ALBUQUERQUE, N.M. - It has all the makings of a potential rangeland disaster: thousands of acres of grass and the herds of cattle, antelope and other animals that need to eat it versus an emerging army of spiky caterpillars with voracious appetites. But ranchers hope quick action will take care of the caterpillars _ also known as range worms _ before they can destroy pastures in northeastern New Mexico.

Some ranchers are trying to keep the caterpillars at bay using crop-dusters, pickup trucks outfitted with foggers and an insecticide called permethrin.

They've already sprayed 108,000 acres and have at least 40,000 acres to go, Union County extension agent David Graham said Tuesday.

"We're trying to knock them back," he said as he prepared for another day of spraying.

While ranchers in northeastern New Mexico regularly live with range caterpillars, Graham said the latest invasion was "the worst case we've had in 10 years."

Entomologists say recent moisture and the resulting green pastures could help this year's caterpillar population, but ranchers' efforts and natural predators could keep them in check.

"They're boom and bust insects," said David Richman, a science specialist at New Mexico State University in Las Cruces. "They come up and they go down. They come up and they go down. It's hard to predict."

One problem is the caterpillars' appetite. Graham said research has shown that just a few of them can eat as much grass as a yearling cow.

Don Schutz, a rancher in the Wagon Mound area, has witnessed the caterpillars' technique.

"It's just this mass of moving worms" when the population becomes overwhelming, "and they form these windrows and you can have a foot of green grass in front and desert behind it," he said.

Another problem is that the yellow caterpillars, which can grow to a couple of inches long, are lined with tiny hairs shaped like tree branches that sting.

That can be painful to yearling cattle eager to put their noses in the grass and start chomping, as well as to people who handle the caterpillars or kneel in infested grass, Graham said.

The heaviest infestation is in Colfax and Mora counties, he said. There also have been reports of caterpillars in Union and San Miguel counties.

Ranchers are paying about $1.25 per acre to spray for the caterpillars. Schutz has sprayed about 4,000 acres but said others have sprayed as many as 20,000 acres.

That's important, he said, because unless all infested pastures are sprayed, the insects that survive in untreated areas could find their way back to treated pastures.

Schutz acknowledged spraying doesn't always kill all of the pests, but said it at least makes the situation manageable.

Those caterpillars that make it to the pupa stage still have to deal with natural predators like skunks and shrews, Richman said. Beetles, wasps and other parasites also can have an impact during other life cycles, he said.