From the emerald ash borer to Dutch elm disease to spotted lanternflies, there are a number of invasive pests and diseases that threaten trees across Michigan. One other important invasive species that’s currently wreaking havoc on our local trees is the spongy moth.
We’ll share everything you need to know about these insects below, including some of the signs they’re feeding on your trees, which trees are at greatest risk, and what — if anything — you can do to protect your trees.
Spongy Moth 101: The Basics
There are actually a few different spongy moth species and subspecies that threaten plants and trees around the world, but only one — the European spongy moth (Lymantria dispar dispar) — is established in Michigan.
Formerly called the gypsy moth, the European spongy moth is a sexually dimorphic creature, meaning that the adult males and adult females look different. The females are flightless insects with white wings that may measure up to 2 inches across. The males, on the other hand, are smaller, primarily brown in color, and fly quite well.
But it isn’t the actual moth that causes most of the trouble; it is the larval caterpillar stage that causes problems. These voracious insects consume leaves, sometimes completely defoliating plants and trees in the process. In fact, individual caterpillars have been known to consume as much as 1 square foot of leaves while growing.
Where Did Spongy Moths Come From?
The spongy moth is historically native to portions of Europe and Asia. Shortly after the Civil War, a French scientist located in Massachusetts began trying to develop a hardier strain of silkworms. This, unfortunately, led him to accidentally release spongy moths into the wild, where they were able to establish a thriving population.
In their native range, spongy moth populations are kept in check by an array of specialized predators, diseases, and parasites. A few animals consume these moths and caterpillars in the U.S., but there are far fewer, which has allowed spongy moths to expand their range relatively unchecked.
Currently, spongy moths are found across most of the northeastern United States, from Maine to North Carolina, and across the northern United States as far as Minnesota.
How Do Spongy Moths Harm Trees?
During outbreaks, spongy moth caterpillars may defoliate entire trees. This leads to considerable stress, temporarily prevents the tree from photosynthesizing, and leaves the tree vulnerable to secondary threats.
Conifers may die from a single defoliation event, but hardwoods often fare better. If no further stress occurs, many will replace the lost leaves within the same season and go on to recover. But repeated attacks start to weaken trees, and even hardwoods that were initially healthy may enter a decline spiral and die.
Which Trees Are at Risk of Spongy Moths in Michigan?
In total, spongy moths are thought to feed on more than 300 plant and tree species in the U.S. (and some sources claim the total is closer to 500 species). Some of the trees they target most commonly include:
- A variety of oaks, including members of both the red and white oak groups
- Apples, crabapples, and hawthorns
- Sweetgums
- Willows, aspens, and birches
- Witch hazels
- Lindens (basswoods)
- Serviceberries
Additional trees that are vulnerable to the moths without being their preferred hosts include:
- Black gums
- Persimmons
- Dogwoods
- Several hickories
- Pears
- Maples
- Paw paws
- Sassafrases
- Pines and spruces
What Are the Signs and Symptoms of Spongy Moths?
There are a few different signs and symptoms of spongy moth infestations, which homeowners and commercial property managers should watch for.
Egg Masses
Tan-colored, oval-shaped, fuzzy egg masses that measure about 1.5 inches long. They may be spotted on outdoor tree trunks and branches, as well as the sides of homes, outdoor furniture, and firewood. Egg masses may be found anytime between late summer, when they’re deposited, and early spring when they hatch.
Caterpillars
Spongy moth caterpillars are hairy and bear five pairs of blue spots and six pairs of red spots. They are about 2.5 inches long and typically begin emerging in April or May in Michigan. The caterpillars feed for several weeks and then spin brown cocoons in June or July before developing into adult moths over the next 10 to 14 days.
Adult Moths
As mentioned earlier, female spongy moths are white-winged and flightless, while males are smaller, brown, and fly well. They may be seen during July or August, but individual moths only live for a few days. Females will produce one clutch of eggs before dying.
Frass
Frass is a term that simply means insect droppings. In the case of spongy moth caterpillars, the frass may be obvious enough for homeowners to notice, as they produce a large number of small black pellets (the quantity of pellets produced has been characterized as “rain” by some).
Defoliation
In addition to spotting caterpillars in the late spring to early summer, you may also note partial or total defoliation of your tree. The leaves often show small holes initially, before the holes expand to encompass the entire leaf. This defoliation can sometimes happen quite rapidly, sometimes in less than a week.
Spongy Moth Treatments and Protective Measures
While spongy moths are a significant threat to Michigan trees, there are a few treatments and prevention strategies that can help protect your trees, including:
- Physical removal and destruction of the egg masses occur during the late summer through spring.
