Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Stay at Home Mom

Monday, March 1, 2010

As a baby, she will know freedom. She will feel the breeze on her skin as she floats on the air in search of her first meal. Soon she will settle down, build a house and remain there for the rest of her life. She will grow to adulthood in this house. She will mate in this house, lay her eggs and finally die in this house. Though she never sees her offspring, she is a true stay-at-home mom. She is the Evergreen Bagworm Moth.
The eggs of the bagworm moth spend the winter in their mother’s bag. Each egg may be one of a thousand laid by their mother last fall. Although she will never see her offspring, she has provided them the protection they need to survive our cold New England winters. She has lined her bag with a nice insulating layer of silk and sealed it from the elements. Without this protection, her eggs would not survive the cold; for unlike many insect eggs, they don’t contain any antifreeze and are susceptible to freezing.
When the eggs hatch in the late spring, each tiny caterpillar must go in search of a suitable tree on which to feed. This is one area where mother did not provide very well for her young. Many types of moths will fly from tree to tree, laying eggs on the right type of leaves. When the eggs hatch, the young caterpillars get right to work eating. Not so the bagworm. A stay-at-home mom doesn’t have that option. So it is up to the young caterpillars to seek out their first meal. At a quarter of an inch long, they are not able to walk the neighborhood looking for the perfect home. They must get there quickly, so they take to the air. Each caterpillar lets out a strand of silk which is caught by the air currents and carries it downwind. If all goes well, they land on a preferred tree and take up housekeeping. The preferred trees are evergreens; junipers, red cedars or arborvitaes. If they land on another type of tree, they may set out again, with another chance of landing in a good spot. Or they may settle for one of over 100 other, less choice trees. As time passes, however, they are less inclined to venture forth. In fact, the urge to settle down soon overcomes the urge to relocate to a better food source. In that case, the young bagworm will starve to death.

Once a caterpillar lands on an acceptable food source, she gets down to the business at hand – constructing her home. This is a silk lined bag decorated with bits of whatever foliage she comes in contact with. Only then, when she is secure within her bag does she start to feed. The bag is her only means of protection. Without it she is defenseless. bagworm imageShe will remain inside the bag while feeding, sticking her body out only far enough to feed. She will wear her bag as she travels to new feeding sites. As she grows, she will enlarge her bag so she always has room to retreat completely into the bag at the first sign of danger.

Bagworms will feed throughout the summer, reaching a size of about 1 to 2 inches. Sometime in August they will close up the ends of their bags and pupate. In a few weeks, the adult bagworms will emerge from their pupal cases. The female bagworm looks more like a slug than a moth. She has no eyes, no wings, no antenna and no mouth. Her final task in life is to prepare for the next generation of bagworms. Using a chemical attractant - a pheromone - she lures a male to her bag. The male bagworm has wings and antennae. He will leave his bag and fly around in search of the female. When he finds one, he will insert his abdomen into the females’ bag and mate with her. After laying up to 1,000 eggs, she uses her body hairs to plug up the opening of the bag, doing all within her power to help her young survive the coming cold weather. Sometimes the female will worm her way out of the bag before closing it up for the winter. She will fall to the ground, at last free of her house. Defenseless, hungry and completely spent from egg laying, she soon dies.
Other bagworms never make it that far. Some perish within the bag, entombed alongside their developing eggs.

I have occasionally found the bags of another type of bagworm - the grass bagworm - attached to the side of my house. These bagworms grass bagworm imagefeed on grass and seek out a vertical surface to attach their bags before pupating. What attracted them to my house, I have no idea, but for a stay-a-home mom, it is quite a journey.