This "bagworm season" which began in early June has been marked by damaging localized infestations throughout Ohio. Images showing heavy defoliation from bagworms (Thyridopteryx ephemeraeformis) were commonly shared during our weekly BYGL Zoom Inservices. Populations appeared to be higher than has been seen for several years.
Thankfully, a high percentage of the bagworm caterpillars in southwest Ohio have completed their annual "tie-off" in preparation for pupation. The caterpillars tie silk to a twig or other anchorage point and then close-up shop by tightly closing their bags.
This means the overall damage caused by the caterpillars wrapped in silk bags festooned with host plant debris is drawing to a close. It also means that while it's far too late for insecticides to be effective, it's prime-time to deplete populations by plucking and destroying the bags.
Life in a Tote Bag
Both male and female caterpillars spend their entire larval development inside their silk-lined bags camouflaged on the outside with host plant debris. 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. Don't mistake these temporary silk holdfasts for the more robust silk anchor-points used to secure the bags to their plant hosts for pupation.
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. First, male caterpillars, pupae, and bags are much smaller compared to females. Second, 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-like insect. 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
It's a common misconception that bagworms only eat evergreens. In fact, an alternate common name used in many southern states is "evergreen bagworms." However, 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 pickoff and destroy the bags which will eliminate the females. This control method remains effective throughout the fall, winter, and spring to destroy the eggs before they hatch. Bags should be destroyed rather than simply being dropped to the ground; 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-picking efforts on point-source plants so those plants don't support a much wider infestation next season.
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.
The bottom line is that if bagworms were sprayed earlier in the season, don't assume the insecticide application didn't work if plants remain festooned with bags. I sprayed a juniper in my landscape on July 27 but still have bags containing dead caterpillars hanging beneath the stems. That's why we recommend opening some bags a few days after an application to see if the caterpillars are dead or alive.