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On trees and shrubs the tiny bundles of gathered twigs, foliage and silk go unnoticed for months.
In August and September, a bagworm caterpillar's case grows to 2 inches and damage done to a homeowner's tree or shrub begins to show.
More common in the south and central part of the state, the bagworm's range has been expanding to the north, according to Purdue University entomologists.
Bagworms in the larval stage may defoliate arborvitae, junipers and other trees and shrubs.
Bagworms construct spindle-shaped bags to protect themselves against birds and other natural enemies, according to the Purdue Extension Publication: Bagworms, E-27. They withdraw inside if disturbed.
Early in June, the insects hatch from eggs, which wintered in the old bags attached to tree branches. As soon as the young worms appear, they start to spin the bags as a sort of coat. The bags are enlarged as they feed and grow. The caterpillars crawl part way out of the bags to feed. If disturbed, they retreat safely inside, and it is almost impossible to pull them out.
In August, the caterpillars attach the bags to a branch, turn around so they're head down, and pupate for about four weeks.
Emerging as moths, adult wingless females never leave the bag and are fertilized by the winged male. The eggs are laid in the bag where they pass the winter. There is only one generation each year.
If only a few small trees or shrubs are infested, bagworms can sometimes be controlled by picking off the bags and destroying them. This method is most effective during the winter and early spring because it destroys the eggs before they hatch.
Pesticide application recommendations are included in the Purdue publication.
Bagworms eat deciduous and evergreen trees. Some of the more common evergreen host plants include arborvitae (Thuja), fir (Abies), hemlock (Tsuga), juniper (Juniperus), pine (Pinus) and spruce (Picea). Deciduous host plants include black locust (Robinia pseudoacacia), honeylocust (Gleditsia triacanthos), sweetgum (Liquidambar styraciflua) and sycamore (Platanus occidentalis).
