Bagworms an unwelcome neighbor
“All right then, what’s so funny?”
We were sitting across from one another at the kitchen table when Deb started to chuckle, a curious little laugh that seemed half giggle and half a wistful sigh. In her hand was a page turned in by an earnest young man from her 10th grade English class. The straightforward early September assignment had been to write a few paragraphs describing the place where you live.
His home, as he presented it, was situated at the quiet end of a street in town, one of several houses on a little cul-de-sac — which he referred to as a “cuddle sack.”
Stretching the point, you might consider the cocoon of a typical moth as a sort of protective cuddle sack within which the final caterpillar instar (stage) undergoes pupation into a reproductive adult.
An all-too-commonly encountered cocoon for those with heavily infested trees on their property is that of the bagworm. Though actually the larva of a moth rather than a worm, it does indeed live out its entire life within a bag, which in itself is a curiosity. Whereas most moths only spin a cocoon of silk in which to pupate after the last of a series of larval instars, bagworms surround themselves in a silken case on their first day out of the egg.
As the caterpillar grows, it enlarges the cocoon, covering its exterior initially with tiny bits of detritus and later on with pieces of the vegetation on which it’s been feeding. This added layer of plant material serves several useful functions, the most obvious of which is protection from predators.
Not only does the reinforced, fort-like cocoon protect its resident from the biting jaws of many would-be predators, its exterior — constructed of foliage from its host tree — also does a surprisingly good job of camouflaging the bag, which can grow to two inches by summer’s end. Dangling from the branches of a juniper like miniature spindle-shaped Christmas tree ornaments, they are often mistaken for reproductive structures, like the cones on a spruce tree.
As an added plus, the structure’s insulating properties also increases the temperature inside the cocoon, speeding up the larva’s rate of development.
There are some 1,000 species of bagworms, all of which spin ultra-thin silk fibers (quite different in structure from the commercial silk produced by silkworms) from modified salivary glands. One of the most commonly encountered species in the eastern states is the evergreen bagworm, Thyridopteryx ephemeraeformis.
As suggested by its common name, the species is most often encountered on coniferous trees and shrubs, especially arborvitae and junipers, although they have also been found feeding on a wide variety of deciduous trees.
The one-year life cycle of the evergreen bagworm begins when the tiny caterpillars hatch out of their eggs in late spring-early summer. Pinhead tiny as they are, this is nonetheless the time of dispersal. Hanging on to a twig, they spew a length of silk into the air. These first instar larvae are so light that even a gentle breeze can catch the silk and carry them aloft for quite a distance, hopefully to another tree.
In a lovely bit of biological wordplay, the process is called parachuting or ballooning.
The rest of the summer is given over to feeding on the buds and needles of evergreens or chewing holes in the leaves of deciduous trees. Heavy infestations can defoliate a tree, slowing its growth and even killing it in severe cases.
In the fall, the larvae stop feeding, firmly affix their cocoons to a branch and pupate into reproductive forms. After four weeks, stubby dark brown males with transparent wings emerge from their bags to fly (at night) in search of the wingless females, who remain in their cocoons.
After fertilization, the female lays several hundred to a thousand eggs within the protective confines of her bag, emerges and dies. Once the young hatch out, empty cocoons can remain on a tree for several years, making an infestation look worse than it may actually be.
Light infestations can be handled by scissoring off the bags where they attach to a branch and disposing of them in the trash. (Bags simply dropped to the ground still contain viable eggs.) For a heavier infestation, larval stages can be addressed with various biological or chemical pesticides. The timing of application for these treatments, however, is critical, so contact the County Extension office or visit https://extension.osu.edu/ask-an-expert for advice.
Oh, and cul-de-sac, whose original meaning in French was “bottom of the sack,” first was used in English to refer to a dead-end street in the 1800s. J.R.R. Tolkien purportedly named Bilbo’s house Bag End to poke fun at his fellow Brits’ affection for adopting French phrases for everyday concepts.
Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to email@example.com.