You see, bagworms (Thyridopteryx ephemeraeformis) create these pretty amazing bags that are virtually impenetrable. Once the bag is fully formed they must be squished (technical term I’m sure) to be killed.
So, unless there is only a small quantity of bagworms terrorizing your tree or you plan on standing in your yard for hours on end squishing them (technical term again) – you’d better know when you can spray them with an insecticide. This requires a bit of bagworm life cycle knowledge.
In the early fall, the male bagworm emerges as a moth and goes in search of female bagworms. Female bagworms never leave their bags nor do they turn into moths.
In fact, according to the Department of Entomology at Penn State, the maggot-like female lacks eyes, functional legs, mouthparts, wings and, after mating, lays up to 1000 eggs inside her bag. Then she says I’m done, you kids are going to eat me out of house and home. So, she leaves the only home she’s known and dies (exaggerated a bit but she does die at the end of the story sooo…). Seriously gets the short end of the deal – am I right ladies? I mean, you’d think she could at least get a pair of wings…but anyway…
The eggs will remain in the bag through spring when they will then begin to hatch. We typically see this happen in the St. Louis area anywhere from mid-May to mid-June or at about the time Catalpa trees are in bloom.
As the larva emerges from its cocoon, it will begin to form its own bag. The new bag allows the larva’s head and legs to be free which create the perfect living environment for them to munch freely on your plants. As the larva grows, so does the bag.
The larva will feed for about six weeks. Once they have finished their meal (i.e. eaten your entire tree) they will use the silk that their bags are made from and float on the breeze to find a new food source and home. Your neighbor’s bagworm problem? It could become yours.
Around mid-August the bagworm is done feeding and will attach itself to a twig, close it up and pupate. The following spring will come around and the new generation will follow the footsteps of the old.
So, the best time to spray for bagworms? From mid-May to mid-June. The time frame can vary though depending on the weather in the spring, so your best bet is to start looking at your problem areas in mid-May and keep watching until you see the larva.
Bagworms only produce one generation a year, but that generation can do some serious damage in a relatively short time frame. While they do tend to be more troublesome on evergreens, especially Arborvitae and Juniper, no plant is completely impervious. Deciduous trees and shrubs can often regenerate new growth but the damage to evergreens, if not managed properly, is often irreversible.