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. 

Spongy moth's do better in warm, dry conditions - so if we have another warm, dry spring and summer in 2024, we could be in for another high spongy moth year.

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 that can lead to problems.  Plus, the caterpillars can cause an itchy rash in some people - so it is best not to touch the caterpillars directly

The defoliation is troubling to see, especially when its beautiful mature oaks and other beloved trees when they 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 stressor. 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, 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.

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.

 

State of Wisconsin

The State of Wisconsin Department of Natural Resources has a a thorough spongy moth portal that has links to the same UW-Extension site above for yard tree management and also other information about woodlot management and more.

 

Dane County Office of Energy & Climate Change

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

 

Contact a Certified Aborist 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: Property Mulch Techniques
• Penn State Extension: Mulching Landscape Trees

 

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 will be out throughout the winter of 2023-2024 and until the egg masses hatch in spring.

Urban Forestry crews have been spraying 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 are spraying the soybean-based horticultural oil described above.  The oil deprives egg masses of oxygen so they cannot hatch.

 

 

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 all together but it reduced the population and the effect it has on trees. The DNR has ended this program in places where the moth is established (like in Madison).

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 Partnership

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

A single spongy moth pheromone trap will be installed on an existing tree within the known infestation area on the west side of Madison. This trap will not cause damage to the tree.

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

Engineering and Forestry are in the process of determining spongy moth impacts in this area and next steps.

 

have questions about the spongy moth caterpillar?

Send Urban Forestry an email.

 

wait. didn't this moth used to be called something different?

Yes.  It was changed by the Entomological Society of America as part of their Better Common Names Project.