The Spongy Moth: Tracking an Invisible Invasion in Michigan's Forests
How scientists use aerial surveys, ground monitoring, and natural predators to protect Michigan's forests from a destructive invader
The Unseen Army in Our Backyards
Imagine this: it's a perfect summer day in Michigan. You spread a blanket under your favorite oak tree, anticipating a peaceful picnic. But within minutes, you're surrounded not by serenity, but by a rain of BB-sized droppings and hairy caterpillars descending from above. This unsettling scenario has played out across Michigan in recent years, courtesy of an invasive insect that has transformed outdoor living for many residents: the spongy moth 1 .
Serious Ecological Threat
Since its arrival in Michigan in the 1980s, the spongy moth has become one of the state's most destructive forest pests 3 .
Scientific Surveillance
A sophisticated scientific surveillance system helps track and combat this threat to Michigan's forests.
The Invader and Its Impact
From Distant Lands to Michigan Forests
The spongy moth's journey to Michigan began unexpectedly in 1869 when a well-intentioned French artist and entomologist named Étienne Léopold Trouvelot imported them to Massachusetts. He hoped to cross-breed them with silkworms to establish a robust silk industry in North America 3 .
The insect reached Michigan by 1980, first appearing in Midland and Clare counties 3 . Since then, it has established itself in every county across the state, becoming a familiar, if unwelcome, part of Michigan's ecosystem 3 .
Spongy moth caterpillar feeding on leaves
Anatomy of a Leaf-Eater
The spongy moth undergoes four distinct life stages, each with unique characteristics:
Egg Stage
Tan-colored, fuzzy egg masses about the size of a quarter contain hundreds of eggs. These overwinter on any surface 5 .
Pupa Stage
In midsummer, caterpillars spin silken cocoons where they metamorphose into adults over approximately two weeks 5 .
Adult Stage
Male moths are dark-brown with a wavy pattern and can fly, while larger females have cream-colored wings with black markings 3 .
More Than Just Nuisance
A single spongy moth caterpillar can consume one square meter of leaves during its lifetime 5 . In large numbers, this appetite translates to devastating defoliation. While they prefer oak, aspen, and birch trees, they will feed on over 500 different plant species 5 .
Human Impact
The caterpillars significantly impact human activities. Their droppings and the caterpillars themselves can make outdoor spaces unusable. The caterpillar's hairs can cause skin irritation and allergic reactions in some people 5 .
Defoliation caused by spongy moth caterpillars
Michigan's Aerial Surveillance: A Bird's-Eye View of Destruction
The Annual Sky Patrol
Each summer, while most Michiganders are enjoying beach days and barbecues, the Michigan Department of Natural Resources embarks on a critical mission: aerial surveillance of the state's forests. Specially equipped aircraft fly predetermined patterns over approximately 20 million acres of forestland, systematically documenting evidence of spongy moth damage 1 .
Trained observers meticulously map areas showing signs of defoliation—the technical term for leaf loss caused by the feeding caterpillars. From the air, this damage appears as brownish or thinning patches amidst the healthy green forest canopy. These aerial surveys provide the broad-scale data necessary to understand the infestation's scope and trajectory across Michigan's vast forested areas.
Aerial view of Michigan forests showing defoliation patterns
Tracking the Rise and Fall
The data collected from these annual flights reveals the cyclical nature of spongy moth outbreaks. By tracking defoliation acreage over time, researchers can identify population trends and predict future outbreaks.
Spongy Moth Defoliation in Michigan (2020-2024)
| Year | Defoliated Acres | Trend Description |
|---|---|---|
| 2020 | 947,900 | High infestation level |
| 2021 | 1,350,000 | Peak outbreak year |
| 2022 | 386,000 | Significant decline |
| 2023 | 156,000 | Continued decline |
| 2024 | 168,879 | Stable low levels |
This data reveals a clear story: Michigan's most recent spongy moth outbreak peaked in 2021 with a staggering 1.35 million acres defoliated, followed by a dramatic population collapse in subsequent years 1 . This pattern aligns with the known cyclical behavior of spongy moths, where outbreaks typically last two to four years before collapsing due to natural controls 4 .
A Closer Look: Egg Mass Surveys
While aerial surveys map the damage, ground-based monitoring helps predict future outbreaks. Each fall, teams of technicians fan out across Michigan's forests to conduct egg mass surveys. This systematic counting of egg masses on trees, buildings, and outdoor furniture provides a crucial forecast for the following spring's caterpillar population 2 .
