Spongy Moth

Scrape every tan, velvety egg mass into soapy water. Leave the bucket two days. Eggs you dump on the ground still hatch.

Spongy moth is an invasive moth, Lymantria dispar. People still call it gypsy moth. Then the real name. The larva is hairy, with five pairs of blue bumps and then six pairs of red. It does not spin a silk tent. In June, wrap a burlap skirt at chest height and empty it every afternoon. Bt only while those caterpillars are still small. A healthy oak usually leafs out after one June of feeding.

• Scrape masses into soap. Do not scrape them onto the mulch.
• Five blue spots, then six red. No tent in a fork.
• A spray after the oak is already veins-only is too late.

Scrape the egg masses into soapy water

The mass looks like a scrap of dirty sponge stuck to bark, siding, lawn chairs, wheel wells, or a woodpile. It is tan or buff, about an inch to an inch and a half, and felted with hair from the female. One mass can hold hundreds of eggs, sometimes near a thousand. Slide a putty knife or paint scraper under it. Catch it. Drop it in soapy water. Leave it two days, then trash it.

Eggs left on the soil still hatch. Stepping on a mass does not finish it. Wear gloves. The felt can rash bare skin. A labeled horticultural oil can soak a mass you cannot reach with the knife. Motor oil is leftover. It injures the tree. Old, faded, soft masses already hatched. Scrape the firm new ones from fall through the week before oak leaf-out.

Fail if the finish is a scrape onto the ground and a hope that frost will do the rest.

Wrap a burlap skirt and empty it every afternoon

Once the caterpillars are about an inch long, they walk down the trunk in the heat of the day to hide, then climb back at night to feed. Cut a strip of burlap twelve to eighteen inches wide and long enough to go around the trunk at chest height. Tie twine around the middle. Fold the top half down so it hangs as a skirt. That pocket is where they rest.

Check it every afternoon. Wear gloves, or knock them with a stick, into a bucket of soapy water. Leave them two days. Pupae and the pale, flightless females hide under the same flap in July. They go in the same bucket. Take the cloth down in August so it does not girdle the bark, and scrape any new masses that were laid under it.

In April and May, while larvae are still tiny, a sticky wrap can catch crawlers that ballooned onto the trunk. Put batting in the furrows, then a wrap, then sticky compound on the wrap. Never smear glue on the bark. It stains and injures the tree, and they walk around the dry patches. Switch to the burlap skirt when they are large enough to hide. Empty the skirt. Glue on the bark is leftover.

A healthy oak usually leafs out after one June

The canopy can look winter-bare in June while the maple next door is still green. A healthy hardwood that went into spring with a full crown usually pushes a second set of leaves later in summer. That flush spends reserves. One year is looks and a weaker tree, not an automatic take-down. Wait until the next spring before you decide the oak is dead.

Two heavy years in a row, or one year on a newly planted oak in drought, is the year to treat. Pine and hemlock mixed under oak can die after one heavy feeding, because those needles do not come back the same season. Water through dry weeks. Do not pile fertilizer on a skeletonized canopy as the first move. Wait on a strong oak that still has green interior wood and only one June of this feeding.

Five blue spots, then six red, and no silk tent

The confirm on a grown caterpillar is the row of bumps, not the hair. Five pairs of blue behind the head, then six pairs of red. The body is dark and bristly, up to about two and a half inches. Newly hatched larvae are tiny and black. The spots show after they molt. Tap is leftover. They do not drop on a silk thread the way a looping cankerworm does. They crawl the trunk and the leaves. There is no communal tent.

The female moth is white with wavy brown marks and cannot fly. The male is smaller and brown, with feathery antennae. The egg mass is velvety. A smooth gray mass without that felt is not this moth.

A fork tent is tent caterpillar; a tip web is fall webworm

Eastern tent caterpillars spin a dense nest in the V of a branch in spring, then leave the tent to feed. They have a pale stripe down the back. See tent caterpillar. Do not run their stick-pull here.

Fall webworms wrap the tips of deciduous branches in a loose web in late summer, after this moth has already pupated. Many hairy caterpillars sit inside. See fall webworm. The web is a communal nest, not a hiding skirt you tie on the trunk.

Fall cankerworm is a hairless looping inchworm. Tap a chewed leaf and it drops on silk. See fall cankerworm. Do not run that moth's after-leaf-drop sticky band here. Forest tent caterpillars feed in groups on oak and aspen and do not build a tent. Color will not split every hairy worm. The five-then-six row still belongs to spongy moth. A smooth egg mass is spotted lanternfly, not this felt.

Bt only while the caterpillars are still small

Bacillus thuringiensis (Bt, the kurstaki strain) has to be eaten. Spray when larvae are still under about half an inch, better under an inch, and leaves are out enough to hold the spray. That is usually May, while most of the leaf is still there. Once the canopy is veins-only, a bottle will not rewind the flush. Large larvae take a spray poorly, and a late fog can irritate them so they drop on people even more.

Hatch is staggered, so one pass is often not enough. Follow the label in your hand. Coverage on a tall oak is the missing step, not a brand. Broad leftover sprays that kill wasps can leave you with more caterpillars next year, not fewer. Skip the carton of male moth lure. It tells you males are flying. It does not protect this tree.

Confirm the spots, then scrape, band, wait, or spray young

1. Confirm this moth. Five pairs of blue bumps, then six pairs of red. No silk tent. Tan velvety masses on bark or outdoor gear.
2. Scrape every firm mass into soapy water. Leave it two days. Never onto the ground. Gloves on.
3. If this is one June on a healthy mature oak, wait for the second flush. Water through dry weather. Do not take the tree down in June.
4. Band in June. Burlap skirt at chest height. Empty it every afternoon. Sticky compound goes on a wrap in April, never on the bark.
5. Bt only while the caterpillars are still small, if the tree is young, already stressed, a pine or hemlock under oak, or this is a second heavy year. Fail if the finish is a fog after the oak is already skeletonized.

Related reading

• Tent caterpillar — dense silk in a branch fork in spring, pale stripe, not a blue-then-red row.
• Fall webworm — loose web on deciduous tips in late summer, many caterpillars inside.
• Fall cankerworm — hairless looping inchworm that drops on silk; band after leaf drop, not a June burlap skirt.
• Spotted lanternfly — smooth egg mass, not this velvety felt.
• How to grow oak — the tree these larvae hit first in a June outbreak.