By fall, the spongy moth caterpillars that chewed through Wisconsin trees earlier in the year are long gone. But the next generation may already be hiding in plain sight on your property.
Look closely at tree trunks, firewood, outdoor furniture, and other sheltered surfaces, and you may find their masses tucked into protected spots. Each one can hold hundreds of eggs that will remain there through the winter before hatching into hungry caterpillars next spring.
That makes fall an especially useful time to act. We’ll show you how to recognize spongy moth egg masses, where to look for them around your Greater Madison property, and how to remove and destroy the ones you can safely reach.
What Do Spongy Moth Egg Masses Look Like?
Spongy moth egg masses don’t look much like eggs at all. Instead of individual eggs, you’ll see a tan or buff-colored patch covered in a fuzzy, felt-like coating. That covering helps protect the eggs through Wisconsin’s winter.
Each egg mass can contain 500 to 1,000 eggs. Female moths lay them in summer, and the eggs remain dormant through fall and winter before hatching the following spring.
You may find egg masses attached to tree trunks and branches, but spongy moths aren’t particular about where they leave the next generation. Egg masses can also turn up on firewood, outdoor furniture, vehicles, buildings, and other protected surfaces around your property.
Why Fall Is the Best Time to Find Egg Masses
By fall, spongy moths have completed their active life cycle for the year, but the eggs they leave behind are just beginning a long dormant period. Females lay egg masses in summer, and those eggs remain in place through fall and winter before hatching the following spring.
That gives Greater Madison homeowners several months to look for and remove accessible egg masses before the caterpillars emerge. Fall can be especially convenient as leaves drop and trunks, branches, and other surfaces become easier to inspect.
Removing egg masses now won’t completely eliminate spongy moth from your property, but each mass destroyed can mean hundreds fewer caterpillars hatching nearby in spring. The key is finding them.
And that means knowing some of the less obvious places to look.
How to Find Spongy Moth Egg Masses on Your Property
To find spongy moth egg masses, start by looking at your trees, especially oaks and other preferred hosts. Check trunks from the ground up, paying close attention to rough bark, crevices, the undersides of branches, and other protected spots where a female moth could tuck away an egg mass.
But don’t stop at the trees. Spongy moths can lay their eggs on almost any sheltered outdoor surface.
So, check firewood piles, birdhouses, sheds, fences, play equipment, benches, picnic tables, and patio furniture. If something has been sitting outside all summer, it may be worth a look before you put it away for winter.
How to Remove and Destroy Spongy Moth Egg Masses
Finding an egg mass is only half the job. Simply scraping it off a tree and letting it fall to the ground may leave viable eggs behind, so make sure you destroy the mass after removing it.
For egg masses you can safely reach, use a putty knife or similar tool to gently scrape the entire mass from the surface and into a container of soapy water. Leave egg masses that require climbing or awkward reaching alone.
Be careful when scraping masses from tree bark so you don’t damage the tree in the process. Once you’ve scraped the masses into the soapy water, let them soak for a few days before disposing of them in the trash.
If you’re finding large numbers of masses, particularly on mature trees where many are out of reach, that may be a sign that it’s time to consider professional help.
| Another Option: Horticultural Oil
Scraping isn’t the only way to destroy accessible spongy moth egg masses. A horticultural oil labeled for spongy moth egg masses can be applied directly to the mass to prevent the eggs from hatching. Follow the product label carefully, including application and temperature instructions. Never substitute motor oil or another lubricant, which can damage trees and harm the environment. |
Spongy Moth in Greater Wisconsin: Which Trees Should Homeowners Check First?
If you have oaks (Quercus spp.) on your property, start there. Oaks are among the spongy moth caterpillar’s preferred hosts, making them especially important to inspect for egg masses before the next generation hatches.
Don’t overlook other deciduous trees, though. Spongy moth caterpillars feed on hundreds of tree and shrub species, with birches (Betula spp.), aspens (Populus spp.), crabapples (Malus spp.), willows (Salix spp.), and lindens (Tilia spp.) among their preferred hosts in Wisconsin.
Remember that egg masses aren’t limited to preferred host trees. Female moths can deposit them on many outdoor surfaces, so an oak-heavy property may be a good place to start your search, but tree trunks are only part of the territory you’ll want to inspect.
Can Trees Recover from Spongy Moth Defoliation?
Otherwise healthy deciduous trees can often survive a single round of heavy spongy moth defoliation and produce a new set of leaves within weeks. Losing that much foliage still puts significant stress on a tree, however, and recovery becomes more difficult when heavy defoliation happens repeatedly.
Trees already dealing with drought, disease, root damage, or other stress may also have a harder time recovering.
