Bagworms are a common pest found on arborvitae, causing considerable damage if left unchecked. These caterpillars live in spindle-shaped bags and feed on tree foliage, often leading to defoliation.
Arborvitae, along with other plants such as junipers, cedar, and pine, are susceptible to bagworm infestations. Keeping an eye on these pests is crucial for maintaining your plants’ health.
A notable characteristic of bagworms is their protective, spindle-shaped bag made from silk and bits of foliage, which enhances their camouflage.
These caterpillars will carry their bags while moving and feeding on foliage until they reach the end of their growth, which usually happens towards the end of summer.
Identifying and treating bagworm infestations early can prevent significant damage to your arborvitae.
Bagworms are known to feast on up to 128 different trees across various regions of the United States. In addition to arborvitae, these pests are also found on red cedar, juniper, and spruce trees.
Familiarize yourself with their habits and appearance, and monitor your arborvitae for any signs of infestation to ensure healthy plants in your landscape.
Identifying Bagworms on Arborvitae
Bagworms have a unique life cycle that involves four stages:
- Egg: Female bagworms lay hundreds of eggs in their bags, which overwinter and hatch in late spring.
- Larva: Once hatched, the tiny caterpillars begin to feed on the host plant, such as arborvitae, and create their silk bags.
- Pupa: Towards the end of summer, the caterpillars pupate within their bags for about two to four weeks.
- Adult: Adult male moths emerge from the bags to mate, while the wingless females remain in their bags to lay eggs.
Bagworms exhibit specific morphological characteristics that help in identification:
- Caterpillars: Bagworm caterpillars are about 1/4 inch long, with a black head and reddish-green body1. They build spindle-shaped bags constructed of silk threads and bits of foliage2. These bags blend into the arborvitae, making it difficult to spot them.
- Moth: Adult bagworm moths are small, with fringed wings that are white-grey and speckled with black3.
Understanding Bagworm Infestations
Bagworms are common insect pests that target various host plants. Some of the most susceptible plants include:
- Arborvitae: A popular evergreen shrub for landscaping
- Juniper: Another evergreen that is commonly used in gardens
- Cedar: Large evergreen trees that offer dense foliage
- Pine: A versatile evergreen tree species
These pests may also attack some deciduous trees and shrubs like maple, locust, and boxelder1.
Signs of an Infestation
- Defoliation: Bagworms can defoliate entire branches, leaving only bare twigs behind2.
- Bags: Small, spindle-shaped bags made of silk and fragments of host plant foliage1. These bags house the bagworms and serve as protection against predators.
- Cocoons: Mature larvae pupate within their bags, giving rise to new adult moths1.
|Common Signs of Infestation
|Defoliation, small bags hanging from branches, cocoons attached to branches
|Defoliation, presence of silk and foliage bags
|Significant loss of foliage, spindle-shaped silk bags
|Bare branches, larvae crawling along branches with silk bags attached
Potential Damage to Arborvitae and Other Trees
Effects on Evergreen and Deciduous Trees
Bagworms can cause significant damage to both evergreen and deciduous trees. They are known to particularly attack during July and August, leading to defoliation of these plants 1.
Evergreen trees often suffer due to damage to their needles and branches. For example:
- Arborvitae: Needles and small branches are consumed, causing dieback and defoliation.
- Red Cedar: Foliage becomes brown and may ultimately lead to plant death.
- Spruce: Tips of twigs are eaten, resulting in a stunted appearance.
Deciduous trees like maple and sycamore also face damage, although less severe than their evergreen counterparts. Their leaves are often consumed, causing an overall weakening of the tree.
