What is a bagworm? And why do I need to know?

Just what the heck is a bagworm?

Reports about the pesky insect have been popping up in the garden calendar for the past few months, until being overshadowed by calls about the ever-ravenous Japanese beetle.

While those beetles have been turning plants’ leaves into doilies, the bagworm has quietly been doing its thing, which is eating the needles on your evergreen trees and shrubs. Spruce, junipers and pines are particularly vulnerable.

“We are most concerned about evergreen trees because they don’t reneedle,’’ says Scott Evans, the horticulture program coordinator for the Nebraska Extension in Douglas and Sarpy Counties.

“Once those needles get chewed off, those don’t grow back.’’

The destructive caterpillar lives in a bag made of silk and fragments of needles or leaves. Its head and legs stick out as it feeds.

Males turn into ashy-black moths with transparent wings in late August or early September. That’s when they somehow impregnate the female in her bag, which she never leaves.

“I don’t understand how that happens, and they are very successful,’’ Evans said.

After overwintering in the bag, eggs hatch in mid-May to early June. The larvae parachute out of the bag onto neighboring trees and the cycle starts again.

They don’t limit their appetite to evergreens; any hardwood tree will do.

“But we are not concerned about those trees. They will refoliate next year,’’ Evans said. “They are fine.’’

Trees such as the Colorado spruce are hugely popular in this area, Evans said. People love the color and the similarity to a Christmas tree. But bagworms are fond of them, too, and can kill them if there’s a heavy infestation.

Watch out for browning needles.

To help save the spruces, junipers and pines, it’s best to treat for the worms when they are small. Bacillus thuringiensis and Spinosad are good options with little side effects for other insects. But now that they are larger, your options have changed.

“Bifenthrin and other more traditional products will be the products of choice along with hand-picking and dropping in a bucket of soapy water,’’ Evans said.

The bags need to be open and the caterpillars feeding for insecticides to work. Once the bags are closed, insecticides will not penetrate the bags to kill them.

Another option is to surround vulnerable trees with plants that attract bagworm predators. Asters and daisies are good options as well as fruit and seed-bearing plants that attract birds, which eat the bagworms in late summer.

It’s been an average year for bagworms, just as it’s been for Japanese beetles and june bugs, another topic for callers to the extension office. But that doesn’t mean it’s a good year.

All three bugs have taken their toll on trees, shrubs and plants.

“Every year I keep saying this; I feel like a broken record,’’ Evans said. “This was a tough year. Every year there is a different insect or disease we are trying to manage.’’