Bagworms (Thyridopteryx ephemeraeformis) attack numerous species of evergreens and a few deciduous plants. Blue spruce (Picea pungens), which grows in U.S. Department of Agriculture plant hardiness zones 2 through 8, is among the favored hosts for these damaging pests.
The adult bagworm is rarely seen. The eyeless, wingless, grublike female spends her whole life in the cocoon. Small, black male moths emerge in late summer or early autumn to find mates; males mate with females through small openings in females' cocoons. Females lay 300 or more eggs, which overwinter in the cocoon.
The larvae are the most frequently encountered. These small caterpillars hatch in late-spring or early summer. Some larvae float to nearby trees on small strands of silk while others immediately begin constructing their camouflaged cocoons.
The cocoons are baglike and made up of silk and fragments of leaves and twigs from the host tree. To the untrained eye, these small bags appear to be small sections of dead branch on the host tree. As larvae move about, however, their bag is always with them. The larvae continue adding to their protective bags, making them up to 2 inches long by the end of the feeding season.
The newly hatched larvae are tiny, about the size of a pencil lead, but that doesn't stop them from creating considerable damage on their hosts. These young larvae etch the surfaces of spruce needles and create the first notable symptom: browning needles near the tips of the branches.
As the larvae mature and become bigger, more severe damage results. A large population of bagworm larvae, which is common due to the number of eggs each female produces, can completely defoliate a spruce tree or shrub. In cases where the population isn't as large, the spruce or other host plant will become stressed, have stunted growth and eventually weaken.
Smaller blue spruces allow for nonchemical control. Examples include globe (Picea pungens "Globosa") or dwarf tree varieties such as Sester's Dwarf (Picea pungens "Sester's Dwarf"), both of which grow in USDA zones 2 through 8. You can mechanically pick off the bags of these pesky creatures by hand. While simple, this can become a tedious task as the bags often blend in very well to the rest of the spruce. Place picked bags in a bucket of warm, soapy water or otherwise destroy them.
Natural insecticides are also readily available and offer reduced risk to the environment. These contain natural effective ingredients such as Bacillus thuringiensis (Bt), a naturally occurring bacteria; and neem oil.
Mix 4 teaspoons of a Bacillus thuringiensis concentrate per gallon of water and spray via pump sprayer. Apply as soon as caterpillars emerge -- generally in late-May through June. Mix neem oil insecticide concentrate at a rate of 6 tablespoons per 1 gallon of water; spray with a pump sprayer when larvae begin to emerge and every seven to 10 days as needed. Handle each product with care; wear goggles, gloves and long sleeves to prevent contact with your skin or eyes. Each product requires thorough coverage of the leaf surface to work effectively. Because spruces vary so greatly in size, the exact amount of diluted insecticide needed will vary.
For large trees, professional help may be required. Thorough coverage is essential since both insecticides must be ingested to work. Use a pump sprayer to cover the entire spruce.
- University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources: Bagworms
- University of Kentucky Entomology: Bagworms on Landscape Plants
- North Carolina Cooperative Extension Department of Entomology: Bagworms
- Monrovia: Colorado Blue Spruce
- Missouri Botanical Garden: Bagworms
- Monrovia: Dwarf Globe Blue Spruce
- Washington State University Clark County Extension: PNW Plants: Sester's Dwarf Blue Spruce
- Photo Credit Lynne Furrer/iStock/Getty Images
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