BYGLers reported during yesterday’s Tuesday morning BYGL Zoom Inservice that overwintered Bagworm (Thyridopteryx ephemeraeformis) eggs are hatching or have hatched in Ohio meaning the “bagworm season” is now underway. Look closely at trees and shrubs festooned with last season’s bag abodes to detect newly hatched bagworms.
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.
However, bagworms have a history of “now you see them, now you don’t,” but eventually “now you see defoliated brown shrubs!” Thus, successful management to avoid plant damage must focus on early detection regardless of what happened last season.
Bagworm Behavior and Detection
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.
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 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 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. 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. 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. However, this approach can be problematic.
There’s a general rule with using insecticides that the bigger they are, the harder 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 considered in planning bagworm 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.
The Passive Approach
Biological Control by Connecting-the-Dots
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.
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, an effective insecticide-free long-term bagworm management strategy is to simply plant flowering plants that attract pollinators including parasitoid wasps.
If you plant them, they will come.
Selected References (By Published Date)
Davis, D.R., 1964. Bagworm moths of the western hemisphere (Lepidoptera: Psychidae). Bulletin of the United States National Museum.
Sheppard, R.F. and Stairs, G.R., 1976. Factor Affecting the Survival of Larval and Pupal Stages of the Bagworm, Thyridopteryx Ephemeraeformis (Lepidoptera: Psychidae). The Canadian Entomologist, 108(5), pp.469-473.
Moore, R.G. and Hanks, L.M., 2004. Aerial dispersal and host plant selection by neonate Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae). Ecological Entomology, 29(3), pp.327-335.
Ellis, J.A., Walter, A.D., Tooker, J.F., Ginzel, M.D., Reagel, P.F., Lacey, E.S., Bennett, A.B., Grossman, E.M. and Hanks, L.M., 2005. Conservation biological control in urban landscapes: manipulating parasitoids of bagworm (Lepidoptera: Psychidae) with flowering forbs. Biological Control, 34(1), pp.99-107.
Rhainds, M., Régniere, J., Lynch, H.J. and Fagan, W.F., 2013. Overwintering survival of bagworms, Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae): influence of temperature and egg cluster weight. The Canadian Entomologist, 145(1), pp.77-81.