Scientists have learned to read these egg masses like a storybook, identifying key indicators that reveal population health:
Interpreting Spongy Moth Egg Masses
| Egg Mass Type | Size & Appearance | Population Indicator |
|---|---|---|
| Healthy | Larger than a quarter, firm, tan-colored | Likely heavy infestation next season |
| Virus-affected | Smaller than a quarter | Population decline expected |
| Parasitized | Contains pin-sized holes | Natural enemies at work |
| Spent | Whitish color, falling apart | Already hatched, no threat |
Citizen Science
The Michigan Department of Natural Resources encourages citizens to become frontline observers in this effort. If you spot egg masses on your property, you can report them to local authorities or the MSU Extension service, contributing valuable data to the statewide monitoring network 1 5 .
Nature's Comeback: The Invisible Enemies
A Viral Assassin
Just when spongy moth populations seem unstoppable, nature often delivers a dramatic check in the form of Nucleopolyhedrosis virus (NPV). This naturally occurring pathogen spreads rapidly through dense caterpillar populations, particularly when overcrowding facilitates transmission 2 .
The virus works insidiously, eventually causing caterpillars to die in a characteristic inverted "V" position while clinging to tree trunks 2 . This telltale sign indicates NPV is actively working to collapse the population. The virus is remarkably species-specific, affecting only spongy moths and posing no threat to humans, pets, or beneficial insects 2 .
The Fungal Ally
Another powerful natural control agent arrived in Michigan in 1991: Entomophaga maimaiga, a fungal pathogen specifically lethal to spongy moth caterpillars 2 . This fungus persists in the soil year after year, with moisture helping to activate and disperse its spores.
When caterpillars come into contact with these spores while crawling on the ground or tree trunks, they become infected. The fungus eventually kills them, leaving them hanging straight down on tree trunks—a different position than NPV-killed caterpillars 2 . The beautiful aspect of this fungus is that it remains in the ecosystem as a long-term biological control agent, helping to keep future outbreaks in check.
NPV Virus
Natural pathogen that causes population collapse
Entomophaga Fungus
Long-term biological control in the soil
Natural Balance
Ecosystem self-regulation over time
The Scientist's Toolkit: Managing Spongy Moth Populations
Michigan's approach to spongy moth management has evolved into an integrated strategy that combines monitoring, natural controls, and targeted interventions. While the state doesn't have a uniform statewide spray program, several counties with persistent issues have established suppression programs 2 5 .
Egg Mass Scraping
Physical removal of egg masses into soapy water
Late fall through early spring, before hatchingTree Banding
Burlap wrapped around trunks traps climbing caterpillars
During caterpillar season (spring to early summer)Btk Treatments
Bacterial insecticide applied to leaves, kills feeding caterpillars
When caterpillars are young (May-June)Tree Wrapping
Creates a two-layered skirt that traps caterpillars
During active caterpillar seasonHomeowner Advice
For homeowners dealing with spongy moths, the Michigan DNR offers this advice: "Don't panic, stay safe" 1 . Focus protection efforts on young and vulnerable trees, since healthy mature trees can typically withstand several years of defoliation. Water trees at least once a week during outbreaks to help maintain their vigor 1 .
The most commonly recommended insecticide is Btk (Bacillus thuringiensis var. kurstaki), a naturally occurring bacterium found in soil 1 5 . Btk is highly specific to caterpillars and has little impact on humans, pets, or beneficial insects like pollinators 1 . Proper timing is crucial—it must be applied when caterpillars are young and actively feeding, typically within one to two weeks of hatching 1 .
Lessons from the Front Lines
The story of Michigan's spongy moth survey offers more than just pest management insights—it reveals our evolving relationship with nature in a globalized world. The moth's journey from a well-intentioned experiment to a established forest pest illustrates the unintended consequences of moving species beyond their native ranges 3 .
Yet, there's hope in this story. The natural controls provided by NPV and Entomophaga maimaiga fungus demonstrate nature's remarkable ability to self-regulate when given time 2 . Michigan's forests have shown impressive resilience, with hardwoods typically releafing by mid-to-late July after defoliation 1 .
The spongy moth will likely always be part of Michigan's ecosystem, with outbreaks occurring cyclically every seven to ten years 2 . But thanks to continued monitoring and research, we're better prepared than ever to manage these events. The partnership between scientists, natural resource managers, and informed citizens represents our best strategy for protecting Michigan's forests for generations to come.
As you enjoy Michigan's beautiful forests this summer, take a moment to look more closely at the trees around you. Notice any unusual caterpillar activity? Spot any egg masses on your property? You're now equipped with knowledge that might help researchers track and manage this fascinating invader. The battle against the spongy moth continues, and every observant eye makes a difference.
To learn more about spongy moths or report sightings, visit Michigan.gov/Invasives or contact the Michigan Department of Natural Resources at DNR-FRD-Forest-Health@Michigan.gov 1 .