Board Certified Master Arborist, Wayne White, shows an example of a bagworm sack. This was found on a spruce in Plainfield, IL. This little bag could hold anywhere from 300-1,000 eggs!
What Is It?
Bagworm, contrary to its name, is a moth. It is in the family Psychidae of the order Lepidoptera. This is the second largest order of insects in the world, including all butterflies and moths. The larvae of Lepidopteran insects are most commonly called caterpillars. The most common species of bagworm in our area is Thyridopteryxephemeraeformis Haworth. However, there are about 1,000 species of bagworm in North, Central, and South America. Bagworms are easily identified by the dangling brown bags that they create during their life cycles.
Bagworms can affect over 50 families of evergreens and deciduous trees and shrubs. However, most of the concerns in our area show bagworm infestations on spruce, junipers, and arborvitae.
The only damaging stage of this insect is the caterpillar stage, as the adult bagworms do not feed. Caterpillars, in general, have large heads with well-developed chewing parts. In addition to the unsightliness of the hanging bags that the insect builds, bagworms can cause serious damage to their host tree or shrub. Bagworms consume the foliage of their host. The longer and more severe the infestation, the more damage that will be caused to the tree or shrub. If the infestation goes unnoticed or unaddressed, the amount of damage may be irreversible.
Lepidopteran insects, such as the bagworm, go through four distinct development cycles: eggs, larvae, pupae, and adults. Eggs hatch in early June, and the young caterpillars (larvae) start feeding immediately. Simultaneously, they start constructing small, complex sacks around themselves. These ‘bags’ that hang off the leaves and/or stems of their host plant are what led to the insect’s descriptive name. As they grow larger, they continue to enlarge their protective sack with leaf and twig debris from the host plant. The sacks can measure up to two inches in length, are very tough, and are hard to open. This provides the developing bagworm with a great defense from predators. The insects do not stray from their sack, and they quickly retreat if approached. It would be rare for an insect to emerge further than just the head and thorax. They remain in these sacks during pupation, and after becoming adults, only the males have wings. The females release a pheromone that attracts the males to them for mating. Between 300-1,000 eggs can be laid by each female within her sack, where she then dies. The eggs overwinter within these sacks.
In mid-to-late August, the bagworm’s feeding subsides and it will seal off its bag to begin the pupation process of becoming a moth. Insecticide treatments are most effective if done earlier in the feeding cycle when the caterpillars are small and active. Per the University of Illinois Extension, the best time to spray for bagworms in our area is late June with the option of a second spray in early July. Manual removal of the sacks prior to egg hatch is a very viable option for control, but can become quite overwhelming in heavy infestations.
This University of Florida article features detailed pictures about the different bagworm life stages.