Spongy Moth (Lymantria dispar)

 

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Why were there so many spongy moths around lately? And what makes it such a concern?

In 2023, a combination of environmental factors created opportune conditions for the caterpillar/moth. Those environmental factors came at a time when there was a population spike in this pest.  Every 10 to 15 years there is a boom in the spongy moth population. 

In its caterpillar form, it can eat a whole lot of the leaves off a tree. This is called defoliation. And it is a stressor for trees.  Plus, the caterpillars can cause an itchy rash in some people - so it is best not to touch the caterpillars directly.

With the 2023 population boom, 2024 could result in another difficult year for the caterpillar if conditions are favorable yet again.  The caterpillar thrives in warm, dry times. 

The defoliation is troubling to see, especially when its beautiful mature oaks and other beloved trees go from full canopies to losing so many leaves to these ravenous invasive caterpillars.

 

Mature trees will rebound with new leaves to replace the ones lost to the caterpillars.  But that defoliation is a serious problem.

Trees need a lot of energy to produce more leaves.

If you stack the stress of needing to flush new leaves on top of drought conditions and other issues, like problems from climate change or other pests - it becomes more worrisome about the health of the tree. 

And if the tree has to bounce back from this cycle again and again it becomes harder each time to recover.

Imagine it like a person being sick. 

For a person to fully recover, it's best for them to be well rested and have plenty to drink - "push the fluids" as the doctors always say.  And also time. 

Same is true with trees.  Help them avoid stress repeatedly, and keep them watered.

 

resources for homeowners

If you have concerns about spongy moths on trees on your private property, residents should use these resources.

 

State of Wisconsin

The State of Wisconsin Department of Natural Resources and Department of Agriculture, Trade, and Consumer Protection along with the University of Wisconsin-Madison Extension has a thorough Spongy Moth Resource Center.

On the Spongy Moth Resource Center, you can learn how to distinguish spongy moth caterpillars from native varieties, and other important information.

The Spongy Moth Resource Center also has a hotline you can call with questions.  Their number is 1-800-642-6684. 

You can also email questions to them at spongymoth@wisconsin.gov.

 

UW-Extension

The University of Wisconsin Extension has season-by-season guide for homeowners of things you can do to protect your trees.

More information about the moth covering everything from its life cycle to the species history can also be found on the UW-Extension website.

 

Dane County Office of Energy & Climate Change

Dane County has a webpage of resources and photos for residents.

 

Contact a Certified Arborist for Your Private Trees

You can also contact a certified arborist for help with trees on your property.

 

Water and mulch your trees - this makes a big difference

Help your trees endure the damage caused by spongy moth caterpillars is to reduce it's stress by keeping it well watered and postpone any pruning.  

 

Water Your Trees as Needed

The perfect amount of water is hard to pinpoint for every tree species, soil type, and weather conditions.  The general rule of thumb is that if the soil feels dry then the tree likely needs water.  According to the Arbor Day Foundation, trees need 10 gallons of water for each inch of the trees diameter - but you have to be careful not to over water the tree, too.

The Arbor Day Foundation has an excellent resource with more details when to water your trees and how much they need to thrive (they also have this short three minute video version of the information).

You can also read this post from the Wisconsin DNR Forestry news or jump to page 13 of the USDA "Tree Owner's Manual for Northeastern and Midwestern United States" for more watering tips

 

Deploy Mulch the Right Way

There are right and wrong ways to apply mulch around your tree.  Doing it wrong could actually be harmful.

The basic benefits of mulch is that it helps your trees retain moisture, moderates soil temperatures, and prevents soil compaction.

The basic rules are you want it about 2 to 4 inches deep, and you do not want to mound it up near the tree. In other words, do not create "mulch volcanoes." Piling up excess near the base of the tree is bad.

More detailed resources about how to mulch the right way can be found here:

• How to Mulch: Arbor Day Foundation

  Trees are Good: Proper Mulch Techniques
• Penn State Extension: Mulching Landscape Trees

 

use burlap bands to trap caterpillars

From the late spring through around August, you can wrap a burlap band around the trunk of your tree to prevent caterpillars from crawling into the canopy. 

 

Watch the DNR Video to Learn How

The Wisconsin DNR has an excellent video that shows the steps on how to appropriately install a burlap band and what to do with the caterpillars once you trap them.

Watch the video here.

 

Burlap Application Step by Step

Step One: Wrap a 20-inch wide (or so) band of burlap completely around the trunk of your tree at around chest height.

Step Two: Tie the burlap band to the tree.  Use rope or twine works best.  Position the rope at the center of the band.

Step Three: Fold the top 6 inches of the burlap band down so it flops toward the string around the middle of the band.  This creates a trap where caterpillars crawling up from the ground cannot pass.

