The Bagworm – It Builds a House and Comes To Eat Your Plants

I came across an old acquaintance today while walking near Arcadia,FL, the bagworm. This curious caterpillar of a moth with an unpronounceable name, Thyridopteryx ephemeraeformis , has a very strange habit that makes it very distinctive and easy to identify. As its name implies it makes a case or bag out of silk and bits of plant material and crawls around with only its head and front end exposed. As you can see from the first photo, the bag is an ingenious contraption of small pieces of stems and leaves of the plant that the caterpillar is feeding on. In this case the plant was a hog plum, but this moth feeds on a wide variety of plants, causing it to be a serious pest in some situations. Clearly the bag serves to protect the caterpillar in two ways; it is both a camouflage and a physical protection against attack by predators such as birds. However the caterpillar must come at least partly out to feed and can be attacked then (see second photo taken from the internet).

The breeding habits of the bagworm are quite peculiar- the female is flightless and remains in the case for her entire adult life. The adult male (see photo from the internet) must fly to her location and fertilize her eggs, which are laid in the bag where they remain all Winter. They hatch in the Spring and the young larvae disperse and make their first bags. Although this species is named for a mayfly (which the adult male slightly resembles), there is a greater resemblance to the caddisfly larvae which lives in water but also constructs a case out of sticks, sand or small pebbles. Now although caddisflies and bagworms are somewhat related, they are in separate groups and it seems unlikely that a common ancestor had the habit of constructing larval cases. So we may conclude that their shared habit of constructing cases for their larvae is probably an example of separate but convergent evolution.

So if you see bagworms on one of your valued plants you might just want to move them someplace else since they can do some serious damage. The simplest time to do this is in Winter when the bags contain hundreds of eggs and can easily be picked off the plants. But in any event let us marvel once again at the remarkable diversity of nature.

Bill Dunson, Englewood, FL, and Galax, VA
wdunson@comcast.net

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