The quiet spread of a winged insect may be reshaping forests across continents, one flight at a time.
Deep in the forests of North America, a silent invasion has been unfolding for over a century. It began in 1869 when a French scientist, Étienne Léopold Trouvelot, imported gypsy moths from Europe to conduct breeding experiments in Medford, Massachusetts. When some of these moths escaped, they found suitable habitat and began breeding 8 . What started as a small accident has since grown into an annual defoliation of over one million acres of forest, causing an estimated $868 million in damages each year in the United States alone 3 8 .
$868 Million
Annual damages in the United States
What makes this story particularly compelling to scientists today isn't just the destruction itself, but a biological mystery that could make the problem far worse: why can some female gypsy moths fly while others cannot? The answer to this question may determine whether we can protect our forests from an even greater threat.
The gypsy moth—recently renamed the spongy moth—exists in several distinct subspecies across the globe, with dramatically different capabilities 3 5 .
Flight capable females with potential for hybridization with other subspecies.
Lymantria dispar japonicaWhen female moths can fly, the dynamics of invasion change completely. Flightless females typically lay their eggs near where they emerge, resulting in relatively contained populations that spread slowly, about 2-20 km per year in North America 3 . In contrast, populations with flight-capable females can expand their range much more rapidly as females seek new habitats and host plants.
To understand what makes these moths such effective fliers, researchers in China conducted a comprehensive study in 2022 examining the flight capabilities of Asian spongy moths from five different geographical locations 5 .
Xifeng moths exhibited the most impressive flight performance with the longest duration (0.94 hours) and greatest distance (1.87 km) 5 .
Younger moths consistently outperformed older ones, with flight period and distance decreasing significantly with age 5 .
The researchers identified thorax width and wing loading (the ratio of body mass to wing area) as significant factors influencing flight capability, explaining most of the variation in flight performance (R² = 0.93 and -0.98, respectively) 5 .
While the Chinese study examined the physical aspects of flight, other researchers have been investigating the genetic foundations of this capability. In 2019, scientists sequenced the spongy moth genome—the largest Lepidoptera genome sequenced to date at 865 megabases, encoding approximately 13,300 proteins 6 .
Comparative analyses revealed genetic variations in genes related to:
A more recent study used multiple genetic approaches to identify flight-associated genes, including:
Recognizing the serious threat that flight-capable spongy moths pose, many countries have implemented strict regulations to prevent their introduction 2 4 .
The United States, Canada, Chile, Argentina, Australia, and New Zealand all require ships that have visited regulated areas during flight seasons to undergo rigorous inspection and certification 2 4 .
Ships must provide advance notification of ports called in the previous two years and obtain pre-departure inspections from regulated areas 2 4 .
Vessels found with egg masses may be refused entry, ordered out of territorial waters, or subjected to costly quarantine measures 2 .
The silent flight of a moth may seem insignificant, but as we've seen, it carries enormous consequences for forests worldwide. The difference between flightless and flight-capable females represents a critical threshold in invasive potential—one that could determine the fate of entire ecosystems.
While tools for identifying general origins of introduced spongy moth populations exist, researchers note that we still lack rapid assessment methods to fully characterize key traits in newly discovered populations 3 . Developing such tools remains essential for managing future introductions.
What began as a accidental release over 150 years ago has evolved into a complex scientific detective story, spanning genetics, ecology, and international policy. As researchers continue to unravel the mysteries of moth flight capability, their work illuminates not just the biology of a single insect, but the delicate balance of our forest ecosystems—and how much we stand to lose if that balance is upset.
The future of our forests may depend on what they discover.