The Evergreen Bagworm
Mainly a garden pest east of the Rocky Mountains, the bagworm, or Thyridopteryx ephemeraeformis, can defoliate a wide array of evergreen conifer trees, as well as a few deciduous hardwood trees. They are so-named because of the baglike case-cocoon in which hundreds of insect eggs overwinter. Bagworms often go unnoticed by gardeners in evergreens because the cases look like tiny pine cones or other natural plant features. Does this Spark an idea?
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Pest Life Cycle
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Bagworm caterpillars devour the foliage on evergreens to grow and mature over summer. They construct cocoons of self-made silk meshed with plant parts such as needles and twigs. The 1/2- to 2-inch-long cocoons are golden brown and jagged and hang down from evergreen boughs. In fall, winged male caterpillars emerge from their cocoons and travel to other cocoons that contain the blind and legless females. The male and female mate inside her cocoon and the she lays 300 to 1,000 eggs in her cocoon, crawls out, drops to the ground and dies. The eggs overwinter in the cocoon bag and hatch in spring, creating new crawling caterpillars that form new cocoons on different branches or plants.
Affected Plants
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Bagworms can infest sycamores, oaks and willows and any other edible tree or shrub species in the landscape. Evergreen conifers are more consistently heavily infested by bagworms than deciduous plants. Arborvitae, cedars and junipers are the most heavily damaged evergreens, but it's not uncommon to encounter spruce, fir or pine trees with bagworms, too.
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Movement
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Bagworms are spread mainly by movement of infested plants that grow in nurseries and garden centers. Close quarters of nursery plants during production allow bagworms to crawl or casually drop onto branches of nearby plants. Alternatively, tiny caterpillars in late spring are blown onto nearby trees by wind, more commonly called ballooning.
Damage
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A small number of bagworms will cause virtually unnoticeable chewing of a few leaves on a tree in the first year. If not killed, subsequent generations of caterpillars further devour foliage so that larger branch areas or an entire plant is defoliated by July or August. Seasonal defoliation of a tree may not kill the plant, but repeated defoliation after a couple of years depletes the tree of food-making abilities through photosynthesis and can lead to death. Evergreens typically do resprout needles from twigs stripped of needles but may remain with nude brown branches.
Control
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The least environmentally impactful means to control bagworms is to physically remove the egg-filled cocoon-bags from plants anytime from fall to early spring. The bags must be removed before eggs start hatching anytime from April to early June. Crush, burn or bury bagworm cocoon-bags after harvest. Once the tiny caterpillars are out on the evergreen plants eating, an array of insecticides may be used. Alternatively, the microbial organism Bacillus thuringiensis (Bt) may be sprayed onto foliage for the bagworm caterpillars to consume and then die.
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