- Wrapping burlap straps around tree trunks, then killing the caterpillars that use the wraps as hiding places.
- Wrapping sticky bands around trunks will trap the caterpillars on their way to the leaves.
- Spraying Bacillus thuringiensis kurstaki (Btk) — a bacterium that kills the larvae — on leaves in May.
- Spraying conventional insecticides on the trees, though this is often harmful to beneficial insects.
- Collecting caterpillars and pupae by hand and destroying them.
It’s also important to avoid moving wood materials from place to place, as this is a major route by which the spongy moth spreads.
Spongy Moths in Michigan: FAQ
Still have questions about spongy moths and the threat they present to your trees? We’ve rounded up a few of the most common questions our customers have and answered them below.
What to do if you see a spongy moth?
Always contact a certified arborist to assess your trees and recommend the best treatment options if you see spongy moths or spongy moth caterpillars on your property.
Why are spongy moths a problem?
Spongy moths are a problem because they can defoliate trees, and, as an invasive insect, they don’t have many effective predators, which can allow populations to reach very high densities.
How can you get rid of spongy moths naturally?
Hand removal of egg masses and wrapping trees with burlap or sticky tape are all 100% natural approaches for combating spongy moths. It should be noted that Btk sprays are also naturally occurring bacteria and approved for organic use, but some people don’t consider this a “natural” approach.
How long will the spongy moth infestation last?
Typical spongy moth infestations last about one to three years. These outbreaks tend to occur about once per decade, though this can vary from one location to the next.
Will trees recover from spongy moths?
Healthy trees will often recover from a single spongy moth defoliation event, but repeated attacks can weaken trees and leave them vulnerable to secondary stressors.
Does spraying for spongy moths work?
Yes, Btk spraying for spongy moths can work, assuming it is carried out during the proper time of year (generally May in Michigan).
How much does BTK spray cost?
BTK spray is generally affordable, but prices vary from about $15 to $200, depending on the specific product and concentration you select. Professional application of the spray (something that’s required for tall trees) will add another $100 to $500 per treatment.
Let Longtree Tree Service Help Protect Your Trees
It’s important to act quickly when you notice a spongy moth problem. There are a few DIY approaches you can use to address the problem, but you’ll likely enjoy better results if you solicit the help of an ISA-certified arborist.
Fortunately, that’s easy for residents in Southeast Michigan. Whether you’re in Beverly Hills, Detroit, or Royal Oak, professional arboricultural assistance is available. Simply contact the Longtree Tree Service team, and we’ll be happy to schedule and onsite evaluation and provide you with a free estimate.
We’ve been providing unmatched tree-care services for homeowners and commercial property managers across Southeast Michigan since 2019. And during this time, we’ve built a reputation for being the tree-care team our customers can trust.
Contact us today to schedule your free estimate.
Sources & References
- Washington State Department of Agriculture – “What Is BTK?”
Explains the role of Bacillus thuringiensis kurstaki (BTK) as a biological control agent for invasive moth larvae, how it works, and when it’s used.
https://agr.wa.gov/departments/insects-pests-and-weeds/insects/invasive-moths/btk/what-is-btk - University of Wisconsin–Barron County Extension – “Gypsy Moth: Identifying and Managing Gypsy Moth Caterpillars”
Provides a research-based guide to identifying gypsy moth caterpillars, understanding their life cycle, and recommended management strategies for homeowners and land managers.
https://barron.extension.wisc.edu/files/2023/02/Gypsy-Moth-Identifying-and-Managing-Sypsy-Moth-Caterpillars.pdf - Michigan Department of Natural Resources – “Spongy Moth (formerly Gypsy Moth)”
Offers state-specific information on spongy moth identification, impacts, reporting procedures, and management recommendations for Michigan landowners.
https://www.michigan.gov/invasives/id-report/insects/spongy-moth - U.S. Department of Agriculture (USDA) APHIS – “Spongy Moth”
Provides federal-level information on spongy moth biology, distribution, regulatory status, and integrated pest-management approaches.
https://www.aphis.usda.gov/plant-pests-diseases/spongy-moth - Michigan State University Extension – “Dealing with Spongy Moth Around Your Home or Property”
Presents practical, science-based tips for recognizing, monitoring, and reducing spongy moth populations in residential and recreational landscapes.
https://www.canr.msu.edu/resources/dealing-with-spongy-moth-around-your-home-or-property - Michigan State University Extension – “BTK: One Management Option for Gypsy Moth”
Explains the use of Bacillus thuringiensis (BTK) as a management tool for gypsy/spongy moth caterpillars, including application timing and effectiveness.
https://msu-prod.dotcmscloud.com/resources/btk-one-management-option-for-gypsy-moth
