Bagworms are moth larvae (caterpillars) that develop within silk bags festooned with pieces of their host plants. They never leave their bags throughout their larval development. The caterpillars attach their bags to their plant hosts with a small stand of silk and extend their bodies a short distance out of the bag to feed.
Bagworm populations were on a rising trajectory from year to year in Ohio throughout the late 2010s peaking in 2020. However, the caterpillars were almost a no-show in 2021, 2022, and last year. Bagworms have a history of “now you see them, now you don’t,” but eventually “now you see defoliated brown shrubs!”
Did you know? Bagworms may feed on over 125 species of evergreen and deciduous woody plants in 45 plant families. The Entomological Society of America’s approved common name for the species is “Bagworm.” However, they are commonly called “evergreen bagworms” in the southern U.S. owing to the dominance of evergreens and thus the high likelihood of finding the caterpillars feeding on evergreen hosts.
In Ohio, it’s important to pay close attention to both deciduous trees and shrubs as well as evergreens. Overlooking deciduous woody plants during bagworm inspections allows infested plants to become reservoirs for infestations to spread to neighboring host plants, sometimes spreading from deciduous hosts to evergreens.
As the caterpillars mature, they begin weaving more and more host plant debris into the silk which provides structural stability as well as camouflage. This behavior makes bagworms one of the sneakiest general defoliators found in Ohio landscapes.
The overwintered eggs hatch within the female bags from last season. A percentage of the 1st instar caterpillars will produce a strand of silk to catch the wind and “balloon” the tiny caterpillars to new locations. This behavior is a key reason bagworms often appear on hosts far from trees and shrubs that were infested last season.
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.
Stopping bagworms from producing noticeable damage may include both passive and active approaches to management. The active approach is short-term while passive management is long-term.
The Active Approach
An active approach is to closely inspect susceptible trees and shrubs and apply properly timed insecticides. There’s a general rule with using insecticides that the bigger they are, the harder to kill. This is certainly true with bagworms. 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.
The Passive Approach
A passive approach to bagworm management focuses on increasing the diversity of flowering plants in a landscape. It’s well documented that a wide range of enemies of insect pests are fueled by nectar. They are pollinators as well as predators or parasitoids. In other words, an effective long-term bagworm pest management strategy is to simply plant flowering plants that provide nectar.
