Spongy moth: The insatiable leaf eater

METRO DETROIT — For those who have been to the farther reaches of the region in the summer, a few odd sights stand out to the keen eye. Crop dusters flying low in the morning hours. Bands of burlap or duct tape stretched around trees. Dark green specks covering nearly everything in sight. Scores of trees left bare and unprotected from the coming winter’s harshness.

All of this is the doing of one little bug: the spongy moth.

The spongy moth — scientifically known as the Lymantria dispar, and until 2022 as the gypsy moth — is an invasive insect species brought to the United States in the mid-19th century. According to the United States Department of Agriculture, larvae kept in Massachusetts escaped sometime between 1868 and 1869 and the moths have made their way west ever since. Their travel has been facilitated by the wind, as the species’ larvae are carried away, and the spongy moth makes its presence in an area known with its infamous appetite for nearly all kinds of foliage.

“Generally, the feeding (on leaves) is going to impact stressed trees,” said Cheryl Nelson, a forest health outreach forester with the Michigan Department of Natural Resources. “The healthiest of trees — those that are watered, yard trees that are watered, trees in a forest that have been managed (and) thinned out to leave behind healthy trees — are going to survive a couple years of defoliation by the caterpillars. But what can happen is that without those leaves to capture the sunlight and create more food, stressed trees can die from it.”

The spongy moth larvae’s ability to defoliate trees has become one way state natural resources officials track its spread. The 2021 generation of spongy moths was able to defoliate 1.3 million acres of Michigan forests. That 2021 generation was the spongy moth’s population peak, giving Michiganders their best chance of seeing what has become a campground pest and arborist’s scourge.

“The spongy moths as caterpillars have both blue and red pairs of dots on their backs, so they’re very easy to see and identify,” Nelson said. “Then, as the adult moths, the females are white and do not fly … The males have the big, bushy antennas to seek out those female pheromones. They fly to mate with the female, and then the female will lay egg masses. (Spongy moth eggs) are going to be brown and fuzzy and then there are the individual eggs underneath (the fuzz).”

While the DNR does not undertake mitigation efforts against the spongy moth, the species has found itself in the crosshairs of a litany of mitigation methods. One of the more accessible ways of dealing with spongy moths in an area is by “banding” trees. When spongy moth caterpillars are about an inch long, wrapping large lengths of burlap around affected trees creates an easy spot for knocking or vacuuming the caterpillars off the tree and into buckets of soapy water for disposal. Bands can also be made from duct tape covered in petroleum jelly or commercially available insect barrier pastes such as Tanglefoot. More information about making these barriers can be found in the Michigan State University Extension article “Using Bands to Protect Shade Trees from Spongy Moth.”

Those with more resources at their disposal can arrange to have the microorganism called Bacillus thuringiensis, more commonly referred to as Bt, sprayed to control outbreaks in their area. Bt is a naturally occurring organism that damages the digestive system of spongy moth caterpillars when swallowed. Bt sprays and powders are available for purchase, and larger amounts of Bt can be aerially applied. More information on Bt can be found in the MSUE article “Btk: One Management Option for Spongy Moth.”

But the most meaningful mitigation tool of all is one nobody has to apply — because it’s already inside the Spongy Moth. Nucleopolyhedrosis virus is present within spongy moth communities and, when populations are high during outbreaks, can cause moths to die from the stresses of competing for food and resting space. Alongside the NPV virus is Entomophaga maimaiga, a fungus from Japan that has been used to attack spongy moth populations in the United States since the early 1900s and was introduced to Michigan in the 1990s. When moisture and humidity are high, spongy moth caterpillars can come into contact with Entomophaga maimaiga spores on soil or tree bark. The fungus then grows inside the caterpillars and can kill them within a week.

It is on the back of these mitigation factors that the spongy moth population has shown signs of decline. Since their defoliating heyday of 1.3 million acres in 2021, spongy moths only defoliated around 169,000 acres in 2024. And with one of the mitigation measures being a virus, Nelson says the spongy moth has been naturalized in Michigan.

“Now spongy moths act just like our natural populations of (caterpillars) where we might see outbreak years, but then after two to three years those numbers drop down again because they have those natural biocontrols in the environment to knock them back,” Nelson said. “Spongy moths are now considered naturalized, and we will see the same happen with populations getting high again. We’ll have outbreaks to deal with in the future (and) numbers will drop back.”

People can control the spread of spongy moths by checking their clothes, vehicles and items for any caterpillars and spongy moth eggs after spending time outside. For more information about spongy moths, go to canr.msu.edu/spongy-moth/index.