In Kansas, the spray window for bagworms ran from late May through roughly late June. By mid-August, the larvae stop feeding and seal their bags shut, which makes spraying pointless. Right now, hand-picking bags off your arborvitae, cedars, or junipers is the useful move. The bigger payoff is stripping every bag off through fall and winter, before the next generation hatches in mid-May.
What are bagworms doing on my trees right now?
By early August, bagworm larvae in Kansas have been feeding for close to three months and are close to done. K-State Research and Extension describes a single generation each year: eggs hatch in mid-May through early June, and larvae feed for approximately three months. From mid-August to early September, once the bags reach 1 to 2 inches long, the larvae stop feeding and seal the tops of their bags shut. After that, they pupate inside. If you’re finding bags on your junipers or arborvitae this week, you’re looking at nearly full-grown insects that are winding down, not larvae that are still actively chewing through foliage.
Is it too late to spray for bagworms?
Yes, if you’re spraying now, it won’t work. Insecticides only kill bagworms when larvae are small, 1/8 to 1/4 inch long, back in late May and June. K-State’s guidance is direct: once bagworms reach 1 to 2 inches long, they’re much harder to manage with insecticides, because as larvae mature, feeding declines and that cuts down how much of the pesticide residue they actually ingest. Both effective active ingredients, Bacillus thuringiensis subsp. kurstaki (Btk) and spinosad, are stomach poisons. A larva has to eat the treated foliage for either one to work. A larva that’s stopped eating and sealed itself into its bag isn’t going to touch it.
When should I spray bagworms in Kansas?
Start in late May, once larvae are 1/8 to 1/4 inch long, and reapply weekly for four weeks. K-State recommends this repeat schedule specifically because eggs don’t all hatch at the same time. Cooler spring temperatures can delay hatch and push the whole window later, so watching your own trees in early May matters more than circling a date on the calendar. Spray in the morning or evening, when larvae are most active, and follow the product label. If you’ve got eastern red cedar in a windbreak or a long hedge row, K-State notes that getting full coverage on the upper and inner branches of mature trees is genuinely difficult, and a high-pressure sprayer built to push a large volume of spray may be what it actually takes. That’s the common setup on rural properties west of town, where a mature shelterbelt can run a few hundred feet. Tree and shrub care in Rossville and the smaller towns around Topeka runs into this every season.
What do bagworms on evergreens actually look like?
Look for spindle-shaped bags, roughly 1 to 2 inches long by this point in the season, hanging off branches like small Christmas tree ornaments. They’re often mistaken for pine cones or bits of dead plant material, especially against cedar or juniper foliage. Conifers take the worst of it. K-State notes that conifers are particularly susceptible to bagworm damage because they don’t regenerate new growth once it’s been eaten, unlike deciduous trees, which can push out new leaves the following season. Arborvitae, bald cypress, cedar, fir, juniper, pine, and spruce are the conifer hosts. Bagworms also feed on black locust, crabapple, elm, hackberry, honey locust, linden, maple, oak, poplar, rose, sycamore, and willow. In a Topeka yard, they’ll sometimes attach to the siding, a garbage can, or a deck rail near an infested plant, not just the plant itself. On a blue spruce, check for bags before assuming the browning is one of the other things that brown that species here, since the damage patterns look different up close.
Why does the same hedge get bagworms every single year?
Because the female moth never leaves. Male bagworms become small black moths with clear wings, about 1/2 inch long, and fly off using pheromone scent to find females. Females never get that chance. K-State’s publication is specific: female bagworms don’t have wings, legs, antennae, or functional mouthparts, and they never become moths at all. A female lays between 500 and 1,000 eggs, which stay inside her pupal case, inside the bag, all winter. That means an infestation doesn’t spread out and thin over time on its own. It sits on the same windbreak or hedge row and compounds year after year, because the next generation hatches out of bags that never went anywhere. Missing even one female bag when you’re checking a hedge can mean 500 to 1,000 new larvae the following spring. This is also the mechanism behind the ballooning you may have noticed in June: newly hatched larvae, 1/8 to 1/4 inch long, spin a strand of silk and drift on the wind to nearby plants, which is how a bagworm problem spreads from one shrub to the next row over.
What should I actually do about bagworms this month?
Hand-pick every bag you can reach, on evergreens and any other host plant, and don’t stop at the obvious ones. K-State’s guidance for minor infestations is straightforward: regular hand-picking during the growing season and through winter reduces bagworm populations the following year. Check the whole canopy, not just eye level, since female bags tend to sit at the tops of trees and shrubs while male bags cluster lower down. Drop what you collect into a bucket with a soapy water solution, 1 fluid ounce of soap per gallon of water, and let the bags soak for 15 minutes. Then empty the bucket and pile the bags somewhere in full sun. Don’t toss them in the trash, the compost pile, or leave them on the ground, since eggs inside an intact bag can still be viable. If the infestation is heavy or the trees are too tall to reach safely, K-State’s own guidance acknowledges hand-picking stops being practical at that point, which is where a tree health assessment is worth the call, to get eyes on how bad it actually is and whether next spring’s spray window is worth planning around.
