Common bagworms (Thyridopteryx ephemeraeformis, family Psychidae) are the caterpillars of a native moth. Like many lepidopteran caterpillars, bagworms can generate strands of silk from modified salivary glands. The silk is integral to the bagworm’s lifestyle.
The caterpillars are called “bagworms” because they wrap themselves in silk bags festooned with pieces of host plant material. The arrangement provides highly effective camouflage making bagworms difficult to spot. This presumably helps protect the soft-bodied caterpillars from becoming predator meat morsels.
At the end of the “bagworm season,” the caterpillars transition from life in a tote bag to life in a sleeping bag. The caterpillars tie silk to a twig or other anchorage points and then close up shop by tightly closing their bags to await pupation.
During the bagworm season, the caterpillars use silk to temporarily attach their bag abodes to their plant hosts as they feed. This means the caterpillars don’t need to hang on with their tarsal claws giving them the freedom to extend their bodies outward to feed. The anchor points also allow the caterpillars to retreat into their safe silk havens when threatened without the bags and caterpillars crashing to the ground.
However, bagworms will rappel to the ground on a silk thread before they complete their development if populations are high and they run out of leaf food. The bagworm in the image below isn’t levitating. It’s being held upright on my finger by a silk thread.
Common bagworms have been with us for a while. Overwintered bagworm eggs began hatching in southwest Ohio at the end of May (see “Bagworm Eggs are Hatching: The Game’s Afoot!,” June 1, 2022). However, it’s amazing how long these general defoliators can continue to crawl below our radar as they chomp on evergreen and deciduous trees and shrubs before their cumulative damage and size finally make them apparent.
Aside from being shrouded in camo, bagworms commonly go undetected in Ohio landscapes because landscape managers and homeowners may focus their inspections on evergreens. It is a common misconception that bagworms only eat evergreens. In fact, they are called "Evergreen Bagworms" in many southern states.
However, bagworms may be found feeding on over 130 species of host plants including deciduous trees and shrubs as well as evergreens. Bagworms may be so unexpected on deciduous woody ornamentals that the symptoms may be mistaken for damage produced by other defoliators. For example, the symptoms on European hornbeam (Carpinus betulus) shown in the images below were at first misidentified as Japanese beetle (Popillia japonica) damage.
The images below show a small crabapple (Malus sp.) being severely damaged. The caterpillars began to strip bark to feed on the sugar-rich phloem once they had defoliated the tree.
Female bagworms do not fly. Indeed, they never develop into an insect that even vaguely resembles a moth and they never leave their bags. Male bagworm moths do fly. They develop into winged moths and mate with the bagged females. Once mated, the female’s body fills with eggs, and they die.
The bagworms spend the winter as eggs inside dead females in their bags. A single female can produce 500 - 1000 eggs meaning that populations can climb rapidly. Just a few undetected females can spawn damaging numbers of caterpillars next season.
Bagworms have evolved a highly effective way to disperse given that the females never fly, and the caterpillars can only crawl so far. First instar caterpillars spin a strand of silk to catch the wind and "balloon" to new locations. This dispersal behavior coupled with their cryptic lifestyle why bagworms may seem to “appear out of nowhere” in landscapes on hosts that were not infested last season.
Of course, if a few female caterpillars remain undetected after they land from their balloon trip and complete their development, their location can become a “hot spot.” Future bagworm caterpillars don’t need to balloon, they can just crawl to greener pastures.
However, common bagworms have a serious temperature-dependent Achille’s heel. Research published in 2013 showed that the lethal temperatures for 50% of egg clusters ranged from 6.8F for clusters weighing 0.1 grams, and -0.58F for clusters weighing 0.4 grams.
This helps explain the historical geographical range for common bagworms in Ohio. When I first came to Extension 30 years ago, bagworms were rarely found in Ohio north of east-west I-70. However, bagworms now range well into Michigan as well as Ontario, Canada. Global worming?
Biological Control by Connecting-the-Dots: It’s well documented that a wide range of enemies of insect pests are fueled by nectar. 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 (Dolichovespula maculata) to extract the caterpillar meat morsels inside. I’ve also commonly observed these wasps visiting flowers.
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. As with many enemies of other insects, this parasitoid wasp commonly visits flowers; it’s a pollinator.
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.
Physical Removal: If the bagworms are within easy reach, they can be effectively controlled through physical removal. However, the caterpillars must be killed by squeezing or stomping on them once they are plucked from an infested plant. If simply dropped to the ground, they will crawl back upon a host plant. The images below illustrate the “Bagworm 3-Step Management Program” which provides a certain amount of satisfaction in dispatching the troublesome caterpillars. Thus far, no populations have become resistant.
Plucking and then stomping overwintering bags is also effective in eliminating the eggs inside the dead females. Indeed, it’s always a good idea to give infested trees and shrubs a thorough close inspection during the winter months to find bags that made it through the summer caterpillar dance.
Insecticides: Although insecticide applications can be highly effective, they can also be problematic. As shown in the image below, caterpillar development may be asynchronous perhaps owing to whether the eggs were located on the north or south-facing side of the plant host. All of the bags were collected from the same host on the same day. Bagworm caterpillars in the early instar stages are the most susceptible to topical applications of certain products.
Spraying for bagworms also presents another challenge. Dead bagworm caterpillars may not drop from their host, so a close inspection is required. The assumption that hanging bagworms are alive may trigger another application. Of course, they can only be killed once.
Soil applications of systemic insecticides offer another effective management option. These include the noenicitinoid dinotefuran (e.g., Safari, Transtect) as well as the ryanoid chlorantraniliprole (e.g., Acelepryn) and the organophosphate acephate (e.g., Orthene, Lepitect). The systemics provide much longer control compared to topical applications, so their efficacy is less affected by asynchronous larval development. However, they must be applied much earlier. They are not useful as rescue treatments because it takes time for the active ingredients to translocate to the foliage.
Tim Gibb and Cliff Sadof (Purdue University, Extension Entomology) provide a table showing effective insecticides including biorationals and systemics in their online publication title, Landscape and Ornamentals, Bagworms. You can access the publication by clicking on this hotlink: