Solving the bagworm problem without chemicals

Sooner or later, most landscapes have to deal with a bagworm problem. Sometimes the hungry pests only threaten one or two trees in a landscape. But they can really ruin the party when it comes to the classic, evergreen  privacy buffer. A long line of Arborvitae, Leland cypress, or cryptomeria are the perfect home for bagworms, because of the way the bagworms spread.

Eggs hatch out and the tiny larvae begin feeding or climb to higher parts of the tree, then “balloon” on a thread of silk, hoping to land on a neighboring tree (which is why those lines of screening trees make such a perfect place to keep feeding, generation after generation). Then they spin a protective bag composed of silk and the fine-textured needles or bits of leaves of whatever tree they are eating. Female bagworms never leave their bag, even in the adult stage.

Adult male bagworms can fly, and tend to hang out toward the bottom of the tree, and this important fact highlights a major vulnerability that the eco-conscientious gardener can exploit: Because of their location, these male bagworms are particularly susceptible to attack by two predatory insects.

These tiny predatory insects, Chirotic thyridopteryx and Itoplectis conquisitor, are among the most common of a group of insects called predatory wasps. These predatory wasps generally do not sting humans and do not protect or even form social hives of any kind.

You can attract and support predatory wasps by making sure that your landscape provides a steady supply of nectar throughout the growing season. Try planting a 3’ meadow strip along the sunny side of your privacy buffer that includes long-blooming, nectar-rich perennials like Agastache or Pycnanthemum.

I do not recommend the use of Bacillus thuringiensis (B.t.) in the landscape because even though it is a naturally occurring, organic insecticide, it kills ALL SMALL CATERPILLAR LARVAE. If you are gardening to promote butterflies, this is a no-brainer decision.

The other, even less attractive alternative--using chemical spray or soil drench--is not only harmful to the environment but over time less and less effective because the trees become very dense and tall. The chemicals are not effective unless you can reach the crown of the tree early in June, where the larvae that are most vulnerable to the sprays will be.  Those tough, water-proof bags later in the season effectively protect the bagworms from the spray.

Another environmentally sound way to avoid the bagworm problem is to plant less susceptible trees and shrubs. Although bagworms in captivity have been shown to be able to survive on the leaves of just about any tree or shrub, out in the jungle of real-world landscapes, broadleaf evergreens show much less damage. This might be due to their leaves being not quite as ideal for weaving into a bag to provide camouflage protection for the bagworms, compared to needle evergreens.

Also, deciduous trees, while still edible to bagworms, are both less of a first-choice target and also more likely to bounce back with fresh growth after being defoliated by bagworms. Plus, if you plant trees that provide direct food and shelter for birds, you will attract another bagworm larva predator: birds who have young to feed in June.

So if you can, plant a mixed buffer that includes deciduous trees and broadleaf evergreens (Magnolia virginiana, Morella pensylvanica and Ilex opaca ‘Satyr Hill’ are some of my broadleaf evergreen favorites). Doing this should help to slow the bagworms down to the point that they don’t decimate your privacy buffer.

There are also three common birdfeeder birds that are known to scavenge bagworm eggs from twigs and branches during the winter months: chickadees, nuthatches, and titmice. So keeping that birdfeeder going through the winter months might have some secondary benefits in addition to being something fun to watch.

Since the goal is balance, and not total eradication, this multi-pronged approach of planting resistant species combined with attracting predatory beneficial insects and birds in June and over the winter, can really help.

Summary: 4 steps that can reduce bagworm populations:

1. First, plant a long-season, nectar-producing strip of perennials and/or shrubs near your bagworm hangout, to attract beneficial parasitic wasps.

2. Consider replacing the bagworm-magnet trees with trees that are less attractive but still provide evergreen cover (such as magnolias, hollies, and morellas).

3. Mix some deciduous trees and shrubs into the buffer to provide food and cover for birds that nest in June, which will encourage predation of the bagworm larvae in June when they are most vulnerable.

4. Attract winter birds to your landscape to hunt the bagworm eggs from trunks and branches.

And it surely doesn't hurt that all of these recommendations are also good practices for strengthening any environmentally friendly landscape!

Have you had any battles with bagworms and tried these methods? What were your results?--I'd love to hear about it!


Chris Pax

P.S. The Native Plants for the Landscape COURSE is full (yay!) and started last week. But there will be another one in February, on Saturdays. If you're interested, let me know.