Every August, St. Louis homeowners notice the same thing: an arborvitae hedge or a backyard spruce that was green in June is suddenly brown at the top and hung with dozens of little cone-shaped bundles. Those bundles are bagworms, and by the time they are big enough to see from the patio, most of the damage for the year is already done. The good news is that bagworms are one of the most controllable pests we deal with, as long as you act at the right point in their life cycle. Thompson Tree Care and Landscaping has been assessing pest-damaged trees across the St. Louis metro since 2014, and this guide covers how to identify bagworms, when each control method actually works, and how to tell whether a stripped evergreen can be saved.
What are bagworms?
Bagworms are the caterpillar stage of a small moth, Thyridopteryx ephemeraeformis. What makes them unusual is the bag: as soon as a caterpillar hatches it spins a silk case and glues bits of the host tree's foliage to the outside, so the bag looks like a 1- to 2-inch spindle or miniature pine cone made of the tree itself. The caterpillar carries the bag with it as it feeds, poking its head out to eat and retreating inside when disturbed.
By late summer the caterpillar seals the bag and pupates. Males emerge as small black moths, but females never leave the bag at all. After mating, each female lays several hundred eggs inside her own bag and dies there. Those eggs overwinter inside the bag, protected from cold and predators, and hatch the following spring. That single fact drives everything about bagworm control: every bag left on the tree over winter is a sealed package of next year's infestation.
Which St. Louis trees do bagworms attack?
Bagworms are not picky, but they have clear favorites. In the St. Louis area the worst damage shows up on evergreens:
- Arborvitae — privacy hedges are the number one bagworm casualty in local subdivisions
- Juniper — including the low spreading varieties used in foundation beds
- Spruce — Colorado blue spruce and Norway spruce
- Eastern red cedar — the native cedar that lines fence rows and highway edges across Missouri
They also feed on deciduous trees, including maples, honeylocust, oaks, sycamores, and crabapples. Deciduous trees tolerate the feeding far better because they replace their leaves every spring. Evergreens do not, which is why an infestation that would be a nuisance on a maple can kill an arborvitae outright.
How do you spot bagworms?
Look for these signs, in the order they usually appear:
- Small bags hanging like tiny pine cones from twigs and branch tips. In June they are the size of a grain of rice; by August they are up to two inches long. On evergreens the bags are covered with needles and blend in until you look closely.
- Browning from the top down and from the outside of the canopy inward, where the caterpillars have chewed the needles off.
- Bare branch tips with the needles stripped back, often on one side of the tree first.
- Bags that move. On a warm summer day you may actually see a bag inching along a branch. If the bags are still and firmly attached in fall or winter, they contain overwintering eggs.
Bagworms are often mistaken for pine cones on spruce or for seed pods on juniper. If you can pull one off and it is soft and silky inside rather than woody, it is a bagworm bag.
When do bagworms hatch in Missouri?
Timing is the whole game with bagworms, so it helps to know the St. Louis calendar:
- Late May to mid-June: eggs hatch. Newly emerged caterpillars are tiny, spin a silk thread, and can drift on the wind to nearby trees before settling and building their first bag.
- June through August: caterpillars feed and grow, enlarging the bag as they go. This is when the visible defoliation happens.
- Late August to September: bags reach full size, the caterpillars seal them, and pupation begins. Moths emerge, mate, and females lay eggs inside the bags.
- October through May: eggs sit inside the bags waiting for spring. Nothing is feeding, but every bag left on the tree is loaded.
Hand-picking is most effective from September through May, when the bags are large, easy to see, and full of eggs that have not hatched. Sprays only work in a short window from early to late June, when the caterpillars are small and exposed.
How do you get rid of bagworms?
Match the method to the season and you will win. Use the wrong one at the wrong time and you will spend money for nothing.
Hand-pick the bags (September through May)
For small trees and hedges this is the single most effective control there is. Pull or snip every bag you can reach, drop them into a sealed plastic bag, and put them in the trash. Do not simply knock them onto the ground: the eggs will hatch there in spring and the caterpillars will climb right back up. Check the tree twice, because the bags on the shaded interior are easy to miss.
Spray young caterpillars (June)
If the bags are already crawling, the window for spraying is short. Bt (Bacillus thuringiensis) and spinosad are both effective on small caterpillars and are low-risk for people, pets, and pollinators when applied as labeled. Thorough coverage matters, because the caterpillars must eat treated foliage. Repeat according to the product label while new caterpillars are still hatching.
