When can I spray bagworm?

Bagworms and When to Kill Them

Gardening for many is about the flowers, fragrance, and tranquility.  Unfortunately, there is a darker side of gardening that involves insects and fungi.  And although I wish I could only talk about the prettier side of gardening, alas, this is about gardening’s uglier side – Bagworms.


To be frank, bagworms are the nasty little maggot-like creatures that create the little dangling cocoons you often see on evergreens.  Now I know the title of the article seems a bit violent but if you long for your Arborvitae, Spruce, and Junipers to survive – knowing when you can kill them is of the utmost importance.


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.


What does a bagworm look like

Along with the silk they produce, bagworms use twigs and needles to create their bags.


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…



Young Bagworm

Young Bagworm



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.


Young Bagworm

Sneak peak of a young bagworm.


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.


What does a bagworm look like

Bagworm on Spruce


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.

Further Reading:

UK College of Agriculture, Food and Environment

Penn State College of Agricultural Sciences