Spring Cankerworm

Wrap a sticky band around the trunk in February, before the wingless moth climbs. She spent the winter as a pupa in the soil, so she has to walk up. Pack cotton batting into the bark furrows, cover that padding with a wrap, and put sticky compound on the wrap. Glue smeared on the bark can injure the tree, and she walks around the dry patches.

Spring cankerworm is a native geometrid moth, Paleacrita vernata. People call the larva an inchworm because it loops as it walks. In May, tap a chewed elm or oak leaf. A hairless looping caterpillar drops on a silk thread. Two pairs of prolegs on the back half confirm this moth. Fall cankerworm feeds in the same week, often on the same elm, and it has three pairs. A healthy tree usually pushes a second flush after one spring of feeding, so wait on a strong oak. Spray Bt only while the worms are still small.

Wrap a sticky band in February, before she climbs

This moth leaves the soil on a thaw in late winter, usually February into March. Wrap cotton batting or pipe insulation around the trunk below the first limbs, and pack the furrows so she cannot crawl under. Cover that padding with plastic, tar paper, or roofing felt, then put a sticky tree-band compound on the wrap, about six inches wide. Never smear it on the bark.

Check the wrap every few weeks. Dead moths and debris make a path, so refresh the sticky layer. If this elm's crown touches the next maple, band that tree too, or she walks across. If a band from last fall is still on the trunk, scrape the debris and renew the sticky layer now. See fall cankerworm for that moth's climb after leaf drop. Do not run that calendar here.

Some sheets skip banding because it is messy, and it misses females that already crossed. Use it on the trees you mean to protect. It counts moths on this trunk. It does not finish every elm on the block when last spring stripped the street. Take the wrap off in early May, after this climb is over. Do not leave sticky compound on the trunk over summer.

A healthy tree usually pushes a second flush

Shot holes show first, while the worms are still small. Then the tissue between the veins is gone, and only the midrib hangs. A mature elm, oak, or apple that was healthy going into May usually pushes a second set of leaves by midsummer, because that flush spends reserves. One year is looks and a weaker tree, not an automatic take-down.

Two or three heavy years in a row, or one year on a newly planted elm in drought, is the year to treat. Urban trees with compacted soil, lawn to the trunk, or a dry summer take this harder than a forest tree, so water through the dry weeks. Do not pile fertilizer on a skeletonized canopy as the first move. Wait on a strong tree that still has green interior wood and only one spring of this feeding.

Tap a chewed leaf. Two prolegs confirm this moth

The confirm is the drop and the count, not the color. Green, brown, or almost black larvae all belong to this moth. Tap a chewed leaf. A looping, hairless caterpillar about three-quarters of an inch to an inch long when grown drops on a silk thread. Count the fleshy prolegs on the back half. Two pairs is spring cankerworm. A white or yellow stripe often runs along each side, but stripe and color will not split every mixed year.

In late winter, the female that laid next May's eggs is wingless, gray-brown, and about five-sixteenths of an inch. She does not look like a moth. She hides loose oval clusters in bark crevices of larger limbs, not in neat rows on twigs. Those eggs are easy to miss, so the tap in May is the homeowner check. The male is a small gray flyer.

Three prolegs is fall cankerworm. A silk tent is a different moth

Fall cankerworm larvae hatch in the same week and often feed on the same elm. They have three pairs of prolegs, the front pair short. Their eggs sit in neat rows on twigs, not in bark crevices. See fall cankerworm. Do not run that moth's after-leaf-drop climb here.

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 single looping worm on silk. 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. Many hairy caterpillars sit inside. See fall webworm. The web is a communal nest, not a thread this insect drops on.

Spongy moth caterpillars are hairy, with a row of blue spots then red spots, and they do not build a silk tent. See spongy moth. Do not run a June burlap band here. In the Northeast, winter moth is another looping geometrid on the same hosts. Color will not split them. The two-proleg check still belongs to spring cankerworm.

Spray Bt while the worms are still under half an inch

Bacillus thuringiensis (Bt, the kurstaki strain) has to be eaten. Spray when larvae are still under about half an inch, leaves are expanded enough to hold the spray, and feeding is about ten days in. That is usually the second week after hatch, while at least half the leaf is still there. Once the canopy is veins-only, a spray will not put those leaves back. A late spray can irritate large larvae so they drop on people even more.

Hatch is staggered, so one pass is often not enough. Follow the label in your hand. A dormant oil can smother eggs if you honestly cover every bark crevice. Most home sprays miss the top, and this moth hides eggs in those cracks, so skip the brand aisle if coverage on the leaves you can reach is the missing step. Broad sprays that kill wasps can leave you with more loopers next year, not fewer.

Confirm two prolegs, then band, wait, or spray young

1. Confirm this moth. Tap a chewed leaf. A looping, hairless caterpillar drops on silk. Two pairs of prolegs on the back half.
2. If this is one spring on a healthy mature tree, wait for the second flush. Water through dry weather.
3. Band in February, before she climbs. Batting, then a wrap, then sticky compound on the wrap. Never on the bark. Refresh debris. Band a touching crown.
4. Bt only while the worms are still small and feeding, if the tree is young, already stressed, or this is a second heavy year. A fog after the elm is already skeletonized is too late.

Related reading

• Fall cankerworm — three pairs of prolegs; eggs in neat rows on twigs, not bark crevices.
• Tent caterpillar — dense silk in a branch fork in spring, not a looping drop.
• Fall webworm — loose web on deciduous tips in late summer, many caterpillars inside.
• Spongy moth — hairy larva with blue then red spots; no communal tent.
• How to grow elm — a tree these loopers hit hard in a May outbreak.