Start looking for the hungry bagworms on your trees

  • Pamela Corle-Bennett
  • Contributing Writer
12:00 a.m. Friday, June 23, 2017 Home Garden

Bagworms have hatched in the Miami Valley and feeding is progressing. If you have plants that are sensitive to bagworm, you might want to pay attention to them and inspect closely.

First of all, know how to identify bagworm. They are not the same as tent caterpillars or fall webworms. Gardeners sometimes get these mixed up.

Tent caterpillars form white silky nests in the crotch angles of trees. They come out of these nests and feed on the leaves. They prefer deciduous trees, especially cherries.

Fall webworms form a white silky nest around the ends of branches with the leaves inside the nest. They stay protected inside this nest while feeding.

Bagworms are caterpillars that hatch in early June and feed primarily on evergreens, particularly spruce and juniper, but will also feed on some deciduous trees.

Their feeding on evergreens can be dramatic and eventually lead to the death of the plant if not kept in check or eliminated.

Their feeding on deciduous trees isn’t quite as bad because these trees can afford to lose a few leaves a year and not die. They end up looking pretty ratty. If the feeding is heavy enough, the leaves may dry up and drop this season.

The key characteristic used to identify bagworm is the actual “bag” or protective covering that they create while feeding.

This bag is sort of shaped like two sugar ice-cream cones placed end to end in the middle with the top part attached to the tree. They end up about an inch long and around one-third of an inch wide.

This bag is formed as the caterpillar feeds on the foliage. It takes pieces of foliage and spins with silk-like threads to form a protective cocoon.

At this time, they are still quite small, and range in size from newly hatched to approximately one-fourth inch in size. The larger ones look like one sugar cone hanging from the underside of a leaf or on the branch of an evergreen. The leaf tissue is still green.

Since they are so small and still green, these bags are difficult to spot unless you are looking for them. Start looking!

OSU Extension Program Assistant Kathy McConkey had a sample of bagworm on a spruce this past week. The caterpillars were so small they had likely just hatched. She had to use a microscope to identify them. I saw another sample on a Japanese maple that was about the quarter-inch size.

When they first hatch and before they reach the quarter-inch size, you can spray the plants with Bacillus thuringiensis or Bt, a bacteria that when ingested kills the caterpillars. However, this only works on very small caterpillars.

For the larger ones select a spray labeled for caterpillars that is listed for use on ornamental plants.

I would suggest that you focus on evergreens, as these won’t grow new needles to replace the damaged ones. Spraying deciduous trees is more of a cosmetic choice unless you have evergreens around that may be affected by bagworms.