Bagworm Pests

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Bagworms began emerging this week. These are a significant pest of Leyland cypress and evergreen trees.


A larval bagworm feeding on a maple tree. Photo credit: Adam Dale

If you have Leyland cypress, arborvitae, or other evergreens in your landscape, you should examine your trees for these insects. Bagworms are easy to identify. Take time to look at your trees because the damage can be significant.

The Bagworm, Thyridopteryx ephemeraeformis, is a common pest of conifers and other ornamental plants throughout the eastern United States. The bagworm’s common name comes from the fact that these pests create a silken bag camouflaged with plant debris. This bag is used by the insect for protection. These bagworm bags are sometimes mistaken as little seed cones. When trees are heavily infested, the bags actually look like Christmas ornaments.

These pests go through the winter as eggs within their mother’s bag. The eggs are now hatching into larval caterpillars. As the larvae emerge from their mother’s protective bag, they spin a silken thread. This silk thread can be picked up by the wind and carry them great distances. However, most larvae don’t go far and simply remain on the same plant where they hatched.

The newly emerged larvae will quickly begin creating a tiny bag of silk and plant debris that looks like an upside down ice cream cone. As the caterpillar grows, the bag also grows. In August, the larvae firmly attaches the bag to a branch with a sturdy silk band. Inside the secured bag, the caterpillar transforms into an adult. Male adult bagworms emerge from their bags in September. Females are flightless and never leave the bag. The males actually mate with the female inside her bag. After mating, females lay their eggs inside their bag. The cycle begins all over again the next spring when the eggs hatch and emerge from the protection of their mother’s bag.

One of the most effective, yet time consuming, methods of treatment is hand-picking. When this is impractical, there are pesticides that can be used for control. However, the control should be done before August. If a pesticide is needed, apply it when caterpillars are young. Bacillus thuringiensis, sometimes called B.T., is marketed under many trade names. This product is very effective at controlling bagworm larvae and does not harm beneficial insects. However, Bacillus thuringiensis is effective only when the bagworm caterpillars are young. Spinosad, which is marketed as Conserve, is effective on larger bagworm larvae and is also less damaging to non-target insects.

If you choose to use a pesticide, be sure to read and follow the directions and safety precautions on the label. Pesticides are poisonous and can be dangerous when misused.

To learn more about bagworms, their interesting biology, and control measures, please Google “ENT/ort-81”. This is the publication number for our NC State bagworm factsheet for homeowners and landscape managers.

For answers to your garden and landscape questions, call the Caldwell County Extension Center at 828-757-1290 or visit us online anytime at //