Skip the late-summer spray
Contact insecticides applied to large bags in August mostly fail. By then the caterpillars are big, spend most of their time sealed inside the bag, and are about to stop feeding anyway. The money is better spent on hand-picking that fall.
Prune out heavily infested branches
When one section of a tree is packed with bags, cutting that branch out and disposing of it removes hundreds of bags in one motion. Our tree trimming and pruning crew does this routinely on larger trees where hand-picking is not practical, and it pairs well with a general cleanup trim; see our guide to the best time of year to trim trees in Missouri for scheduling.
Systemic treatments for large or repeat infestations
For tall spruce or cedar where spraying and picking are impractical, soil-applied or trunk-injected systemic insecticides can protect the tree through the feeding season. These products require a licensed applicator and correct timing. Thompson Tree Care does not apply pesticides ourselves; a certified arborist assessment tells you whether treatment makes sense for the tree, and we coordinate the application with a licensed applicator when it does.
Can a bagworm-damaged evergreen recover?
This is the question that matters most, and the answer depends on the type of tree. Deciduous trees such as maples, oaks, and honeylocust almost always recover. They lose leaves, but they set new buds and leaf out normally the next spring.
Evergreens are a different story. Arborvitae, juniper, spruce, and eastern red cedar cannot produce new needles from bare, older wood. Once a branch is stripped, that branch stays bare. A tree that has lost a third of its foliage may fill in over a few seasons and look acceptable. A tree that has lost roughly half or more is very unlikely to recover, and it will stand there brown and dead-looking for years. At that point tree removal and replanting is usually the practical answer, and a dead evergreen near a house or driveway becomes brittle and should not be left standing. Our guide to how to tell if a tree is dead or dying walks through the checks we use on site.
If you are unsure which side of that line your tree is on, do not guess. A free on-site look is faster and cheaper than waiting another season to find out.
How do you keep bagworms from coming back?
- Inspect evergreens every winter. Bare branches make bags easy to spot, and every bag you remove before June is hundreds of caterpillars that never hatch.
- Check the neighbors' side too. Bagworms balloon between properties on the wind, so a shared hedge needs attention on both sides.
- Encourage natural enemies. Birds, especially sparrows and chickadees, eat bagworms, and several parasitic wasps attack them. Avoiding broad-spectrum insecticide sprays helps keep those allies around.
- Replant with less-preferred species. When an arborvitae hedge has been lost, consider a mix of species rather than a single row of the same evergreen. Bagworms find a monoculture hedge and work straight down it.
- Keep trees healthy. Well-watered, properly mulched evergreens tolerate a light infestation far better than stressed ones.
Bagworms are just one of several pests and diseases that hit St. Louis yards every summer. Our guide to common tree diseases and pests in St. Louis covers the others, from oak galls to emerald ash borer, so you can tell what you are looking at.
Thompson Tree Care and Landscaping has completed more than 500 jobs across St. Louis, Kirkwood, Chesterfield, Florissant, O'Fallon, and Belleville since 2014 and holds a 4.8-star Google rating from 60+ reviews. We are licensed and insured, estimates are free, and we offer senior and veteran discounts. If bagworms have gotten into your evergreens, call (314) 712-3035 or request a free assessment online and we will tell you honestly whether the tree can be saved.
Frequently Asked Questions
Hand-picking is the most reliable method. From September through May, pull every bag off the tree, seal them in a bag, and put them in the trash so the eggs inside cannot hatch. If you miss that window, spray the tree with Bt (Bacillus thuringiensis) or spinosad in June while the caterpillars are still small. Sprays on large late-summer bags rarely work.
In the St. Louis area, bagworm eggs typically hatch from late May through mid-June. The tiny caterpillars begin building bags immediately and feed through August, so the bags you notice in late summer have been growing for two to three months. Sprays are only effective in the first few weeks after hatching.
Deciduous trees such as maples and oaks usually leaf out again the following spring. Evergreens are different: arborvitae, juniper, spruce, and eastern red cedar cannot regrow needles on bare wood. If roughly half or more of an evergreen has been stripped, it is unlikely to recover and removal is usually the practical choice.
Yes. Newly hatched caterpillars spin silk threads and drift on the wind to nearby trees and shrubs, a behavior called ballooning. One infested arborvitae hedge can seed the evergreens on both sides of a property line within a season, which is why removing bags before they hatch protects your whole landscape and your neighbors.
