By up-armoring and camouflaging its traveling bivouac, an insect invader is waging war against defenseless suburban trees.

A relative latecomer, along with the more infamous borers and beetles wreaking their own havoc, the bagworm is a formidable threat, devouring evergreen needles as its preferred treat, but also content to munch on the foliage of deciduous trees. 

“Usually, it’s more of an insect pest we see farther south, but with recent warmer winters, they come up on nursery stock and can overwinter successfully,” said Donna Danielson, Plant Clinic assistant at The Morton Arboretum in Lisle. “We’ve been hearing about a lot of bagworms in all the suburbs.”

She suggests homeowners regularly inspect their trees, both evergreens and deciduous, including favored varieties such as maple, honey locust and bald cypress.

The black caterpillars travel along dragging their bag as shelter until they are ready to pupate in late summer, the males metamorphosing into a black moth, the females into a blind, wingless, pale egg-laying machine that never leaves its bag until — reproductive duties done —  it exits the pod to die.

A Plant Clinic leaflet explains that the bagworm caterpillars are 1/8- to 1/4-inch long when they first hatch, eventually growing to an inch. As the insect feeds, it creates a silken case covered with leaves made from the host plant as an effective disguise.

“Right now, their bags are a good size, about an inch and a half to 2 inches long,” Danielson said. “They kind of look like cones to the untrained eye. When they first hatch in June, they’re little tiny things and really hard to find.”

A Westmont reader alerted this writer to their flare-up in his town, concerned that many homeowners are oblivious to the sneaky pest.

Because evergreens don’t push out a new set of leaves like deciduous trees can, a large number of the voracious insects can be fatal for evergreens. Danielson said a single female lays as many as 1,000 eggs inside her bag, where they overwinter.

Newly hatched, the bag worms generally start feeding at the top of trees, and when they are small, can be carried from to other trees when wind currents catch their silken parachutes.

“One of my first jobs was to pick them off the trees in high school,” recalls Karen Kadolph, who works in sales at Hinsdale Nurseries, Inc., 7200 S. Madison St. “Once you know what you’re looking for, they are very easy to spot.”

She said one client aptly compared the home of an evergreen-dwelling bagworm to a pine cone in appearance.

“A good time to look for them is in the winter,” said Kadolph about taking advantage of the bare limbs of deciduous trees. “They’re just hibernating in the winter. Pull them off and destroy them.”

She mentions smashing or burning the bags, and Danielson talks of dumping them into a bucket of soapy water and flushing them.

If trees are too tall to safely remove the bagworms, spraying is an option. The insecticide will be consumed by the feeding caterpillar. Danielson said the beginning of July, when the bagworms are smaller and more vulnerable, is the optimal time to spray.

The bags become more impermeable as the worm grows. Repeated sprayings are possible in late summer, but not as effective.

Kadolph recommends homeowners seek expert advice for insecticides.

The Illinois Urban Pest Management Handbook (University of Illinois Cooperative Extension Service) has a complete listing of chemical recommendations in insect control.

Other combat theories include planting members of the aster family, which attract a wasp that attacks the bagworm, but not humans. Other parasites or bacterial agents also can be introduced.

When removing the bags by hand, Danielson advises care.

“We cut the bags off; they are stuck on there with a strong strand of silk and you could do some damage if you try yanking them off,” she said. “It’s shockingly strong. (If we want to open one,) we have to use a really sharp razor blade.”

Arboretum shares expertise

The Morton Arboretum’s Plant Clinic, a helpful resource for tree care, can be reached at (630) 719-2424. The internationally recognized outdoor museum boasts collections of 4,117 kinds of trees, shrubs and other plants from around the world at I-88 and Route 53 in Lisle. Check out its scenic, educational and entertainment offerings at www.mortonarb.org.