Overwintered Bagworm (Thyridopteryx ephemeraeformis) eggs are hatching in southwest Ohio meaning the “bagworm season” is now underway. Look closely at trees and shrubs festooned with last season’s bag abodes.

Bagworms are moth larvae (caterpillars) that develop within silk bags festooned with pieces of their host plants. They never leave their bags throughout the 7 stages (instars) of 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 as well as last year.

Of course, there continued to be isolated pockets with damaging populations throughout Ohio in 2021 and 2022, but widespread populations were confined to the northern part of the state. Thomas Dehaas (OSU Extension, Erie County (Sandusky)) reported on several occasions that bagworms were a continual problem in the counties bordering Lake Erie.

However, bagworms have a history of “now you see me, now you don’t, now you see brown shrubs!” Thus, successful management to avoid plant damage must focus on early detection regardless of what happened last season.

Bagworm Behavior and Detection

It is a common misconception that bagworms only eat evergreens. Indeed, they are frequently called "evergreen bagworms" in many southern states although “bagworm” is the Entomological Society of America’s approved common name for the species. However, the caterpillars may be found feeding on over 125 species of evergreen and deciduous woody plants in 45 plant families.

It’s important to pay close attention to deciduous trees and shrubs as well as evergreens. Overlooking deciduous trees and shrubs during bagworm inspections allow infested plants to become reservoirs for infestations to spread to neighboring host plants, sometimes from deciduous hosts to evergreens.

The 1st instar caterpillars of this native moth are very small with their bags measuring around 1/8" in length. 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 bagworm 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. 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 “founding” females from last season can spawn damaging numbers of caterpillars this season.

The “founder effect” with bagworms often translates into hotspots. These can be seen both within infested plants as well as between infested plants.

Management

Stopping bagworms from producing noticeable damage may include both passive and active approaches to management. A passive approach is to design landscapes that have a diverse range of flowering plants. But more on that later.

An active approach is to closely inspect susceptible trees and shrubs and apply properly timed insecticides. However, this approach can be problematic.

There’s a general rule with using insecticides that the bigger they are, the harder they are to kill. This is certainly true with bagworms. The smaller early-instar caterpillars are easier to kill compared to the larger later-instar caterpillars.

However, 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. This creates asynchronous development with bagworms meaning it's common for a wide range of instars to be present at the same time. This is illustrated in the image below and must 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.

Bagworms may also be managed with topical insecticides. However, those selected should have a minimal impact on the natural enemies of bagworms. There are at least 11 species of parasitoid wasps that have been found to attack bagworms. Baldfaced hornets (Dolichovespula maculata) are also a known nemesis of bagworms. Thus, so-called “bio-rational” insecticides are a good choice.

Biological Control by Connecting-the-Dots: 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. Thus, an effective long-term insecticide-free bagworm pest management strategy is to simply plant flowering plants that provide nectar.

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 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, if you plant them, they will come.

Ron Wilson

In The Garden With Ron Wilson