Bagworms in Central PA: The Three-Week Window That Decides If Your Evergreens Survive

“Bagworms in central PA are one of those pests that don’t look like a pest.” By the time a homeowner notices something is wrong with an arborvitae or a spruce, the damage is usually already done: and the worst part is that the window to actually do something about it is only about three weeks long. We’re inside that window right now.

Bagworms have been a southern Pennsylvania problem for years. Penn State Extension has been tracking their spread north for a while, and around Lock Haven and the broader central PA area, we’re now seeing them on properties where they weren’t an issue even a few years ago. Most homeowners here don’t know what they’re looking at, and the bag itself looks so much like part of the tree that it routinely gets missed until the damage is severe.

This article covers what bagworms are, why early June is the only window that really matters, how to identify them on the evergreens around your house, what works and what doesn’t, and the honest answer on when to call someone.

What bagworms actually are

Despite the name, a bagworm isn’t a worm, it’s the larval (caterpillar) stage of a moth. The reason they’re called bagworms is the thing they build around themselves: a small spindle-shaped “bag” made of silk and pieces of the host plant, bits of needle, leaf, and twig woven into a camouflaged case.

The disguise is the whole problem. A bag full of green arborvitae bits looks exactly like a small clump of arborvitae. A bag covered in spruce needles looks like a tiny pinecone hanging from the branch. Until you know what you’re looking for, you walk past them.

The life cycle is straightforward and that’s what makes the timing critical. Each fall, a single female lays between 500 and 1,000 eggs inside her bag, then dies. The eggs overwinter inside that protective bag, completely beyond the reach of any spray, and hatch the following spring. Hatching happens in late May and early June in Pennsylvania. The newly hatched larvae crawl out, immediately start building their own miniature bags, and either stay on the host plant or “balloon” on silk threads to a neighboring tree on a windy day. By midsummer, they’re feeding hard, growing fast, and getting harder to kill.

Why early June matters and not later

That’s why this isn’t a “any time in summer” pest. It’s a three-to-four-week window in late May and early June, and then it’s effectively over until next year. Treatment in May or early June kills the larvae before they do real damage. Treatment in July kills some of them while the rest finish defoliating the tree. Treatment in August does almost nothing.

The Lock Haven and central PA angle

Bagworms historically belonged to southern Pennsylvania. Penn State Extension has tracked them moving north over the last decade, and they’ve been reported in increasing numbers north of Interstate 80, which runs right through our area. We’re now seeing them on properties around Lock Haven where homeowners have lived for twenty years without ever encountering one.

That’s the practical problem. Most central PA homeowners have never had to identify a bagworm. They don’t know what the bags look like, they don’t know to check, and by the time the damage is obvious, the treatment window has passed. The single most common way bagworms kill a mature arborvitae in central PA is that nobody knew to look until August, and by August, it was too late.

The plants worth checking on your property

Bagworms will feed on dozens of species, but their real damage is concentrated on a handful of common landscape plants. If you have any of these around your house, they’re worth a close look this week

If you have arborvitae, juniper, or spruce anywhere near the house, the most important property task this week is walking over and actually looking at them. Closely.

How to actually spot bagworms, what you’re looking for

Here’s the practical check. Walk up to each evergreen on your property and look closely at the branches, especially the outer few feet of growth where light hits and where caterpillars feed. You’re not looking for a “worm.” You’re looking for small hanging shapes:

Shape: spindle-shaped, fat in the middle, tapered at both ends, like a tiny carrot or a small upside-down pinecone. Usually a quarter-inch to half-inch long this time of year, growing to one or two inches by July.

How it hangs: from a short silk strand at the top, with the bag dangling down. The bag itself moves slightly if you bump the branch, that’s the giveaway. Real pinecones don’t sway like that.

What it’s covered in: bits of the host plant. On arborvitae, it’s covered in tiny green arborvitae fronds. On spruce, it’s covered in spruce needles. On juniper, juniper bits. The whole point of the bag is camouflage, it’s deliberately wearing the tree it’s eating.

Where to look first: the outer branch tips, the south and west sides (where sun hits and caterpillars are most active), and any spot where the foliage looks thinner or browner than the rest. Heavy infestations often start at the top of the tree and work down.

If you find one, stop and look harder. Bagworms are rarely solo, where there’s one, there are usually dozens. On a heavily infested tree you may find a hundred or more after a careful look.

What works in this window and what doesn’t

If you find bagworms, what to do this week

When professional help is the right call

For small shrubs you can reach with a pump sprayer, the DIY path is reasonable, Bt and spinosad are available at hardware stores, and if you catch the larvae small, a thorough application can clear a hedge.

For tall arborvitae, mature spruces, or any plant where you can’t physically reach the top branches with a sprayer, the math changes. Bagworm treatments only work if the spray reaches the larvae, and incomplete coverage on a 15-foot evergreen often means the bottom is clean and the top is still being eaten. That’s where it makes sense to bring in someone with the right equipment.

The same is true if you’re seeing heavy infestation across multiple plants. A homeowner with a hand sprayer can manage a few bags on one shrub. A row of infested arborvitae along a property line is a different scale of problem, and once any plant has been heavily defoliated, the question shifts from “can we save it” to “is this plant worth saving, or are we replacing it.” That’s a different conversation.

Sources: Penn State Extension, “Bagworm” fact sheet; University of Maryland Extension, “Bagworms on Trees and Shrubs”; published bagworm life cycle and treatment timing research. Geographic spread data current as of 2026.

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.