Manage bagworms now so they don’t harm trees

“Why are the pinecones on my tree moving?” a client asks.
“Because those aren’t pinecones, they’re bagworms,” I reply.  

Dangling from evergreens like teardrop-shaped Christmas tree ornaments, bagworms cause many a homeowner to scratch their head in wonder. Pinecones that dance?

Bagworms dangle from a juniper branch.
Credit: Erik Rebek, Bugwood

But the tell-tale thinning of trees that can follow is no laughing matter. Covered with bits of needles and leaves, the bags that give bagworms their name serve as protection for the caterpillars inside. Bagworm caterpillars are the juvenile form of a moth.  

That sounds innocent enough, but like all caterpillar-like insects, they are born hungry. Walking stomachs, they prowl among your trees, munching away to cause sometimes serious defoliation. They particularly enjoy evergreens such as arborvitae, cedars, junipers, and pines. But they will also dine on maples, locusts, lindens, and other deciduous trees.  

Eggs hatch out from bagworms’ bags in May. Tiny larvae spin an eighth-inch bag with bits of needles or leaves glued together with webbing. Like tiny backpackers, bagworms tote those bags around as they feed. As they grow, the bags get bigger and bigger and your tree foliage gets thinner and thinner.  

I don’t know if bagworms have an adventuresome streak, but they do a bit of hang-gliding. They spin a fine web and use the wind to glide to other trees in a stunt called “ballooning.” 

By August or September, the bags – and bagworms – are 1- to 2-inches long.  They stop wandering and feeding and tie up to a twig using tough silky threads. In late summer, they transform into moths. But get this, ladies, only the males have wings.  So the gals just hang out in their bags and wait for the boys to, um, visit. Post rendezvous, each female bagworm lays 200 to 1,000 eggs in its bag. Next spring, the eggs hatch to start the cycle over again.

Stopping that cycle is important and now is a crucial time. Mid-June to mid-July is the best time to treat trees with bagworms with a very effective organic control called Bt. A naturally occurring soil bacteria, Bt or Bacillus thuringiensis kills only small caterpillars. And guess what? Bagworms are just the right size right now.  Bt doesn’t harm humans or animals and is easy to find. It’s sold in hardware stores and garden centers under names like Dipel and Thuricide. Applying Bt is a do-it-yourself job if you can reach your trees with a sprayer. If not, call in a pro.

Even easier is picking off the bags if you have only a few bagworms. Snip them off with scissors or pruners, bag and trash them. Don’t leave them on the ground where the eggs can hatch. Since there can be as many as 1,000 eggs in each bag, removal is important. Get those bags gone. And gone before the eggs hatch in May.  

Tiny bagworms are hard to spot when they first appear in May on plants such as this Colorado blue spruce.
Credit:  Dave Lantz

Learn more about bagworms and see some great photos on the HGIC website.

You can beat bagworms and keep your trees safe. Fortunately, this is one insect for which there is an easy – and organic – fix.  So get out there and bag some bagworms.

By Annette Cormany, Principal Agent Associate and Master Gardener Coordinator, Washington County, University of Maryland Extension. This article was previously published by Herald-Mail Media. Read more by Annette.

This article was previously published by Herald-Mail Media.

What’s Eating My Tree? Bagworms or Webworms?

Have you noticed the large webbed sacs in the trees along the sides of the road lately? They do prompt questions to the Home & Garden Information Center’s Ask an Expert service. Often they are referred to as bagworms which causes some confusion because bagworms are a different insect.

So what are webworms and bagworms? Will they cause damage to trees? Let’s take a look at them both.

fall webworms
Fall Webworms. Photo: Kelly Oten, North Carolina Forest Service, Bugwood.org

Fall webworm (Hyphantria cunea)

Fall webworms show up every year but their populations can vary in size. Often they are insignificant in number and not too noticeable, but every so often there is a large population. The sacs can be numerous and quite large, leading one to assume they will devour an entire tree’s worth of leaves.

fall webworm
Fall webworm (Hyphantria cunea). Photo: Milan Zubrik, Forest Research Institute – Slovakia, Bugwood.org

But, thankfully, beneficial insects such as parasitoids and predators such as birds, who love to eat the caterpillars, keep populations in check and spraying is not necessary. Granted, the webs are not pleasing to look at. If they are within reach they can be pruned out and destroyed.