Impact on the Ecosystem
Bagworms may affect the ecosystem by damaging trees native to North America, such as arborvitae and red cedar 2. This damage can result in the following:
- Decreased tree health
- Stressed trees becoming more susceptible to diseases and pests
- Reduced aesthetics of trees and shrubs in both ornamental and natural settings
Comparison of Bagworm Impact on Evergreen and Deciduous Trees
|Commonly Affected Trees and Shrubs
|Potential Effects on Ecosystem
|Arborvitae, Red Cedar, Spruce, Junipers
|Tree health decline, increased susceptibility to diseases/pests, reduced aesthetics
|Maple, Sycamore, Locust, Boxelder, Linden
|Overall weakening of trees, reduced growth
Prevention and Control Methods
There are several natural predators of bagworms that can help control their population:
- Birds: Woodpeckers and other insectivorous birds feed on bagworms, reducing their numbers.
- Hornets and wasps: Vespid wasps and hornets prey on bagworm larvae, providing further control.
Mechanical control methods for bagworms include:
- Handpicking: Remove bagworms from arborvitae trees manually and drown them in soapy water 1. This method is effective, especially for isolated or urban trees.
- Pruning: Prune infested branches to reduce the spread of the bagworms throughout the tree.
There are several chemical control options to treat bagworms on arborvitae:
|Effective, low toxicity to mammals
|May harm beneficial insects
|Bacillus thuringiensis (Bt)
|Targets caterpillars, safe for humans
|Not effective on older larvae, multiple applications required
|Strong and effective
|Toxic to fish, bees, and other beneficial insects
|Provides long-lasting residual control
|Harmful to bees and aquatic life
|Fast-acting, kills a variety of pests
|Toxic to aquatic life and bees
|Highly toxic to bees and other non-target organisms
|Organic alternative, relatively safe
|May require multiple applications, not as effective as synthetic pesticides
To optimize results:
- Timing: Apply treatments early in the season, preferably in May or June, when bags are still small and larvae are more vulnerable.
- Application: Follow label instructions for proper dosage and safe application.
Important Tips and Considerations
Monitoring and Treatment Timing
- Observe your arborvitae regularly for signs of bagworm infestation.
- Start monitoring in spring to catch early signs of activity.
- Bagworm eggs primarily hatch in late May to mid-June, depending on the region.
- Use the National Phenology Network as a helpful resource to track pest activity timing.
A sudden surge in the population of small bags on your arborvitae is indicative of bagworm eggs hatching.
Comparison between stages:
|Observe for signs of new activity, prepare treatments
|Apply insecticides within a few days of eggs hatching
|Monitor for defoliation, apply additional treatments if needed
Safety and Compliance
- Always follow label directions for safe and effective use of pesticides.
- Consider using Bacillus thuringiensis, which is a biologically-based option.
- Wear proper protective gear while applying chemicals.
Features of Bacillus thuringiensis:
- Naturally occurring bacteria
- Safe for humans and pets
- Effective against bagworm caterpillars
Pros and cons of Bacillus thuringiensis:
|May require multiple applications
|Safe for humans and pets
|Works mostly on young bagworm caterpillars
|Targeted specifically at pests
|Weather dependent (rain can reduce effectiveness)
Bagworms feed on the leaves and branches of arborvitae and other evergreen plants. They make silk bags that protect them from predators and weather.
They can cause severe defoliation and plant death if left untreated. Bagworms can be controlled by hand-picking, pruning, or spraying with insecticides.
Hand-picking and pruning are effective for light infestations, but insecticides are needed for heavy infestations. The best time to spray is in late spring or early summer, when the caterpillars are small and vulnerable.
The most recommended insecticide is Bacillus thuringiensis (Bt), a bacterium that kills the caterpillars without harming beneficial insects
- Common Insect Pests of Arborvitae – University of Kentucky ↩ ↩2 ↩3 ↩4 ↩5 ↩6
- Bagworms – Purdue University ↩ ↩2 ↩3
- Bagworms on Trees and Shrubs | University of Maryland Extension ↩
Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about bagworms. Scroll down to have a look at some of them.
Letter 1 – Bagworms
”pine cone” pod
Location: Southern MO (central), just north of AR
February 4, 2012 2:29 pm
Dear Mr. Bugman,
I have just moved into a new home and hanging from the shrubs outside are little pods about 2” long and 1” wide. They look just like a seed pod and I didn’t notice them at first until I found one hanging from my deck, attached with a type of silk.