Kansas bagworm calendar
| Timeframe | What the insect is doing | What to do |
|---|---|---|
| Mid-May to early June | Eggs hatch, tiny larvae (1/8 to 1/4 in) emerge and balloon on silk to nearby plants | Start scouting; this is the window to prepare for spraying |
| Late May through late June | Larvae still small; peak feeding begins | Spray weekly for 4 weeks with Btk or spinosad, morning or evening |
| July into early August | Larvae growing, bags lengthening toward 1 to 2 inches | Spraying loses effectiveness as feeding declines; hand-picking still works |
| Mid-August to early September | Larvae stop feeding, seal the tops of their bags, begin to pupate | Spraying is no longer useful; hand-pick and soak bags in soapy water |
| Fall through winter | Bags remain on branches; females’ eggs overwinter inside | Best window for thorough hand-picking, since every bag is finished and each one you pull is eggs removed |
| Following mid-May | New generation hatches | Scouting starts again |
How do bagworms compare to other pests we cover in Topeka?
Bagworms are a real, common problem on Topeka hedges and windbreaks, but they’re a yard-scale issue, not a regional emergency. The Kansas Forest Service’s own 2025 forest health report lists bagworm among the routine landowner inquiries its Forest Health Program fielded that year, and describes that whole category of inquiries as minor or localized problems that still deserved a look, not widespread threats. Compare that to emerald ash borer in Shawnee County, which the state tracks as a confirmed, spreading regional threat to the ash population. A hedge full of bagworm bags is worth handling well. It isn’t the same category of problem.
One thing worth ruling out while you’re checking conifers: bagworms strip foliage, but they don’t turn a whole pine brown in a matter of weeks. If a pine specifically is browning fast rather than looking chewed, that’s a different problem, and a pine tree turning brown in Topeka covers what pine wilt looks like and why the removal deadline falls before May 1.
Bagworms aren’t the only pest that gets missed on a walk past the yard. Our spring tree health checklist is built around catching early hatch and other seasonal issues before they compound, and if cedar is part of what you’re managing, cedar trimming timing in Topeka covers when to prune those same trees without adding stress on top of a bagworm year.
Frequently asked questions
Can I still spray for bagworms in August?
No. By mid-August, larvae have stopped feeding and sealed the tops of their bags to pupate, and both effective insecticides, Btk and spinosad, only work if the larva eats treated foliage. A larva that’s no longer feeding won’t ingest the residue, so spraying now won’t reduce the population.
What kills bagworms best?
Insecticides containing Bacillus thuringiensis subsp. kurstaki (Btk) or spinosad are effective, but only when larvae are small, 1/8 to 1/4 inch long, which in Kansas means late May into June. Apply weekly for four weeks, since eggs hatch on a staggered schedule and not every larva is the same size at once.
Why do I get bagworms on the same trees every year?
Female bagworms never grow wings, legs, or the ability to fly, so they never leave the plant they hatched on. Each female lays 500 to 1,000 eggs that overwinter inside her bag, right on that same branch, which is why an untreated infestation compounds on the same hedge row or windbreak year after year instead of spreading out and fading.
Do bagworms kill trees, or just damage them?
They can kill a tree, particularly conifers. K-State notes that conifers can’t regenerate foliage once it’s eaten, so extensive bagworm feeding over several years in a row can fully defoliate and kill a well-established tree or shrub. Deciduous trees generally tolerate bagworm feeding better because they push new leaves the following season.
Should I just leave bagworm bags alone and let birds handle it?
Not if you want the population to actually drop. Birds and parasitic wasps do prey on bagworms, but K-State is clear that these natural enemies don’t kill enough of them to affect the population or prevent plant damage on their own. Hand-picking and proper disposal is the control that actually moves the needle for a minor to moderate infestation.
Get a second opinion on your trees before next spring
Fall and winter are the best time to strip bagworm bags off a hedge. Every bag is finished by then, nothing is still hatching, and each one you pull is 500 to 1,000 eggs gone before the next hatch in mid-May. If you’ve got a windbreak or hedge row that’s dealt with this for more than one season, or conifers thick enough that hand-picking every bag isn’t realistic, Topeka Tree Pro connects you with insured local crews who handle everything from hedge and shrub care to a full arborist consultation on what your trees actually need heading into fall. Crews in the Topeka network also handle tree trimming if pruning damaged growth is part of the plan. Call (785) 333-4052 to get matched with a crew this week.