Spongy moth is an invasive caterpillar that strips the leaves off oak, aspen and birch, and it is working its way toward the Twin Cities from the east and southeast corners of Minnesota. It is not established in Minnetonka, Wayzata or the rest of the west metro yet, but egg masses travel easily on firewood and outdoor gear, so it pays to know what to look for before it shows up in your yard.
What Is Spongy Moth?
Spongy moth (Lymantria dispar) is an invasive forest pest introduced from Europe. It feeds on more than 300 species of deciduous trees and shrubs, with oak, aspen and birch at the top of its host list. One caterpillar does little harm. A full outbreak is different: once a population establishes itself, it cycles through waves of heavy defoliation every eight to twelve years, stripping trees bare in early summer and leaving them stressed for the rest of the season.
Is Spongy Moth in the Twin Cities Yet?
Not established, at least not yet. Here is where things stand:
- Trapping since 1973: the Minnesota Department of Agriculture has run detection trapping for decades and has eradicated every isolated infestation it has found.
- Active quarantine: the current emergency quarantine covers Lake and Cook counties in the northeast.
- Where it turns up: most often in the southeast and northeast corners of the state, with the invasion front moving in from the east.
- What that means locally: Minnetonka, Wayzata, Maple Grove and Brooklyn Park sit ahead of the front for now, but egg masses hitchhike on firewood, campers and outdoor furniture, which is exactly how isolated populations appear somewhere new.
What Does Spongy Moth Damage Look Like?
The clearest sign is defoliation in June, when a tree that leafed out normally in spring suddenly looks thin or skeletonized. Watch for:
- Chewed, ragged leaf edges and a canopy that thins out in early summer
- Caterpillars climbing trunks and siding
- Frass (caterpillar droppings) collecting under the tree
- Tan or buff colored egg masses about the size of a quarter, usually on the underside of branches, on outdoor furniture, or on stacked firewood
A single season of defoliation rarely kills a healthy tree. Repeated defoliation over consecutive years is what does the damage, because the tree never fully recharges its energy reserves before it is stripped again.
Which Trees in Minnesota Yards Are Most at Risk?
- Highest risk: oak and aspen
- Also on the host list: birch, basswood and crabapple
- Highest risk of all: any of the above already stressed by drought, construction damage, girdling roots or a prior storm
A stressed tree has far less reserve to recover from a defoliation event, which is why our crews assess overall tree health, not just the pest, when we look at a property in Minnetonka or Edina.
Spongy Moth vs. Emerald Ash Borer
Twin Cities homeowners are already dealing with one confirmed invasive pest, so it helps to know how the two compare.
| Pest | Status in Twin Cities metro | Trees at risk | How it kills |
|---|---|---|---|
| Emerald ash borer | Confirmed and established | Ash trees only | Larvae tunnel under the bark, cutting off nutrient flow |
| Spongy moth | Not established; quarantine active in Lake and Cook counties | Oak, aspen, birch, 300+ species | Repeated defoliation stresses the tree until it cannot recover |
What Can Homeowners in Minnetonka and Wayzata Do Now?
- Do not move firewood. That single habit is the fastest way an isolated population turns up somewhere new, which is why the state quarantine restricts moving regulated articles out of infested counties.
- Destroy egg masses properly. Scrape them off the tree, fence post or woodpile and drop them into soapy water rather than leaving them in the yard.
- Keep trees healthy. A tree already stressed by drought or root damage is the one that suffers most if defoliation arrives.
- Get an ID when in doubt. Thinning leaves in June, chewed foliage or an unfamiliar egg mass can be settled in one arborist visit.
Our crews inspect trees across the west and north metro, and the visit that flags pest pressure is usually the same visit that catches a hazard limb or a tree that needs tree trimming before it becomes a bigger job. When a tree is too far gone to save, whether from pest pressure, storm damage or age, our tree removal crews handle it safely from Minneapolis to Wayzata.
Frequently Asked Questions
Will spongy moth kill my tree in one season?
Rarely. A single year of defoliation stresses a tree, but most healthy trees releaf and recover. The real risk is repeated defoliation over consecutive years, or defoliation on a tree already stressed by drought, construction damage or root issues.
How do I know if it is spongy moth and not another pest?
Egg masses, caterpillars in early summer and June defoliation on oak, aspen or birch are the signature combination. Emerald ash borer, by comparison, only affects ash trees and shows canopy dieback and D-shaped exit holes rather than defoliation. If you are not sure, an arborist can identify it on site.
Should I spray my trees as a precaution?
Not without a confirmed sighting. Spongy moth is not established in the west metro, and preventive treatment against a pest that has not arrived is money better spent on general tree health, which is the biggest factor in whether a tree survives a defoliation event when one does happen.
What should I do if I find an egg mass on firewood?
Do not move the firewood. Scrape the mass into a container of soapy water and dispose of it, then burn or use the wood on site rather than transporting it, which is how isolated infestations spread to new counties.
If you are seeing thinning leaves, chewed foliage or anything you cannot identify on trees in Minnetonka, Wayzata, Edina or the surrounding suburbs, call Dynasty Tree Experts at (763) 301-9185 or request an inspection online. For more detail on the state’s monitoring program, see the Minnesota DNR spongy moth page.