If you see helicopters hovering at tree-top levels in parts of Riverside, North Riverside and Brookfield in the next couple of weeks, don’t alert Jack Bauer or Homeland Security. It’s not an invasion.

Rather, they are there to prevent one?”from gypsy moths.

The Illinois Department of Agriculture (IDA) will begin spraying areas of the three villages, mainly near the forested areas along the Des Plaines River, where gypsy moths have been observed in alarming numbers in recent years.

According to James Cavanaugh, gypsy moth project coordinator for the IDA, the gypsy moth threat has been moving steadily westward since the insect was introduced in the United States from Europe.

Since 1974, the IDA has been trapping moths to gauge the extent of their numbers here in Illinois. Cavanaugh said that the moth has overtaken Michigan and parts of northern Indiana and has a resident population in most of Lake County and northern Cook County in Illinois.

“There has been millions of acres of defoliation” in places like Pennsylvania, according to Cavanaugh, while in Illinois he has “seen it on a scale of a block or two in Illinois.”

According to the United States Department of Agriculture Web site, the gypsy moth is “one of North America’s most devastating forest pests,” where in its caterpillar stage, it feeds on “hundreds of species of plants … but its most common hosts are oaks and aspen.”

The moth in Illinois has tended to spread along riverways, and for the past two to three years, the IDA has been working with the Cook County Forest Preserve District to try to control the spread of gypsy moths.

During the 2005 trapping season from June to August, the IDA identified the forest bordering the southern portion of the Hollywood section of Brookfield as particularly affected by the pest. Other large concentrations were found in the forest preserves on either side of First Avenue from 31st Street to Ogden Avenue. Notable concentrations were also found in North Riverside south of 26th Street and in Riverside’s First Division.

Cavanaugh said the moths prefer to lay their egg masses in oaks, willows, poplar and fruit trees.

“But when the populations get heavy they’ll eat over 250 species of trees and shrubs,” Cavanaugh said.

Spraying will likely begin the week of May 22. Originally, the IDA hoped to begin spraying the week of May 15, but anticipated cooler temperatures will probably delay the effort for a week.

“The timing has to be perfect and the weather has to be right for spraying,” said Cavanaugh, who couldn’t predict an exact date or times for spraying.

“Hopefully we’ll be in and out of your area in one day,” he added. “Seven to 10 days later, we’ll come back for a second treatment.”

The IDA sprays a substance called BTK, a bacteria that occurs naturally in the soil, over the affected areas. According to Cavanaugh, BTK contains “no toxicity to animals or people.” However, it is toxic to certain species of butterflies and moths. Any gypsy moth caterpillars feeding on leaves that have been sprayed with the substance will die.

Cavanaugh added that the IDA will spray about a half-gallon of the insecticide per acre, and that within 10 to 15 minutes the spray will be out of the air totally.

The IDA has never sprayed this particular area to control the spread of gypsy moths, although it has done similar spraying in Oakbrook and areas just north and south.

In June the IDA will return again, this time in the central area of Riverside to release, again via helicopter, a “pheromone treatment” intended to disrupt mating of adult gypsy moths. The treatment creates “a cloud of pheromone [emitted by the female gypsy moth], so when the male moth flies into it, it doesn’t know which way to go. They can’t find the females so there are no eggs,” Cavanaugh said.

The pheromone treatment is only effective in areas where there are low numbers of the insect, however.

The IDA will resume its trapping effort in June as well to identify whether the moth has spread or decreased in any area.

Riverside Village Forester Michael Collins said he’s familiar with the spraying program, having witnessed it first-hand when he was working in the far west suburbs.

“The whole program helps slow the spread,” he said. “You know you can never completely eradicate it, where you see heavy infestations, you need to knock those down, so they don’t completely devastate those areas.”

Adult gypsy moths lay tan-colored egg masses of up to 1,000 eggs, Collins said. The silver-dollar-size masses are noticeable on tree trunks, he said, especially during winter months when no leaves are on the trees.

Once hatched the caterpillars are very distinctive. About 1.5 to 2 inches in length, the caterpillars are brown, with red and blue dots on the tops of their bodies.