Oaks deserve particular attention because repeated defoliation can weaken them and leave low-vigor trees more vulnerable to secondary pests such as the twolined chestnut borer (Agrilus bilineatus).
That’s another reason fall egg-mass removal can be worthwhile. Reducing the number of caterpillars that hatch nearby next spring may help limit another round of feeding on trees that are still recovering from previous damage.
When to Call a Greater Madison Arborist About Spongy Moths
Finding a few spongy moth egg masses within easy reach doesn’t necessarily require professional help.
With that said, an arborist’s perspective can be useful when egg masses are widespread or high in the canopy, valuable trees have experienced heavy or repeated defoliation, or you’re seeing thinning foliage, branch dieback, or other signs that a tree may be struggling.
An ISA-certified arborist can evaluate both current spongy moth pressure and the overall health of your trees.
Depending on what they find, the best approach may include continued monitoring, supportive tree care, or professionally timed pest management before caterpillars cause significant feeding damage in spring.
Frequently Asked Questions About Spongy Moth Egg Masses
Spongy moth egg masses can remain on outdoor surfaces for months, leaving plenty of time to wonder what you’ve found and what to do about it. Here are answers to some common questions about managing them around your Madison-area property.
How many eggs are in a spongy moth egg mass?
A single spongy moth egg mass can contain 500 to 1,000 eggs. Destroying even a few fresh egg masses can therefore reduce the number of caterpillars that hatch nearby the following spring.
When do spongy moth eggs hatch in Wisconsin?
Spongy moth eggs generally begin hatching in spring as temperatures warm and trees begin producing new leaves. In southern Wisconsin, hatching can begin as early as mid-to-late April, although timing varies with weather from year to year.
Will scraping spongy moth egg masses onto the ground kill them?
No. Simply scraping an egg mass from a tree and leaving it on the ground may not kill the eggs. Scrape accessible masses into soapy water and allow them to soak before disposing of them.
Can spongy moth egg masses survive a Wisconsin winter?
Yes. Spongy moths overwinter in the egg stage, and the protective covering around each mass helps the eggs survive cold winter conditions before they hatch in spring.
Are old spongy moth egg masses still viable?
Not necessarily. Old, hatched egg masses can remain attached to surfaces after the caterpillars have emerged. Fresh masses are generally tan and firm, while older masses may appear faded, weathered, or softer.
Does removing egg masses guarantee I won’t have spongy moth caterpillars next spring?
No. Removing egg masses from your property can reduce the number of caterpillars that hatch nearby, but it cannot prevent caterpillars from arriving from surrounding areas. Young spongy moth caterpillars can disperse on silken threads and be carried by the wind.
What should I do if spongy moth egg masses are too high to reach?
Leave egg masses alone if removing them would require climbing or unsafe reaching. An arborist can assess egg masses that are inaccessible from the ground and help determine whether monitoring or professional pest management is appropriate.
HH Accurate Arborists: Spongy Moth Inspection and Treatment
Finding spongy moth egg masses this fall doesn’t mean another round of heavy defoliation is inevitable. It means you’ve found the pest at a stage when there’s still time to take action before caterpillars begin feeding next spring.
If egg masses are widespread, difficult to reach, or concentrated around trees already under stress, our ISA-certified arborists can assess spongy moth activity, evaluate the health of your trees, and recommend monitoring, treatment, or supportive care based on what they find.
HH Accurate Arborists serves homeowners throughout the Greater Madison area. Schedule an inspection with our team to assess spongy moth activity and help protect your trees before spring feeding begins.
Sources & References
- University of Wisconsin–Madison Extension – “Spongy Moth”
Wisconsin-specific guidance on spongy moth identification, life cycle, preferred hosts, tree impacts, egg-mass removal, horticultural oil, and other management options.
https://hort.extension.wisc.edu/articles/spongy-moth/ - Wisconsin Department of Natural Resources – “Spongy Moth”
State guidance on spongy moth history and establishment in Wisconsin, preferred host trees, defoliation and tree recovery, population monitoring, and egg-mass control.
https://dnr.wisconsin.gov/topic/foresthealth/spongymoth - Wisconsin Department of Natural Resources – “DNR Encourages Property Owners to Make Spongy Moth Treatment Plans Early”
Guidance on identifying fresh and old egg masses, where to find them around Wisconsin properties, preferred host trees, egg-mass abundance, physical removal, horticultural oil, and when professional treatment may be appropriate.
https://dnr.wisconsin.gov/newsroom/release/68686 - Wisconsin Department of Natural Resources – “Insecticide Options for Homeowners”
Wisconsin guidance on spring egg hatch and treatment timing, including typical mid-to-late April hatch in southern Wisconsin, homeowner treatment options, and professional management for larger trees.
https://dnr.wisconsin.gov/topic/foresthealth/spongyMothInsecticides