Fall webworm facts

  • There are two generations each year. One is active in May and the second larger population is active starting in late summer into the fall. However, the sacs can be noticeable in the trees all winter.
  • They feed on over 100 species of trees. Preferred hosts include walnut, oak, hickory, willow, apple, and other fruit trees.
  • The adult is a ¾ inch-long moth, white or white with black spots.
  • After eggs hatch on a host tree, the caterpillars produce a fine web over the ends of branches. They feed only within the web.
  • Mature caterpillars are about an inch long with noticeable long, silky hairs. They come in two colors, the ones with black heads are yellowish-white and the red-headed ones are brown.

Refer to our webpage about fall webworm

Bagworms (Thyridopteryx ephemeraeformis)

By far bagworms are the more destructive of these two insects and need to be managed. They have voracious appetites and devour the needles of evergreens– particularly arborvitaes, junipers, Leyland cypresses, and cedars. We hear the cries of desperate residents wanting to know if the dead areas on their trees will regrow. Unfortunately, that answer is no.

Bagworm facts

  • There is one generation per season.
  • In addition to conifers, they will feed on deciduous trees (ones that shed their leaves in the fall) such as sycamore, maple, locust, boxelder, and linden, but the damage is not as significant.
  • Eggs hatch in late May or early June. Caterpillars feed and create bags made from pieces of the plant they are feeding on. The bags enlarge as the caterpillars feed during the summer.
  • The caterpillars pupate in late summer. Adult male moths emerge and fly to female bags that contain wingless female moths, then mating occurs. Females can lay 200-1,000 eggs which overwinter in the female bag.

    Bagworm damage
    Bagworm damage on Arborvitae. The brown portion will not recover. Photo: University of Maryland Extension

    bagworm
    Evergreen bagworm. Photo: Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org

Controlling bagworms

  • Fall, winter, and spring: Remove and destroy bags containing overwintering eggs. Pinch them at the tip or use a small clipper to remove the ones you can reach from the tree. Do not drop them on the ground near the trees. Dispose of them in the trash.
  • Beginning in late May into early June. Begin to look for small caterpillars moving on the trees. As they feed, small bags will form. The best time to spray is when the caterpillars have just emerged and are small. Bacillus thuringiensis (Bt), an organic insecticide, will help to control the young caterpillars.
  • By mid-July the caterpillars will be too large for Bt to be effective. Look for a registered insecticide labeled for bagworms at a plant nursery or hardware store. Contact an arborist or landscaper to treat large trees or if the bags are located high up in the trees.

    young bagworms
    Young bagworms can be controlled with an organic insecticide, Bacillus thuringiensis (Bt). Photo: Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org

Other tent-making caterpillars

There are two more tent-making caterpillars that are sometimes mistakenly referred to as ‘bagworms’. They are eastern and forest tent caterpillars. Both are active in the spring and not in the summer or fall.

By Debra Ricigliano, Lead Horticulturalist, University of Maryland Extension Home and Garden Information Center

Yard and Garden Tips and Tasks for July

Spotted Lanternfly

Spotted lanternfly adults may be found feeding on many hosts, especially tree of heaven (Ailanthus altissma). Report any finds to the Maryland Department of Agriculture immediately, collect a sample or take digital photos of what you have found.

 

BagwormBagworm caterpillars are now very active. Look for little bags crawling around on evergreen trees and shrubs and be prepared to spray infested trees with the microbial insecticide, Bt by mid-July. Mature bagworms are not well controlled with Bt They are best collected by hand and destroyed or sprayed with insecticides containing spinosad.

 

Proper lawn mowing is critical to help it survive the summer. “Mow ‘em high and let ‘em lie” should be your mowing strategy. Cut your cool-season turf (fescues and bluegrass) to a height of 3-4 inches and leave the clippings on the lawn where they will naturally decompose.

 

BroccoliSow seed for fall transplants of broccoli, kale, turnip, and cauliflower in flats or containers by the 3rd to 4th week in July. Late crops of squash, beans, and cucumbers can be direct sown into your garden through the end of July.

 

More Tips and Tasks for July

Q&A: Do bagworms kill trees?

Bagwoom cocoon
Bagworm cases are made with evergreen needles and leaf material

Q: Do bagworms kill trees?

A: If there is a large infestation of bagworms on an evergreen tree, it is indeed possible for them to kill the tree if the defoliation is severe. Bagworms can be a problem on deciduous trees as well, but they typically do not kill them.

Bagworms are a common pest in Maryland and we receive a lot of questions about how to deal with them. Fall and winter are the best time to remove and destroy the bags that contain the eggs.

Here are a few tips about the lifecycle of bagworms and how to control them. Continue reading