Is It Too Late for Winter Moth Treatment Timing on the Northshore Eastside?

Winter moth treatment timing starts in the fall, when adult moths emerge across Bothell, Kenmore, and Woodinville from October into December. Flightless females climb tree trunks to lay eggs, and the caterpillars that hatch from them damage buds and leaves from March through mid-summer.

Trunk bands are the only step that has to happen in fall. October is ideal, but the bands that go up in early to mid-November can still catch climbing females.

Key Takeaways

• Moths at your lights in November mean adults are emerging and females are laying eggs on trunks, not that treatment is over.
• Trunk bands are the only fall step, and they work best when put up in October, though early to mid-November still helps.
• Dormant oil goes on during a mild, open spell in February or early March, and never in freezing weather.
• Young caterpillars are hard to control while they feed inside unopened buds, so spring treatment works best once they move out onto expanding leaves.
• Banding, oil, and caterpillar treatment each have their own window between October and June, so a plant health care program can take the timing off your plate.

Bands go up best in October, though early to mid-November can still catch climbing females.

What Is the Best Time to Treat Winter Moths?

Ideal winter moth treatment timing runs from October all the way to June. Here’s when each step should take place in Bothell and the surrounding Northshore Eastside:

• October to Mid-November: Put up trunk bands.
• October to December: Males fly while flightless females climb trunks to lay eggs.
• February to Early March: Apply dormant oil during a mild, open spell of weather.
• Early Spring at Bud Break: Eggs hatch and larvae start feeding inside buds.
• Just After Bud Break: Treat small caterpillars while they are still exposed.
• June to July: Larvae drop to the soil to pupate.

Do Sticky Tree Bands Work?

Sticky tree bands can help, but they aren’t a guaranteed fix. The later you put them up, the fewer females they can catch. When you choose to use bands, keep these specifics in mind:

• Timing: Band in October, though moths stay active through December.
• Material: Use Tanglefoot or a similar sticky material on a polyethylene strip secured around the trunk.
• Fit: Wrap the strip tightly so nothing can crawl underneath, and put the sticky material on the wrap, never directly on the bark, since it can harm thin bark.
• Detection: Check the band for females, and if you find them, plan a dormant oil application in late winter to smother the eggs.
• Limits: Bands cannot stop caterpillars from drifting in from nearby trees.

Dormant oil goes on during a mild, open spell in February or early March, never in freezing weather.

Does Dormant Oil Work on Winter Moth Eggs?

Dormant oil can help when it comes to winter moth eggs, but it can still miss some of them. There are a few factors that affect how well it works:

• Timing: Oil smothers the eggs, and the best window appears to be a mild, open spell in February or early March. That timing comes from blueberry research, so treat it as a guide for landscape trees rather than a strict protocol.
• Coverage: Eggs tuck into bark crevices, so oil misses some of them.
• Temperature: Don’t spray oil during freezing weather, or the plant can be damaged.

Caterpillar treatment works best once the buds have opened and young caterpillars are feeding on expanding leaves.

When Should You Treat Winter Moth Caterpillars in Spring?

Treat winter moth caterpillars once they’re out of the buds to feed on expanding leaves. The youngest caterpillars burrow into unopened buds, where they’re hard to control. You can only visually gauge damage after the buds break.

There are a few effective insecticides and biological products, but homeowners shouldn’t make foliar applications to trees that are over 10 feet tall. That’s where an arborist should come in.

The Trees That Need It Most

Landscape trees can tolerate early feeding, so treatment is more important for trees that get hit hard every year. Repeated defoliation leads to problems like:

• Dieback: Three or more years of heavy defoliation can cause branch dieback and death.
• Slower Recovery: Defoliation can also slow growth, slow wound closure, and impair roots.
• Secondary Problems: That stress can open the door to wood borers and decay fungi. Birch owners should read our guide to the bronze birch borer, and our overview of common fungal tree diseases covers the decay side.

Who Should Handle Winter Moth Caterpillar Treatment

On trees over 10 feet tall, oil applications and caterpillar treatments should be handled by a licensed applicator, since both are dependent on precise timing and complete coverage. Book a visit sooner rather than later if a tree meets any of these conditions:

• It lost its leaves to caterpillars in more than one year.
• It had heavy bud damage last spring.
• It is too large or hard to reach to spray safely.

An arborist consultation gets the tree inspected, and the treatment dates set before the new window even opens.

Frequently Asked Questions About Treating Winter Moths

How can I tell if winter moth caterpillars are on my tree?

Look for pale green inchworms with a light stripe down each side, up to about 1/2 inch long. Tattered leaves tied together with silk threads are another sign, and an arborist can confirm the cause before you treat.

Can biological control keep winter moths in check?

Only partly. A parasitic fly, Cyzenis albicans, brought winter moth populations down in British Columbia and Oregon after releases in the late 1970s and early 1980s, but winter moth still damages maples and other trees often enough to need regular management here.

What trees do winter moths attack in Washington?

Winter moth feeds on a wide range of deciduous trees, and oak, bigleaf maple, paper birch, willow, aspen, and balsam poplar are all listed hosts in the Pacific Northwest.

Let Seattle Tree Care Handle Your Winter Moth Treatment

Our plant health care program covers every window (from fall banding to spring caterpillar treatment), so you don’t have to keep track of them yourself. We provide tree service in Bothell, Woodinville, and across the Northshore Eastside. Request a quote today, and we’ll build a plan specifically for your trees.