Common bagworms (Thyridopteryx ephemeraeformis, family Psychidae) are native moth caterpillars that live in silk bags festooned with plant debris. It’s the perfect camouflage allowing them to remain undetected until revealed by their voracious appetites.
The caterpillars are summer pests; however, their bags remain visible through the fall, winter, and spring. Indeed, old bags can linger for years.
The “bagworm season” in southwest Ohio began around the end of May when overwintered eggs hatched. You can read the BYGL Alert heralding the event that was posted on June 1 and titled, “Bagworm Eggs are Hatching: The Game’s Afoot!” by clicking this hotlink: https://bygl.osu.edu/index.php/node/1795
The bagworm season ends when 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.
This means the plant damage caused by the caterpillars is drawing to a close. It also means that it's far too late for insecticides to be effective. However, it's prime time to deplete populations by plucking and destroying the bags containing quiescent caterpillars, pupae, and later dead females with live overwintering eggs.
Life in a Silk Sock
Both male and female caterpillars spend their entire larval development inside their silk bags. The bags have two openings at opposite ends. The large upper opening is used by the caterpillars to poke their heads out to feed and enlarge their bag abodes. They avoid tumbling from their plant hosts by anchoring their bags to their plant hosts with a small bit of silk.
The lower opening serves as a toilet; it allows the caterpillars to shove out fecal pellets (frass). Otherwise, their bags would gradually become loaded down with frass and the bag-o-poo would eventually pull caterpillars from their hosts towards extinction.
The male and female caterpillars take very different paths as they develop. Male caterpillars, pupae, and bags are much smaller compared to females. Male pupae look much like other moth pupae and the resulting male moths have wings and are capable fliers. Their wings lack scales causing them to superficially resemble dark-colored flies.
The males also have large antennae used to detect and track the eau de l’amour wafting from the female bags. Male bags will soon be identifiable by pupal skins sticking out of the bottom of the bags.
The adult females never develop into a moth. They remain inside their bags and develop into something that looks more worm-like than moth-like. The mature bagworm female moths have no wings, no apparent mouthparts, no antennae, and three pairs of very short, dysfunctional legs. Her abdomen terminates in an ovipositor (egg-laying structure) used for depositing and packing her eggs into her pupal case which is her main function in life.
The mature females emit a chemical attractant (sex pheromone) that draws in the males and mating occurs with the females remaining in their bags. Soon after mating, the female produces overwintering eggs that are laid snug inside of their mother's old bag. Each female is capable of producing 500 – 1,000 eggs which explains why populations can build rapidly.
A Host of Hosts
Although common bagworms are sometimes called “evergreen bagworms, it's a common misconception that bagworms only eat evergreens. In fact, the caterpillars can feed on over 130 different species of plants including a wide range of deciduous trees and shrubs.
Feeding symptoms on deciduous hosts are sometimes overlooked or mistaken for damage caused by other general defoliators. However, overlooking bagworms on deciduous trees and shrubs allows the plants to become reservoirs for infestations to spread to neighboring host plants.
It's too late for insecticide applications to have any meaningful impact. The vast majority of the plant damage has already occurred. Also, the bagworms are out of the reach of insecticides once they sequester themselves in their bags.
The only control option that is currently effective is to pick off and destroy the bags which will eliminate the females and later the overwintering eggs. This control method remains effective throughout the fall, winter, and spring. Bags should be destroyed rather than simply being dropped to the ground because eggs will still hatch from bags on the ground.
Since females don't fly, early bagworm infestations are often concentrated on a few plants; sometimes just a single plant. This is why it's important to concentrate your bagworm-plucking efforts on point-source plants, so those plants don't support a much wider infestation next season. It’s also important to look around to find bagworms attached to structures.
There is a wide range of insecticides that kill bagworms; however, it's common for caterpillars that are exposed to a toxic dosage to tie their bags to their hosts before succumbing. It's deceptive because the bags containing dead caterpillars look just like bags with live caterpillars that pupate and carry on the population next season.
If you sprayed earlier in the season but still see bags hanging on the targeted plants, you should check things out by opening a few bags. An alternative approach is to simply give the bags a tight squeeze to see if the essence of bagworm is exuded. Of course, finding live bagworms means management must focus on plucking-bagworms.
Long-Term Control: The Power of Pollinators
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.
A scientific paper published in 1976 (see Selected References below) 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 ofI. conquisitorexceeded 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 long-term bagworm pest management strategy is to simply plant flowering plants that attract pollinators including parasitoid wasps.