Bagworms on Arborvitae and Spruce: The Hanging Cocoons Killing Rochester Evergreens

By Linda Marsh, Pests & Diseases. Last updated: June 25, 2026

If you have noticed small brown "ornaments" dangling from your arborvitae hedge or blue spruce and the foliage near them is thinning, you are almost certainly looking at bagworms. They are not pinecones, and they are not seed pods. Each bag is a living caterpillar that drags its protective case along as it feeds, and on Monroe County privacy hedges they can hollow out a row of arborvitae in a single season.

What do bagworms on arborvitae and spruce actually look like?

A bagworm bag is a spindle-shaped or teardrop-shaped case, roughly 1 to 2 inches long when mature, built from silk and woven with bits of the host plant's own foliage. On an arborvitae the bag is studded with green scale-like leaves, so it blends in almost perfectly until it browns. On a blue spruce or Norway spruce the case is studded with short needle fragments instead.

The tell that separates bagworms from cones or galls is simple: cones and galls are fixed in place, while a bag hangs by a strand of tough silk and the open end faces down. Early in the season the bags are tiny, a quarter inch or less, and pale green, which is exactly why they get missed until the damage is done. If you are seeing widespread evergreen browning and are not sure of the cause, our Rochester homeowner's diagnostic guide to a sick tree walks through how to narrow down the other common culprits, including winter burn and root stress.

Why are bagworms so damaging to Rochester evergreens?

Bagworms feed on foliage, and arborvitae and spruce hold a hard truth that deciduous trees do not: most conifers do not readily push new growth from bare, old wood. A maple stripped by caterpillars in June will often releaf. An arborvitae stripped to brown twigs usually will not. The buds simply are not there to recover from.

This is why a bagworm infestation that looks cosmetic in July can be a death sentence by the following spring. A heavy population can defoliate a plant completely, and on a tightly planted privacy screen the caterpillars move plant to plant, so one infested arborvitae becomes a brown gap in the whole hedge. Spruce tolerate partial defoliation a bit better than arborvitae, but repeated years of feeding still thin and weaken them, often opening the door to secondary problems like needlecast. If your spruce is browning from the inside out rather than from bagworm feeding, that pattern usually points to fungal needle disease instead, covered in our piece on blue spruce needlecast in Rochester.

When is the right time to treat bagworms in New York?

Get the timing wrong and even the right product fails. There are two distinct windows.

The first is the dormant window, late fall through early spring (roughly October into April). Through the cold months the bags you see are overwintering egg cases: each female bag can hold hundreds of eggs that will hatch the following spring. Removing bags now, by hand, physically removes next year's population before it starts.

The second is the early-summer spray window. In Upstate New York, per Penn State Extension and Cornell, the eggs typically hatch in late May into early June, and the newly emerged caterpillars are tiny and feeding actively. This is the only stretch when Bacillus thuringiensis (Bt), a low-impact biological insecticide, works well, because Bt has to be eaten by small, actively feeding larvae. Wait until midsummer and the caterpillars are larger, sealed deeper in tougher bags, and far harder to kill. By August they have stopped feeding entirely and no spray will reach them. Getting the calendar right is so important that it is one of the most common reasons homeowners end up wanting a professional read before treating tree insects.

How do you remove bagworms by hand?

For a small number of bags on a reachable shrub, hand-picking is genuinely effective and free. Here is the approach:

1. Do it in winter or very early spring, before any May hatch.
2. Pull or clip each bag off, snipping the tough silk band that anchors it to the twig (that band can girdle and kill a small branch if left for years).
3. Drop the bags into a sealed bag or a bucket of soapy water. Do not just toss them on the ground, since the eggs can still hatch.
4. Throw them in the household trash, not the compost or brush pile.

Check the plant thoroughly, including the interior and the top, because bags hide against the foliage. On a tall arborvitae screen or a large spruce, hand-picking every bag is rarely realistic, and that is where the June Bt window or a professional treatment becomes the practical answer.

Can a bagworm-killed arborvitae come back?

Usually not, and this is the part Rochester homeowners find hardest to hear. If an arborvitae has been browned to bare wood by bagworm feeding, the branches that have lost all their green foliage are typically dead and will not refoliate. A plant with patchy damage and surviving green sections may slowly fill back in over several seasons if the bagworms are removed and the plant is kept healthy, but a fully stripped specimen is generally a replacement, not a recovery.

That asymmetry, easy to prevent and nearly impossible to reverse, is the whole reason early identification matters so much. A ten-minute winter walk along the hedge with pruners can save an entire privacy screen.

FAQ

Are those hanging bags pinecones or bagworms? If the case hangs from a strand of silk with the open end pointing down and is built from the plant's own foliage, it is a bagworm. Cones and galls are rigid and fixed in place and do not dangle.

Will my arborvitae grow its needles back after bagworms? Branches stripped completely to brown wood almost never refoliate, because conifers do not readily bud from bare old wood. A plant with surviving green may slowly recover, but a fully browned arborvitae is usually replaced.

When should I spray for bagworms in Rochester? Roughly late May into June, right after the eggs hatch, while the caterpillars are still small and feeding. Bt and other treatments lose effectiveness once the larvae mature in midsummer, and by August feeding stops entirely.

Can I just pick the bags off? Yes, for a small, reachable shrub. Remove them in winter or early spring, cut the silk band, and seal the bags in the trash so the eggs cannot hatch. For tall screens or large spruce, hand-picking everything is rarely practical.

Do bagworms spread to other plants? Yes. On a tightly planted hedge the caterpillars move from plant to plant, and they also feed on many other species, so an untreated infestation tends to widen each year.

Sources

• Monster Tree Service of Rochester
• Penn State Extension, Bagworm
• Cornell Cooperative Extension, Cornell University
• USDA Forest Service