The relatively mild weather we’ve been having this fall has been wonderful for hiking. Unfortunately, it’s also good weather for outdoor chores, so I don’t have freezing cold and snow as excuses to let the windows go unwashed. As I brushed off spider webs, I noticed small brown somethings stubbornly stuck to the windows and frames. The objects were each a tiny cluster of dead plant stems formed into a cylinder and glued to the surface with a circle of white at one end.
This jogged a memory, and I thought back to our moth workshop last summer. Kyle Johnson, Minnesota Biological Survey (MBS) Entomologist, found an impressive number of caterpillars and moths for us to learn about in just 24 hours. During that workshop, at the end of July, we spotted one of these little brown cases on the surface of a leatherwood leaf. Kyle explained that they were movable camouflage for the larvae of bagworm moths. I stored that information away for future use.
The future is here! I snapped some photos of the bagworm cases before scraping them off the windows with my fingernail, and scrubbed the glass clean. As I started investigating the bagworms further, my searches turned up some dire-sounding warnings about how they can defoliate evergreen trees in your yard. The photos of the cases didn’t look quite right, though. Eventually, I discovered that there are two species of case-forming insects sometimes called common bagworms. One has the tongue-twister name of Thyridopteryx ephemeraeformis and is also known as the evergreen bagworm. Although they can be destructive to juniper and white cedar, they are native to North America. The other insect is Psyche casta, also known as the grass bagworm. This matched the cases I found much better! Both types of bagworms are the larvae of tiny moths.
While they don’t seem to cause problems, grass bagworms were introduced from Europe around 1931 and have spread from Boston across southeastern Canada, and as far west as Minnesota. Modern information about them is pretty sparse, but in 1934, when they were new and interesting, Donald W. Farquhar of Harvard University wrote a comprehensive observation of their unusual life cycle.
Farquhar observed the larvae hatching in June and July. Their first act is to spin a silken sleeping bag and then decorate it with bits of their surroundings. Grass bagworms use little straws of dry grass, which also creates excellent camouflage. This stage reminds me of caddisflies, which are unrelated aquatic larvae who also use silk and local materials to build a protective case. In both groups of insects, the cases are distinctive enough to help with identification. I had split open one of the cases after scraping it off my window, and I did find white silk lining the case of grass.
Once protected, the bagworm sticks their head and thorax out of the case to crawl around and feed. Although the evergreen bagworm seems to cause whole trees to turn brown, I think that grass bagworms just skeletonize patches on broader leaves. Farquhar reports that they eat grasses, mosses, lichens, and “other low plants.” He also observed them eating the scale insects who cause beech bark disease out East, and even cannibalizing their peers in a laboratory setting. As with quite a few moth species, they must get all of their eating done in childhood, because the adults have no mouths!
This larval childhood is also when they do their traveling. Sometimes, just after spinning their tiny silk bag, a larva will drop down on a thread and catch a ride on the wind, similar to a spiderling ballooning on their silk. Humans transport them even farther as we move the objects they call home. Even with tiny legs, walking places is an option, too. Full-grown larvae will climb up trees, buildings, fence posts, stone walls, and my first-floor windows to get ready to pupate and go through metamorphosis. Monarch butterfly caterpillars also go through this wandering phase.
According to Farquhar, all of that takes 11 months. The larvae spend the winter under rocks or in the crevices of tree bark. Fresh spring food allows them to grow faster, and by May they are ready to pupate and become adults.
Female bagworm moths have no wings. They emerge from their case, hold onto the bag, and in Farquhar’s words, she “liberates the attractant which summons males within perceiving distance.” In other words, she releases pheromones. Males, who do have strong wings, follow her scent upwind and then search on foot to find her. The two mate, and then the female reinserts her ovipositor into her empty case and squirts in about 150 eggs. She closes up the end of the case with white fuzz, then drops to the ground to die. The eggs hatch after a couple of weeks, and the cycle begins again.
Although I’ve noticed bagworm cases before, I didn’t know anything about their life cycle, or why I find their old cases on walls and windows instead of plants. Now I do! So I’m grateful that this warm fall weather allows me to check things off my to-do list.
Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.
For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is open with our brand-new exhibit: “Anaamaagon: Under the Snow.” Our Fall Calendar is open for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.
Last Update: Nov 20, 2024 9:47 am CST
