Most of the Common Bagworms (Thyridopteryx ephemeraeformis) I looked at yesterday in southern Ohio and central Indiana had initiated their annual "tie-off" in preparation for pupation. Bags are tightly closed and tied with silk to a twig or other anchorage point. It means the damage caused by these caterpillars wrapped in silk bags festooned with host plant debris is drawing to a close.
Of course, you may still find a few bagworms that are relatively small and continuing to feed which is presumably the result of extended egg hatch earlier this season. However, these late developers usually represent a small percentage of the overall population.
Life in a Rucksack
Both male and female caterpillars spend their entire larval development inside their silk-sock bag abodes camouflaged with host plant debris. The bags have two openings. The caterpillars poke their heads out of the upper opening to eat plants and enlarge their living quarters. Their bags are constructed from the bottom up which is why they are somewhat cone-shaped. The lower end is more constricted because it housed a much smaller caterpillar.
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. Imagine heavy frass-filled bags pulling caterpillars from their hosts towards extinction.
The male and female caterpillars take very different life cycle paths at pupation. Male pupae look much like other moth pupae. Adult males have wings and they are very capable fliers. Their wings lack scales causing them to superficially resemble dark colored flies. The males also have large antennae that they use to detect and track the "scent of the females." 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 body is creamy-white with only a few areas that are sclerotized (colored and hardened) and a band of tan colored hairs around her body toward the end of her abdomen. 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; 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.
It's a common misconception that bagworms only eat evergreens. Indeed, 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.
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 don't support a much wider infestation next season.
There are a wide range of insecticides that kill bagworms; however, they should have been applied much earlier in the season. Late instar bagworms can detect insecticide toxicants causing them to hasten pupation, but they do not die. However, they do stop feeding which leads to the perception they were killed. Early-pupating females produce fewer eggs, but they still produce enough to continue the infestation next season. The deception explains why damaging bagworm populations may recur the following season in landscapes where insecticide applications were made too late during the previous season.
NOTE: bagworm bags don't go away even if the caterpillars were successfully killed with an insecticide earlier in the season! I've gotten several e-mail messages this season describing bags still clinging to plants long after an insecticide application. The senders all assumed the insecticide failed and in a few cases, repeat applications were made.
A large percentage of the caterpillars tie their bags to their hosts before succumbing to an insecticide which means bags containing dead caterpillars remain attached. It's deceptive because they look just like bags with live caterpillars.