The caterpillars kept their presence hidden until Lynne Johnston and her neighbor were talking one day under the tree.

That's when one of the insects came barreling down on its long, silk thread.

"This green worm came down really fast," Johnston said. "She jumped out of the way, or it would have gone down the back of her shirt."

The small caterpillars with pinpoint eyes and soft green bodies are defoliating many ash and dogwood trees around Helena this spring.

They descend from the trees on spindles of thread, doing a caterpillar's work -- and giving anyone who crosses them goosebumps.

Soon, they'll turn into an army of moths and flutter off on the warm evening air.

But while the trees look bad and the caterpillars number in the thousands, experts say their population should die back naturally and the trees should survive.

"They're called leaf tiers, and what they do is tie leaves together, feed on the leaves, and then pupate," state entomologist Patricia Denke said after examining a photo of the insects. "While they may be feeding on the foliage, and even removing most of it, they will not kill the trees unless they occur over several years in a row."

After pupating, the green caterpillars, in the Tortricidae family, will emerge as small brown moths.

Their lifespan is short, and the insects themselves are more of a nuisance than a threat to Helena's urban forest and backyard barbeques.

"It's kind of minor thing right now," said Larry Hoffman, the Lewis and Clark County extension agent. "It's just seasonal. Their population depends on how our winters go, and how the insects enter the fall."

While the insects tantalize entomologists, they cause alarm in those who stray unknowingly into their silky webs, or discover a few dozen caterpillars inching down their shirt.

The tall ash trees in Kristin Best's backyard have been ravaged by the caterpillars. The green insects dangle down on threads. A few have found their way into the house, starting at the bathroom window.

A few blocks away, Johnston and her neighbor have already moved their barbecue. They also avoid the tree and its host of caterpillars.

"They are so gross," Johnston said. "You can sit under the tree and actually hear them eating."

Both Denke and Hoffman said that while the caterpillars should die off and the trees should recover, a good pest control agent may be able to help in the meantime.

"These things can be on one tree one year and not even be there the next year," Hoffman said. "But you can spray the lawn for when they drop down."

Reporter Martin Kidston can be reached at 447-4086 or at mkidston@helenair.com.