The vast majority of Common Bagworm (Thyridopteryx ephemeraeformis) caterpillars in southwest Ohio have initiated their annual "tie-off" in preparation for pupation. Bags are tightly closed and tied with silk to a twig or other anchorage point. Likewise, male bagworms in the northwest part of the state have tied-off; however, some female caterpillars are continuing to feed.
Of course, we're painting with a broad brush. It's common at this time of the year to find a number of much smaller male and female bagworms that are actively feeding. They were the result of eggs that hatched a bit later, presumably because the overwintered eggs were kept cooler owing to their location on host plants. However, these late bloomers usually represent a relatively small percentage of the overall population.
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 eventually pulling 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 the females. Second, male pupae look much like other moth pupae and the resulting male moths have wings and are highly 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 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.
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.
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 is 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.
Even if properly timed insecticides were effective, a percentage of the caterpillars exposed to a toxic dosage will 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, the infested plants should still be closely monitored next season. There is simply no way to gauge insecticide efficacy based on the continued appearance of bags on sprayed plants. They may contain dead caterpillars or females caterpillars that pupated early and will produce eggs.