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Bagworms Begin Their Feeding Frenzy in May and June

Bagworms ride the wind attach themselvs to a trees foliage with a silken thread. Once there, it builds it's house & gorges on the trees tender growth.
With its house secured in place with silken threads, this bagworm grasps onto Eastern Red Cedar foliage and gorges on tender growth.

Published May 29, 2014 By BILL SEAMAN


Bagworms may be best known by the company they keep. So frequently found on Eastern Red Cedar and Arborvitae, both conifers have the colloquial names of “bagworm trees”. That is unfortunate for two trees that have much more to offer Texas landscapes than the unusual insect pests associated with them. While bagworms do have a reputation of devouring “cedars” when present in large enough numbers, they are opportunistic and will chow down on almost any plant where the wind will take them—literally.

The bagworm’s life cycle begins in May and early June when eggs hatch into caterpillars that are a mere 1/25 of an inch in length. So small and light weight, they spin a silken thread that allows them to drop to green foliage below their familial birthplace; or on a breezy day, use the thread like a broken kite string and ride the wind to a new host plant. Since crawling is an inefficient method of transportation for a caterpillar that is forever living out of a travel trailer, riding the wind while young proves to be the better technique for moving through the garden or landscape.

Less than a half inch long and still in its “dunce cap” juvenile posture, this young bagworm feeds on Cedar Elm leaves and decorates its house with leftovers.

Once the youngster stakes a claim, it begins feeding and constructing its namesake structure. On conifers such as Juniper, Cypress, and Arborvitae, the juveniles migrate to the tender new growth at the tips of twigs and stems. On broadleaf trees and shrubs like Bur Oaks, Honey Locusts, Sycamores, and Roses, they venture out on any lush foliage. At this stage, they carry their house around like dunce cap, keeping it upright, and keeping their head and mouthparts close to the food. For the first few weeks, they may only be able to eat the upper most cells on the leaf, leaving a skeletonized leaf in their tracks.

That all changes once they become between a quarter to one half inch in length, when they take a bagworm’s traditional posture and they begin gorging and leave behind only the toughest leaf veins. When they reach this stage on conifers, the tree’s outermost green needles or scales begin to disappear, exposing a leafless interior. Left unchecked, bagworms can defoliate and kill Italian Cypress, Arborvitae, Eastern Red Cedar, and like conifers. Deciduous trees including Bald Cypress, Bur Oak, Sycamore, and Elms are much more resilient and can generate a new flush of growth once the bagworm feeding frenzy stops. However, defoliation has a negative effect on any tree’s general health.

By August, the bagworm’s home reaches one and a half to two inches in length. As the house has grown, the bagworm has decorated the exterior of the silk bag with evidence of the host plant. On close observation, bags removed from Eastern Red Cedar have been shingled with leaf scales and short needles along with an occasional berry placed toward the top. A bag plucked from a Honey Locust has been decorated with leaflet pieces carefully arranged like shingles on the gable of a Victorian style house. Each bag features the siding materials provided by the host.

At almost two inches long and adorned with Honey Locust leaflets, this bagworm has secured its home in preparation to pupate.

By September the feeding stops. The damage to trees is done. The bagworms seal their homes and spend a few weeks in a pupae stage. For the males, the transformation changes them into a 1-inch long black moth with clear wings. For the females, the transformation converts them into an egg producing grub with no wings, legs, antennae, or functional mouthparts. Once fertilized by the males, the adult female bagworms lay from 400 to 1000 eggs inside their structure—and then die. The eggs remain in the bag until the next year’s caterpillars hatch the following May, completing the cycle.

Managing bagworms can be somewhat of a challenge. Plucking the “ornaments” from the tree is the first line of defense. However, this approach is only practical when the host tree is small and the bagworms can be reached. If this method is doable, always dispose of the bags. If the bags are dropped to the ground, the feeding caterpillars will drag themselves and their homes back up into the tree you removed them from, or find other plants to feed upon. If bags are plucked between September and May, again, dispose of them since those bags once occupied by female bagworms can contain hundreds of eggs awaiting a spring hatch.

An adult male bagworm moth has left behind its house and pupae covering (protruding out the base of the bag) to seek out a mature female.

Bagworms can be successfully managed with insecticides. A spray application of BT (Bacillus thuringiensis) on the foliage of the host tree is the old standard, particularly on young caterpillars. Reapply BT should rain wash the material off the foliage before the bagworms have time to ingest it. There are numerous other insecticides labeled for managing bagworms. Your local independent nurseryman can help you determine which product best meets your needs.

If you have had bagworms in your Texas garden or landscape, now is time to make a close inspection of trees and shrubs that have hosted the pests in previous years. If you are one of the fortunate gardeners who has managed to evade the bizarre caterpillars, count your lucky stars—and do a quick inspection just the same. It’s time to send those bagworms packing.

If you need more information on managing bagworms, or if you want to leave the task to the professionals, contact your Arborilogical Services certified arborist. He or she will design an insect management plan to fit your specific needs.

About the author

Mr. Bill Seaman

Mr. Seaman is a sixth generation Texan, degreed horticulturist, and a retired member of the certified arborist team at Arborilogical Services, “The Experts Your Trees Deserve.”®

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