Upon further inspection, I found tons of these little pods hanging from every shrub in the yard. I have attached a picture and am immensely curious as to what they are. If you could let me know, I would be quite grateful!
These are the cocoons of Bagworms, a species of moth in the family Psychidae. The caterpillars of the Bagworms begin life constructing a small bag which increases in size as the caterpillar grows. The caterpillar eventually pupates and overwinters in the bag. Female Bagworm moths are flightless and never leave their bags. See BugGuide for additional information.
Letter 2 – Bagworms
Subject: Have no idea what this is?
Location: Walls and windows
December 4, 2016 5:16 am
I’ve seen these all over my house when I moved to Florida. I’m from Michigan and have never seen these? Please let me know what they are.
Signature: Y. Diaz
Dear Y. Diaz,
You have Bagworms, caterpillars from moths in the family Psychidae that construct a “bag” from silk and plant material from their host plants. Bagworms live inside the bag and when it comes time for metamorphosis, they frequently leave the plant upon which they have been feeding and anchor the bag to a sheltered location where they pupate. We suspect these stationary Bagworms are in the pupal state.
Letter 3 – Bagworms on Pyracantha
what is this caterpillar/cocoon
Hi. I have had these weird things all summer invading the pyracantha bush. (However you spell that) They mostly hang, but occasionally they will be crawling around dragging there home with them. Very curious, can you identify it? Thanks.
Once again we are not enforcing our threat to immediately trash all letters without locations, but only because in the interest of our readership, we feel you photos demonstrate an important documentation. These are Bagworms in the family Psychidae. Bagworms are caterpillars and pupae of moths. The caterpillars feed on a variety of plants, cedar, juniper and arborvitae being a favorite hosts, but pyracantha is also listed as a host. Bagworms construct bags from the leaves and twigs of their host plants, and we are amazed to see your photos of bags costructed of Pyracantha berries. We haven’t located another image online demonstrating the use of Pyracantha berries in the bag construction, but we just conducted a quick search.
So Sorry about not mentioning my location. I must have missed that threat somewhere on your page. I was just so excited to find someone who might know what it was. We are located in Midwest City, Oklahoma. This is just east of Oklahoma City. Thanks for the info.
Letter 4 – Bagworms
Subject: Agonoscelis puberula? Larvae?
Location: Houston, Texas
July 30, 2017 9:37 am
Hello. Our potted thyme plant has hundreds of what at first look like brown dried flower heads moving in the breeze. But on closer inspection, they are alive! They appear to be shells or cocoons that each house a small larvae, which pokes its head out from one end, and which spins a sort of attachment fiber, like a spider. Could these be larvae for Agonoscelis puberula? I’ve not found any pictures on the Web yet of such shells.
Signature: John in Texas
Letter 5 – Bagworms
Subject: Found on Arizona Cypress
Geographic location of the bug: Boca Raton, FL
Time: 05:00 PM EDT
Your letter to the bugman: After seeing my Cypress being infected by something – it seemed like it occurred overnight – I checked it carefully and found this pine cone looking creature on my cypress, pulled it off and it MOVED in my hand! When I put it on the ground, a caterpillar-looking creature stuck it’s head out. I actually removed thousands which I think can be called a severe infestation. (Is it a saw fly larvae.)
How can I stop further destruction to my tree? And avoid cross contamination to another cypress nearby.
How you want your letter signed: Carol in Boca
Dear Carol in Boca,
You have Bagworms, the larvae of a moth in the family Psychidae. According to BugGuide: “Larvae (bagworms) construct spindle-shaped bags covered with pieces of twigs, leaves, etc., and remain in them — enlarging the bags as they grow — until they pupate (also in the bag). Adult females remain in the bag, emitting pheromones which attract adult males to mate with them. Eggs are laid inside the bag, and when they hatch the larvae crawl away to begin construction of their own individual cases.” We do not provide extermination advice.