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Bagworms are destroying local landscapes

Shown is the damage done by bagworms. (Submitted photo)

Destructive bagworm populations have exploded in our area over the last few years and are sadly destroying many people’s backyard landscapes. I am writing this article to help save your valuable trees and shrubs Now is the time to look closely at your trees and shrubs and be checking for the bagworm silk bags before the eggs and larvae inside the bags hatch and destroy your plants. Each bag can contain hundreds and thousands of eggs and caterpillar larvae that will emerge soon and begin feeding and cause extensive damage.

So what are the symptoms of a bagworm infestation? Bagworm caterpillars consume the tree’s leaves and produce small, spindle-shaped, protective bags made of silk, debris and portions of leaves, that are camouflaged and often go unnoticed until serious plant damage has occurred.

Heavy infestations may lead to a tree being completely stripped of all leaves. In evergreens, no re-growth occurs and attacked branches and trees often turn brown and die. So which tree species are infested? Bagworms attack more than 120 species of both deciduous and evergreen trees and shrubs. Some of the more popular hosts include: juniper, arborvitae, cedar, spruce, honey-locust, linden (Basswood), willow, maple, oak, birch, elm, poplar, and many other species.

Management of bagworms by homeowners may involve hand picking of the bags when they are easily accessible. For the best results, make sure that all bags are removed and destroyed, since even one bag left behind could cause re-infestation. Lift branches, clip off bagworm egg sacks and drop them into a bucket of soapy water. Make sure they are submerged fully. Dump the soaked bagworms into a sealed plastic sack and throw them in your dumpster. Repeat this procedure every fall, winter and early spring to reduce bagworm populations before the eggs hatch.

When hand picking is not practical due to an extreme number of bags or they are out of reach, an insecticide treatment should be used and we want to spray just at the time the “eggs are hatching and the small larvae appear”. According to various references and researchers, the bagworm eggs should start to hatch in late May or early to mid-June.

One method for finding the best time to treat is to frequently and thoroughly inspect trees that were infested last year. This will take time and diligence as the newly emerged caterpillars and their bags are quite small. Also, bags are made of silk and bits of plant foliage and newly-constructed bags with fresh plant bits are effectively camouflaged in among the needles and are hard to find.

A second method for predicting when new caterpillars will be feeding on the trees is to use phenology. Phenology is the study of recurring biological events, especially natural plant and insect rhythms that occur earlier or later in the year in response to seasonal and climatic changes in the environment.

Researchers noticed that when Catalpa trees and Japanese tree lilacs in the area are in bloom, the bagworm eggs hatch and the caterpillars appear on infested trees at that same time. These showy, common landscape plants became known as phenological indicator plants and by watching the indicator plants, we can accurately predict specific insect activities and the ideal time to spray them with an insecticide. Note that the indicator plants are not infested with the pest in question. They are merely an indicator of what might be happening in your landscape.

When hand picking is not practical, an insecticide treatment should be used. While spraying, thorough leaf coverage is critical. Two applications, two weeks apart, may be necessary for heavy infestations.

Bagworm insecticides such as Bacillus thuringiensis (Dipel) , Spinosad, Sevin, and pyrethroids such as Talstar, or Permethrin will be ineffective and a waste of time if they are applied too early or if they were applied too late last summer. Insecticide control becomes less effective as the season progresses and the larvae increase in size. So we want to spray just at the time the “eggs are hatching and the small larvae appear”. Trees should be protected from bagworm defoliation by spraying in mid-June to mid-July when Catalpa and Japanese Tree Lilac are blooming.

Carlson is a retired 30-year KSU landscape professor and ISA Certified Arbonist and consultant. He can be reached at 330-853-0742.

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