Fall Cankerworm

Wrap a sticky band after the leaves drop. The moth that lays next spring's eggs has no wings. She crawls up the trunk. Batting under a wrap, and sticky compound on that wrap, stops her. Glue smeared on the bark is leftover. It can injure the tree, and she walks around the dry patches.

Fall cankerworm is a native geometrid moth, Alsophila pometaria. People call the larva an inchworm because it loops as it walks. In spring, tap a chewed oak or elm leaf. A hairless looping caterpillar drops on a silk thread. Three pairs of prolegs, the front pair short, confirm this moth. A healthy tree usually pushes a second flush after one spring of feeding. Bt only while those worms are still small.

• Band after leaf drop. Sticky compound goes on the wrap, not on the bark.
• Tap a chewed leaf. The larva drops on silk.
• A spray after the canopy is already veins-only is too late.

Wrap a sticky band after the leaves drop

Wait until most of the leaves are off. Leaves stuck to a sticky strip make a dry bridge, and the moth walks across. Wrap cotton batting or pipe insulation around the trunk below the first limbs. Pack the furrows so she cannot crawl under. Cover that padding with plastic, tar paper, or roofing felt. 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. Refresh the sticky layer. If this oak's crown touches the next maple, band that tree too, or she walks across. Keep the wrap up through early May if spring cankerworm also uses this trunk. See spring cankerworm for that moth's late-winter climb.

Minnesota Extension does not sell banding as a neighborhood finish. It is messy, and it misses females that already crossed. Use it on the trees you mean to protect. Fail if glue goes on the bark and the wrap is never checked.

A healthy tree usually leafs out after one spring

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

Three heavy years in a row, or one year on a newly planted oak 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. 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: an inchworm drops on silk

The confirm is the drop, not the color. Green, brown, or almost black larvae all belong to this moth. Darker worms can mark a heavier year. Tap a chewed leaf. A looping, hairless caterpillar about an inch long when grown drops on a silk thread. Count the fleshy prolegs on the back half. Three pairs, with the pair closest to the head clearly shorter, is fall cankerworm.

In winter, look on small twigs for neat rows of barrel-shaped eggs, gray with a dark spot and a lighter ring on top. About a hundred eggs sit in a compact mass. That is next May's hatch. The female that laid them is wingless, gray-brown, and about five-sixteenths of an inch. She does not look like a moth. The male is a small gray flyer.

Two prolegs is spring cankerworm; a silk tent is a different moth

Spring cankerworm larvae hatch in the same week and often feed on the same elm. They have two pairs of prolegs, not three. Their eggs sit in bark crevices, not in neat rows on twigs. See spring cankerworm. Do not run that moth's February 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 three-proleg check still belongs to fall cankerworm.

Bt only while the worms are still small

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 bottle will not rewind the flush. 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 twig mass. Most home sprays miss the top. Skip the brand aisle if coverage on the leaves you can reach is the missing step. Broad leftover sprays that kill wasps can leave you with more loopers next year, not fewer.

Confirm the loop, then band, wait, or spray young

1. Confirm this moth. Tap a chewed leaf. A looping, hairless caterpillar drops on silk. Three pairs of prolegs, the front pair short.
2. If this is one spring on a healthy mature tree, wait for the second flush. Water through dry weather.
3. Band after the leaves drop. 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. Fail if the finish is a fog after the oak is already skeletonized.

Related reading

• Spring cankerworm — two pairs of prolegs; eggs in bark, not neat rows on twigs.
• 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 oak — the tree these loopers hit hardest in a May outbreak.