Étienne Léopold Trouvelot left behind two legacies. One deserves admiration and respect, while the other earns him contempt and criticism.
As an amateur French entomologist, his most infamous mistake was introducing the spongy moth to North America, an invasive species that continues to wreak havoc on forests to this day. Fortunately, Trouvelot was also an artist and astronomer, whose mastery of both the paintbrush and the telescope resulted in thousands of stunning astronomical illustrations. His depictions of the cosmos were among the most accurate of their time. It is through these works that Trouvelot remains esteemed, even as history struggles to overlook his ecological misstep.
Étienne Léopold Trouvelot was born in 1827 in the Aisne region of northern France. Little is known about his early life, but he appears to have been involved in Republican politics, an affiliation that made him an enemy of the state after Louis-Napoléon Bonaparte seized power in 1851. To avoid arrest, Trouvelot fled to the United States with his family, eventually settling in Medford, Massachusetts. A passionate artist and astronomer, he initially supported himself through his artistic and scientific pursuits before turning his attention to entomology, with devastating consequences.
Trouvelot began raising silkworms, a practice possibly inherited from his native country, where silk production was well established. By the end of the Civil War in 1865, he claimed to be feeding a million young silkworms on five acres of woodland behind his home. However, these caterpillars were vulnerable to disease, so Trouvelot decided to experiment with the more robust spongy moths (formerly known as gypsy moth), or Lymantria dispar, a native European species that feed on tree foliage.
Sometime around 1867, Trouvelot brought several spongy moth larvae to his home in Medford and began rearing them in the woods. The story goes that one night there was a storm and a gust of wind tore apart the nets where Trouvelot kept thousands of specimens, allowing the hairy caterpillars to escape into the night.
Caterpillar of the spongy moth. Photo credit: Wikimedia Commons
Native to temperate forests in Western Europe, the spongy moth is a notorious pest. It’s a voracious eater that can completely defoliate entire trees eventually killing them. Today, it devours thousands of square kilometers of forest in the northeastern United States each year, putting up to 300 different species of tree and shrub at risk. In the past half-century alone, the spongy moth has stripped more than 83 million acres of forest, with its range now stretching from the Atlantic coast to central Wisconsin.
The silk produced by the spongy moth turned out to be worthless, and Trouvelot lost all interest in entomologist. In 1870, he left his farm and relocated to Cambridge, Massachusetts, a few miles to the south. That October, a powerful solar storm caused auroras to appear at unusually low latitudes. Inspired by the rare phenomenon, Trouvelot used his lithographic skills to make stunning illustrations of the Massachusetts night sky, illuminated by streaks of green and blue light.
Trouvelot’s artistic and scientific talents soon caught the attention of Joseph Winlock, director of the Harvard College Observatory (HCO), who invited Trouvelot to join the institution. While in Cambridge, he produced hundreds of detailed drawings of celestial objects, using HCO’s 15-inch telescope as well as a smaller 6-inch telescope he kept at home.
To accurately reproduce what he observed, Trouvelot employed the classic grid technique used by artists. He placed a ground glass plate with an incised network of squares between his telescope lenses, allowing the image to be projected onto the grid. He then sketched the celestial features onto a matching grid on paper, meticulously transferring the image one grid at a time.
Recognizing Trouvelot’s expertise as an observer, the renowned telescope makers Alvan Clark & Sons granted him access to the 26-inch refractor they were building at their Cambridgeport workshop for the Leander McCormick Observatory at the University of Virginia. In 1875, he was invited to spend a year at the United States Naval Observatory (USNO) in Washington, D.C., where he worked with the observatory’s 26-inch telescope—the world’s largest refractor at the time. His reputation as an astronomer was further solidified when he joined the USNO’s official mission to document the total solar eclipse of 1878.
Trouvelot had a particular interest in solar phenomena and was among the first to study “veiled spots,” a variant of sunspots. His research on the subject was published in the Proceedings of the American Academy of Arts and Sciences, marking a significant contribution to the field. For a self-taught astronomer, his impact was remarkable—he published 50 scientific papers and an academic manual documenting his observations.
Over the course of his life, Trouvelot created approximately 7,000 high-quality astronomical illustrations. Fifteen of his finest pastel works were displayed at the United States Naval Observatory’s exhibit at the Philadelphia Centennial Exposition in 1876. Determined to share his work with a wider audience, he later had these illustrations printed and published by Charles Scribner’s Sons in 1881.
Trouvelot’s otherworldly illustrations, created a decade before the advent of color photography, were regarded as the most accurate depictions of the solar system at the time. However, even as he refined his craft, his work faced growing competition from advancements in astrophotography. Though Trouvelot acknowledged that “no human skill can reproduce upon paper the majestic beauty and radiance of the celestial objects,” he remained skeptical of astrophotography’s ability to capture fine details, arguing that its images were “in general so blurred and indistinct that no details of any great value can be secured.”
In 1882, Trouvelot returned to France, where he was awarded the Valz Prize for astronomy by the French Academy of Sciences. He joined the Meudon Observatory in Paris, continuing his observations of the solar system.
He died in 1895 in Meudon, blissfully unaware of the ecological disaster his spongy moth experiment had unleashed on North America. By the time the Massachusetts government fully grasped the scale of the infestation, Trouvelot was long gone.
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