to tell what has caused the allergy - the bacillus itself or the detergent used as a surfactant to make it stick to the leaves, he says.

The bacteria are easily washed off cars, toys and picnic tables, says John Anhold, a Forest Service entymologist. If he lived in the spraying area, though, says Anhold, "I'd definitely bring in all my kids' toys" from the yard before spraying began. And, just to be on the safe side, if he had allergies he would "probably go for a drive that day."

Spraying must be timed to avoid rain showers and to target the right stage of the gypsy moth caterpillar: either the second or third molting (or what entymologists call "instar"). It's at those stages that the caterpillar is making its way up the tree, armed with a big appetite.

If spraying is delayed past the third instar it will take longer to kill the caterpillars; by then, says Anhold, they will have munched through a noticeable number of leaves.

Gypsy moths are particularly fond of scrub oak. But, says Anhold, "like a lot of moths, they're generalists." When scrub oak pickings are lean, they'll dine on aspen, river birch and box elder. When the population of gypsy moths gets really big they'll even eat conifers.

A population explosion of gypsy moths can occur in the space of just a few years, which is why the trapping of a measly 47 gypsy moths last year made the UDAF and the Forest Service anxious.

Entymologists figure that the 47 moths trapped in the Holladay area - in an area near Holladay Boulevard and 6200 South known as Knudsen's Corner - represented 1 percent or 2 percent of the real population. And because each female gypsy moth can lay up to 500 eggs, within four years the gypsy moth population would number in the millions. The gypsy moth, which was introduced into New England from France in 1859, has no natural enemies in the United States.

Ancestors of this year's gypsy moths may have been survivors of previous sprayings in the Olympus Cove area. Or, more likely, they were brought into the area from New England - via egg masses stuck to a car tire, camping equipment or lawn furniture, says Larry Gillham, a forester with the U.S. Forest Service.

The UDAF will begin spraying for gypsy moths as soon as the size of scrub oak leaves indicate that the caterpillars are at the right stage of development - probably at the end of April, says Anhold. The spraying will be completed by June, "unless the weather is weird."

The area to be sprayed includes 916 acres near Knudsen's Corner and at the mouth of Little Cottonwood Canyon. Residents will be notified of exact spraying dates.