Bagworms (Thyridopteryx ephemeraeformis, family Psychidae) are moth larvae (caterpillars) that develop within silk bags festooned with pieces of their host plants. As the caterpillars mature, they begin weaving more host plant debris into the silk, which provides structural stability as well as camouflage.
The disguised bagworms are one of the sneakiest general defoliators found in Ohio landscapes. Heavy infestations may be overlooked until the caterpillars have produced substantial feeding injury.
The caterpillars never leave their bags throughout their larval development. They feed by using a small strand of silk to attach their bags to their plant hosts, and then they extend their bodies a short distance out of the bag to consume foliage.
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 simply “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.
A helpful strategy for detecting a bagworm infestation in the early stages of development at this time of the year is to look for bags left from last summer. Both males and females attach their bags to their hosts with strong bands of silk at the end of the season.
Male bagworms develop into moths in late summer. Female bagworms also mature in late summer, but they never acquire a form that resembles a moth, and they never leave their bags. The females mate with the mobile males while inside their bags. Once mated, the female produces eggs that fill her body, and she dies. The eggs are the overwintering stage.
The overwintered eggs hatch within the female bags from last season. An excellent phenological indicator that bagworm eggs are beginning to hatch is the full bloom of northern catalpa (Catalpa speciosa).
First instar caterpillars of this native moth are very small with their bags measuring around 1/8" in length. The bags are constructed with pieces of tan to reddish-brown sawdust-like frass (excrement) stuck to the outside of silk. First instar caterpillars tend to hold them upright, looking like tiny "dunce caps."
A percentage of the 1st instar caterpillars will produce a strand of silk to catch the wind and "balloon" the tiny caterpillars to establish new bagworm beachheads. This behavior is a key reason bagworms often appear on hosts far from trees and shrubs that were infested last season.
A single female can produce 500 - 1000 eggs, meaning that populations can climb rapidly with just a few caterpillars arriving on the wind. The “founder effect” with bagworms often translates into hotspots. These can be seen both within infested plants as well as between infested plants.
Immediate Management
Insecticides are an effective option at this time of the year to prevent damage. However, their use can be problematic. There is a risk of killing non-target arthropods that help keep bagworms as well as other pests such as mites and scale insects in check.
The risk can be reduced by selecting “biorational” insecticides that have a narrow target, preserving beneficial bio-allies. Active ingredients that are considered biorational include, but are not limited to, azadirachtin (e.g., Azatrol), spinosad (e.g., Converse), Bacillus thuringiensis var. kurstaki (Btk) (e.g., Dipel, Thuricide, etc.), and chlorantraniliprol (e.g., Acelepryn).
The efficacy of these and other insecticides is subject to the general rule that the bigger they are, the harder to kill. In general, the smaller early-instar bagworm caterpillars are easier to kill compared to the larger later-instar caterpillars.
However, bagworms commonly have asynchronous development. Overwintered eggs may hatch over an extended period with eggs on the south side of an infested plant hatching earlier than those that are shaded on the north side. It is common for a wide range of bagworm instars to be present at the same time, as illustrated in the image below. The bagworms were collected at the same time from the same shrub.
Thus, it is important to read and follow label directions regarding mixing rates and residual activity. Applying at a lower rate than recommended on the label may only kill small caterpillars. If the residual activity does not protect plants the entire time eggs are hatching, multiple applications may be required. For example, Btk products have short residual activity, meaning that two or more applications may be required to cover the extended egg hatch.
Effective Long-Term Management
A long-term 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. Many 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 to bagworm enemies.
Common bagworms are commonly targeted by an array of predators, parasitoids, and pathogens (the 3-Ps). I've observed bagworm bags ripped open by baldfaced hornets (Dolichovespula maculata) to extract the caterpillar meat morsels inside. I’ve also commonly observed these wasps visiting flowers. They are pollinators and predators.
A scientific paper published in 1976 showed that the parasitoid wasp, Itoplectis conquisitor (family Ichneumonidae) accounted for almost 76% of the parasitism observed on common bagworms in the study. This parasitoid wasp commonly visits flowers to sip nectar. It's a pollinator and a parasitoid.
A study published in 2005 showed parasitism rates of I. conquisitor exceeded 70% in bagworm-infested plants that were next to a central flower bed, but less than 40% in infested plants with flower beds further away. In other words, an effective insecticide-free long-term bagworm management strategy is to supply parasitoid wasps with nectar by planting flowering plants.