Evergreen Bagworm

Spring is a time of awakening when the landscape is green again, flowers are blooming, birds are making their way back home and the bagworms are hatching. One of the banes of homeowners, landscapers, and arborists is the bagworm. Tree branches decorated with spindle-shaped ornaments are not exactly what we had in mind when we planted our trees. So, what is a bagworm exactly? To me, they are both irritating and fascinating.

In our region, the most common one you will encounter is the Evergreen Bagworm, but Worldwide there are over a thousand different species. They go by several common names including Common Basketworm, North American Bagworm, Bag Moths, and my personal favorite Cuddle Sacs. This reminds me of the term Cul-de-sac, which is French for “bottom of the sack.” What an apt nickname for this unique little insect.

Their lifecycle begins in spring when the eggs hatch within the bag and leave the security of their winter home. Many will remain on the host tree, while others spin a thin thread of silk and wait for the wind to carry them to a new host tree or plant nearby. This phenomenon is called ballooning or parachuting and there is another animal that also uses this mode of transportation, spiders. Do you remember the end of Charlotte's Web when all of Charlotte’s babies, except one, ballooned away to new homes leaving Wilbur behind? Is there a quicker way to arrive at a destination than flying?

After arriving at their new host plant, the tiny larvae will spin a silken cocoon around itself and begin decorating it with bits and baubles found on the host. This sack will effectively camouflage the tiny larvae inside protecting it from predation. Now their only job is to eat and grow. The larvae stick their head and thorax out of the bag to munch on leaves and can retreat quickly if danger is nearby.

If you watch a bagworm closely you can see it move as the larvae wiggle around inside. They can move about freely carrying their sack with them, an insect version of a mobile home. Often we do not know we have a bunch of bagworms using our evergreens (or other trees) as their all-you-can-eat buffet until mid to late summer when the bags have grown exponentially and are now visible. They almost seem to appear overnight! The sacks have somewhat pointed ends, one end is for feeding, one end is for pooping. By August or September, they are ready to attach themselves permanently to their host tree and will use surprisingly strong silk to do so. If you have ever tried to remove a bagworm from a tree branch you can attest to just how strong this silk is, as it takes some effort to pull it free.

Everyone loves a love story, in the days of legends and myths there was a beautiful princess by the name of “Psyche”, and she fell in love with the son of Venus who as most of us knows was “Cupid”. Venus did not approve of this love relationship between her son and his beloved Psyche. Venus made life miserable for the poor princess. After some time, Venus felt guilty for her ill treatment of Psyche, and it was within her power to grant a great gift upon her, this gift was“immortality” Many moths are in the family of Psychidae and are called Psyche moths, which is a perfect name for moths believed to be the spirit of the lovely Princess Psyche.

At this point, they will pupate inside their bags and appear a few weeks later as adults. This is where things get really interesting. Males leave the sack and seek females who are still securely sequestered in their bags. Females cannot fly, they lack wings altogether. Therefore, the male must go to them. Some females will leave the bag and mating will occur after which she returns to the safety of the bag.  Although many more remain in the bag where mating takes place in what I can only imagine is a very awkward moment. The male then flies off in what I would describe as love them and leave them sort of way. Mother Nature, however, has a plan for our tiny Don Juan, he is destined to die shortly after mating. The female still stuck in her bag, having never seen the world like her lousy boyfriend, will lay up to a thousand eggs, and then she too will perish. The eggs overwinter in the bag and the cycle starts over again in the spring. Some females do not lay eggs, instead the larvae emerge directly from her body come spring. No one said life was fair. If all of this isn’t strange enough, in some species the females will lay eggs that are fertile without male fertilization in a phenomenon known as parthenogenesis. Saves the female those awkward love them and leave them moments.

Male Evergreen Bagworm---photo by K. Leeker

The most common bagworm in our region is the Evergreen Bagworm, and they are often found on juniper, cedar, and pine, although other hosts will be used as well such as sycamore, box elder, maple, and locust. For the past several years they have been on my St. John’s Wort plant.

For most of us having bagworms on our trees is just unacceptable and unsightly. I can certainly understand that feeling. Fortunately, the vast majority of the time they cause no lasting harm, in those cases your tree just may not be as aesthetically pleasing to look at. There are a few populations, however, that can be a serious problem. In those situations, when a severe outbreak is occurring, control will need to be used. If the problem is within the normal range handpicking the sacks off will be effective, although time-consuming. In the case of a severe infestation, you will need to use chemical sprays that contain Bt. Be sure to implement your control measures at the right time of the year for the most effective result. Whether handpicking or spraying they need to be removed in early to mid-winter. This is when the eggs are secured in the bag, therefore you can kill them before the eggs hatch. If handpicking, do not drop them to the ground, they will still hatch and find another host plant nearby. The bags must be destroyed to effectively control them

Besides ballooning, these tiny moths can spread by other means. Remember me saying that some females do not lay eggs, but rather the eggs remain inside the mother and the larvae emerge later from her body? Birds find them a tasty treat, and they will seek out these egg-laden females for food. The eggs have a very hard shell for an insect and often pass through the digestive system of the bird and remain unharmed, effectively spreading them far and wide, only to hatch and create a new generation. Not only do birds find them tasty and a great source of protein so do some people around the world. In primitive villages, the bagworms are encouraged to grow and then later harvested for a protein-rich snack.

If you decide the little spindle-like ornaments hanging from your trees are just not fitting in with your design vision, I recommend hand-picking as a means of an environmentally safe control option. If, however, your infestation is too severe chemical measures will need to be taken, just remember to follow directions exactly as given. While we are all busy cursing and controlling the dreaded bagworm, maybe we can take a few minutes to appreciate the uniqueness of the cuddle sack and its lifecycle.