Look closely at trees and shrubs festooned with last season’s Common Bagworm (Thyridopteryx ephemeraeformis) bag-abodes. Overwintered bagworm eggs are hatching in southwest Ohio meaning the “bagworm season” is now underway.
The 1st instar caterpillars of this native moth are very small with their bags measuring around 1/8" in length. The 1st instar bags are constructed with pieces of tan to reddish-brown sawdust-like frass (excrement) stuck to the outside of silk and look like "dunce caps."
As the caterpillars mature, they begin weaving host plant debris into the silk which provides structural stability as well as camouflage. Indeed, this behavior makes bagworms one of the sneakiest general defoliators found in Ohio landscapes. Heavy infestations are commonly overlooked until the caterpillars have produced substantial feeding injury.
The overwintered eggs hatch within the female bags from last season. A percentage of the 1st instar caterpillars will crawl from the old bags and produce a strand of silk to catch the wind and "balloon" the tiny caterpillars to new locations. This behavior is one of the reasons bagworms often appear on hosts that were not infested last season. However, heavy rain and high winds quickly destroy the delicate silk strands.
Although bagworm caterpillars may waft in on the wind to establish new bagworm beachheads, looking closely at trees and shrubs with last season’s bags is a good way to detect this season’s crop of bagworms. A single female can produce 500 - 1000 eggs meaning that populations can climb rapidly. Just a few females from last season can spawn damaging numbers of caterpillars this season.
Pay close attention to deciduous trees and shrubs as well as evergreens. It is a common misconception that bagworms only eat evergreens. In fact, they are called "evergreen bagworms" in many southern states.
However, the caterpillars may be found feeding on over 130 different species of deciduous trees and shrubs. Overlooking deciduous trees and shrubs during bagworm inspections allow infested plants to become reservoirs for infestations to spread to neighboring host plants.
Bagworm eggs may hatch over an extended period and eggs on the south side of an infested plant usually hatch earlier than those that are shaded on the north side. Consequently, it's common for 1st, 2nd, and sometimes 3rd instars to be present at the same time; bagworm caterpillars develop through 7 instar stages. This needs to be taken into account in planning management strategies.
For example, early instar bagworms are highly susceptible to the naturally occurring biological insecticide Bacillus thuringiensis var. kurstaki (Btk) (e.g. Dipel, Thuricide, etc.). Caterpillars are much less susceptible once bags surpass 2/3" in length. It's appealing to use Btk products because they do not kill bio-allies such as predators and parasitoids that help provide natural control of bagworm populations.
However, Btk products have two limitations. The active ingredient must be consumed to kill caterpillars and the products have relatively short residual activity. Thus, timing is critical; products should not be applied before the eggs hatch. Even with proper timing, two or more applications may be required to cover the extended egg hatch. Of course, once bags exceed 2/3" in length, standard insecticides will need to be used to suppress heavy infestations.