Step Four: Check the burlap band every afternoon for trapped caterpillars.  This is a daily task you must complete.  Scrape any caterpillars you find into a jar of soapy water to drown them, and then discard them into the trash.

 

When Do You Remove the Burlap Band?

You should remove them in August.

The caterpillars move to their next stage of development at this time, and the bands will no longer be effective.

Remember in the fall, you can begin destroying egg masses you find.

 

What Happened to the Free Burlap?

Burlap was available while supplies lasted, and both locations that provided burlap to residents ran out as of June 7, 2024.

 

destroy egg masses

 

To help control the moth population, residents can destroy the egg masses.

The best time to do this is from October to April.

Masses look like fuzzy brownish patches that can be found on tree trunks, under picnic tables, along sheds, RVs, and many other flat surfaces. Here are some sample photos to help you know what they look like.  Here is a zoomed in photo from the WI DNR. Here is a photo from the Dane County spongy moth website showing the egg masses more true to size found under a picnic table.

Egg masses can hold up to 1,000 eggs, so destroying them on your property can help control this pest.

Here are the two methods that are most effective at destroying the egg mass.

You cannot simply scrape them and try to squish them.  They are too strong to be ground by the heel of your shoe, and if left on the ground they will still hatch.

Option 1: Scrape - Soak - Trash

Use a putty knife or similar flat edged scraping tool, remove the egg mass and submerge it within a container of soapy water.  Use household dish soap.

Keep the eggs submerged for two days. Then toss the soggy masses into the trash.

Option 2: Golden Pest Oil / Horticultural Oil

Apply a product called golden pest oil to the egg masses. This is a natural soybean oil that can be found at garden centers, though it may be hard to find in amounts appropriate for a homeowner and it can be quite expensive. But the oil, when correctly will prevent the egg masses from hatching.

If you elect this method, you must use the oil as directed. The oil directions will tell you to mix it with water before applying to the eggs. You need to do this step so it will soak into the egg mass. 

 

Mailing flyers

The Urban Forestry Division mailed flyers to 3,411 homes around the Kenosha Greenway area in the winter of 2023 and also the Westmorland Park area early spring of 2024.

The flyer, which you can see in the image above, shows how homeowners can destroy egg masses.

From the fall through the spring, you can undertake the steps to find, scrape, soak, and trash egg masses around your property to help control the spongy moth population.

 

city oiling egg masses from the ground

Spraying the oil can only occur when temperatures are above 40 degrees, and crews were out throughout the winter of 2023-2024 when temperatures allowed.

Urban Forestry sprayed egg masses on trees within the right of way spaces in the areas around Whitney Way and Mineral Point Rd areas where the largest concentration of egg masses in the city was noticed in 2023.  Parks Division crews applied oil to known egg masses in select parks as well..

The crews sprayed the soybean-based horticultural oil described above.  The oil deprives egg masses of oxygen so they cannot hatch.

 

 

oak tree injections

Starting in May 16, 2024, Urban Forestry crews began treating oak trees identified by the Parks Division with Tree-age R10 (which has the active ingredient emamectin benzoate).  This is the same product used to treat ash trees from the emerald ash borer. 

This oak tree treatment provides control against the spongy moth caterpillar from feeding on the leaves this year.

The treatment also provides protection for the oaks from the two-lined chestnut borer. The two-lined chestnut borer is a "secondary pest" that attacks stressed oak trees.

Oak tree injections were completed in early June 2024.

Staff performing the treatment are all Pesticide Applicator Certified by the Wisconsin Department of Agriculture, Trade, and Consumer Protection.

what about aerial spraying?

In past years, the Wisconsin Department of Natural Resources (DNR) had offered communities the option to pay for a state-arranged aerial spray program for both public and private property for this pest as part of a suppression program. 

This suppression program did not eliminate the pest. It reduced the population.

The DNR has ended this program in places where the moth is established.  Since the moth is established here in Madison, the DNR no longer supports aerial spraying in our community.

A different state agency, the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) has a separate spraying program, but it focuses solely on counties near the Iowa and Minnesota border to help slow the spread of the moth into those states.

 

Spongy Moth Genetic Research

The City of Madison is participating in a genetic study of spongy moths conducted by the James Lab at the University of Toronto.

This is not a population control effort like other initiatives.  This is a study.

In 2023, a single spongy moth pheromone trap was installed on an existing tree within the known infestation area on the west side of Madison. This trap did not cause damage to the tree.

The type of trap is not used for mass removal, but is used to collect species as part of scientific research and monitoring. 

The moth collection for the study may return in 2025.

 

have questions about the spongy moth caterpillar?

Send Urban Forestry an email.

 

Same Moth; New Name

In 2021, the name of this species was changed by the Entomological Society of America as part of their Better Common Names Project.