Bagworm

Poke the hanging cone on an arborvitae. If silk holds it to the twig and bits of that plant's own foliage dress it, that is a bagworm, not a woody cone and not a silk tent in a fork.

Bagworms are moth caterpillars that live inside a spindle of silk they cover with the needles or leaves they eat. One generation a year. Eggs spend the winter in the mother's bag. Evergreens that lose those needles do not grow them back on the same wood.

• Poke the bag. The caterpillar ducks inside.
• Pick bags from fall through early spring. Destroy them. Do not drop them on the ground.
• A spray in August will not finish a bag that is already tied shut.

Poke the hanging cone: silk holds the bag to the twig

The bag is silk dressed in the plant's own foliage. On arborvitae it looks like a small brown cone. On maple it looks like a bundle of chewed leaves. Poke it. If a caterpillar ducks back inside, or if a silk band ties it to the twig, it is this insect. Woody cones have scales. They snap. They are not silk.

Young bags in late May and June are the size of an ice-cream cone tip and still move as the caterpillar feeds. By August they are one and a half to two and a half inches long and tied shut to the stem. Look inside the hedge, not only at the skirt. Camouflage is the reason most people wait until the brown tips already show.

A silk tent in a fork is a different caterpillar

Eastern tent caterpillars spin a dense nest in the V of a branch in spring, then leave the tent to feed. That is not a bag. See tent caterpillar. Do not run their stick-pull here.

Fall webworms wrap the tips of deciduous branches in a loose, dirty web in late summer. Many hairy caterpillars sit inside. See fall webworm. The web is a communal nest, not a spindle the insect carries.

A case on drywall, stucco, or a garage ceiling is a household casebearer people still call a plaster bagworm. It is a different moth. It eats spider webs and lint. Vacuum it. It does not eat arborvitae.

Evergreens do not grow those needles back

Bagworms chew needles and leaves from inside the bag. On arborvitae, juniper, cedar, and Leyland cypress, a heavy year can strip a limb for good. Those plants do not flush a new set of needles on the same wood. Bare brown wood stays bare. A maple or sycamore that loses leaves can push a new flush and live.

Wait on cutting an evergreen that still has green interior wood you can treat. A few bags on a large deciduous tree are usually looks. Do not wait on an arborvitae hedge until August and hope the brown fills in.

Pick every bag, then destroy it

From fall through early spring the bags hang still, full of eggs. That is the easy window. Clip or pull each bag. Cut the silk band so it does not girdle the twig later. Soak the bags in soapy water or seal them in a trash bag. Do not compost them. Do not toss them on the mulch. Eggs still hatch on the ground, and a crawler can walk back up.

You can pick in summer too, while the bags still move. That stops this year's feeding. Winter pick is what cuts next year's hatch. On a tree you cannot reach, pick the skirt you can reach. Partial pick still removes hundreds of eggs. A ladder on a tall specimen is leftover; hire help or wait for a young-larva spray you can actually cover.

One missed bag is next year's hatch

The female never leaves her bag. After mating she lays about five hundred to a thousand eggs in that bag and dies. Miss one fat bag on an arborvitae and you invite a hedge-full next June. Young caterpillars also spin a silk thread and balloon on the wind to the next evergreen.

If this plant had bags last year, scout it again in late May. Check the neighboring juniper and Leyland, not only the one that looked worst. Ballooning is why a clean pick on one plant is not a neighborhood finish.

Bt only while the bags are still tiny

Bacillus thuringiensis (Bt, the kurstaki strain) has to be eaten. It works on young bagworms that are still grazing needles. It does not soak through a sealed bag. Spray when bags are under about half an inch, usually late May into June, and finish before mid-July in most of the East. Hatch is staggered, so one pass is often not enough. Follow the label in your hand.

Do not spray because the calendar says June if you have not found tiny bags. Iowa notes that catalpa and Japanese tree lilac bloom in the same window — use that as a reminder to look, not as a reason to fog an empty hedge. Skip the brand aisle if coverage on every needle is the missing step. Broad leftover sprays that kill wasps can leave you with more bagworms next year, not fewer.

A spray will not go through a closed bag

Once the caterpillar ties the bag to the stem and stops feeding, a bottle on the foliage does nothing. That is August into fall for most yards. Pick those bags. Wait until winter if you missed the young window. A fog from the driveway wets the skirt and leaves the interior bags dry.

Fail if one August pass is the whole plan and the bags are already cinched to the twigs. Soap in a bucket is for bags you already picked. A kitchen mix sprayed on the hedge is leftover, not a method.

Confirm the bag, then pick or spray young

1. Confirm the bag. Silk plus the plant's own foliage. Poke it. The caterpillar ducks inside.
2. Pick every bag you can reach, fall through early spring. Cut the silk. Destroy the bags. Do not drop them.
3. Scout the next evergreen. Ballooning moves crawlers. One fat missed bag is next June.
4. Bt only while bags are still tiny and feeding, before mid-July, if you cannot pick them all. Repeat on a staggered hatch. Fail if the finish is a closed-bag spray in August.

Related reading

• Tent caterpillar — silk in a branch fork in spring, not a spindle on an evergreen.
• Fall webworm — loose web on deciduous tips in late summer, many caterpillars inside.
• How to grow arborvitae — the hedge these bags prefer; brown tips after feeding do not leaf out again.