Minnesota sees record spongy moth infestation

Duluth will be on the front line for aerial spraying this summer.

spongy moth
A spongy moth caterpillar on a tree leaf. The Minnesota Department of Agriculture will be conducting more aerial spraying this summer, including in some Duluth neighborhoods, to try to slow the invasive species' spread to the west.
Contributed / Minnesota Department of Agriculture

DULUTH — Minnesota entomologists found a record number of spongy moths in the state in 2022, and plans are developing for another summertime aerial attack against the leaf-eating foreign invader.

The aerial spraying, set for mid-July, will be aimed at three Duluth neighborhoods and a large area around Holyoke as the Northland continues to be on the front line in the continental battle to slow the westward march of the insect formerly known as the gypsy moth.

spongy moth
A spongy moth.
Contributed / Iowa Department of Agriculture and Land Stewardship

More than 101,000 male spongy moths were caught in traps in 2022 in Minnesota, eight times the number caught in 2021, the Minnesota Department of Agriculture reports. Most moths were caught in Lake and Cook counties, but more are being found in Duluth and Carlton County as well.

The 2022 catch broke the previous record set in 2013 when about 71,000 male moths were trapped.

Summer spraying, which has occurred for several years in the area, is planned across 27,000 acres, including 14,000 acres near Holyoke in southeastern Carlton County and 9,700 acres in the Duluth area, reaching into parts of Proctor and Hermantown.


One of those areas includes the Endion, East Hillside, Duluth Heights and Kenwood neighborhoods in Duluth.

Another includes Piedmont and Denfeld neighborhoods and other western portions of the city and into Proctor.

The third block includes Midway Township areas just north of Interstate 35 and west of Proctor to about Midway Road.

spongy moth treatment areas.jpg
Gary Meader / Duluth News Tribune

But the spray planned for this summer is not an insecticide and can’t harm or kill anything. Instead, it’s a nontoxic synthetic pheromone flake that fools male spongy moths. When an area is treated with the pheromone, the scent of the female moth floods the area and confuses male spongy moths so they can't find females. Because the spongy moths are at the end of their life cycle, they die without reproducing.

Danielle DeVito, who heads the state agriculture department’s spongy moth efforts, said the areas picked for spraying this summer have shown slowly growing numbers of male moths in recent years.

“Especially in that Holyoke area, there have been gradually higher trap numbers in recent years,’’ she said.

The European native moths are considered a major problem for North American trees because they have few natural enemies here and can overwhelm patches of forest, defoliating trees quickly. They will munch on more than 300 species of trees and bushes, including aspen, birch and oak.

Earlier plans to treat trees have been thwarted by the pest's rapid spread and a lack of resources.

Spongy moth caterpillars can defoliate trees several times in one growing season, unlike native pests like the forest tent caterpillar which generally defoliate trees only once per season. Most of the trees survive losing all their leaves, but some — up to 20 % in one Wisconsin study — succumb to the stress and die.


Since 1970, more than 83 million acres, an area equal to 37 Yellowstone national parks, have been defoliated by the spongy moth in the U.S., about 700,000 acres annually in recent years, according to the U.S. Department of Agriculture.

“The spongy moth is a serious threat to our timber, nursery and tourism industries, and the insect can be a public nuisance during major outbreaks,” said Kimberly Thielen Cremers, manager of the agriculture department’s Plant Pest Regulatory and Mitigation Section. “We need to slow the insect’s spread into Minnesota to protect our natural resources.”

spongy moth defoliation
An area of Rhode Island forest defoliated by spongy months in 2016. Most of the trees will survive, but a percentage of defoliated trees will succumb to other ailments and die.
Contributed / NASA

The name "spongy," from the common French word "spongieuse," refers to the moth's egg mass, which have the color and texture of a sea sponge. The insect's name was changed in 2022 because its former name was considered offensive to the Romani people of Europe.

Spongy moths first came to the eastern U.S. from Europe in the 1860s, arriving by ship, and have been expanding ever since. They travel slowly on their own but have riding west as egg clusters on cars, trucks, trains, trailers and campers. They have been in eastern Wisconsin since the 1970s and have now spread across the entire state and into eastern Minnesota.

The moth does damage when it's in its caterpillar stage. Forest health experts say the moths can't be stopped. But their westward movement can be slowed, and outbreaks can be kept smaller, with annual aerial spraying efforts where the largest concentrations of moths are located.

spongy moth range
Spongy moths are considered established across much of the eastern U.S. — the area colored blue on this map — but are still moving farther west. The Northland is now the front line in the battle to prevent the spread of spongy moths farther west, with active control efforts underway in the areas colored green.
Contributed / Minnesota Department of Agriculture

The Minnesota Department of Agriculture has been setting traps across eastern portions of the state for years, using scent to lure male moths in, and last summer they came in record numbers. The high number of male moths trapped wouldn’t necessarily indicate that large-scale defoliation is certain.

Males can disperse long distances, but females are flightless, and in the past, there haven’t been enough females around in Minnesota to lay many eggs. That’s why Minnesota forests have mostly escaped spongy moth defoliation so far.

But, for the first time, 2022 saw several public reports of both spongy moth egg masses and caterpillars in the Arrowhead region. And state entomologists found egg masses “at almost every location surveyed’’ along the North Shore, according to the Department of Natural Resources annual forest pest report. Because each egg mass can contain 500-1,000 eggs, infestations that may start small can grow rapidly.


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More surveys are planned this spring as the snow recedes and before egg masses hatch, DeVito noted.

In Wisconsin, the Department of Agriculture, Trade and Consumer Protection hasn’t yet unveiled its 2023 spraying plan for spongy moths, but it likely will be similar to 2022 when spraying occurred in Washburn, Bayfield, Sawyer, Barron, Burnett, Buffalo, Chippewa, Crawford, Dunn, Eau Claire, Grant, La Crosse, Lafayette, Pepin, Rusk, Trempealeau and Vernon counties. Bayfield County has seen an especially high number of spongy moths in recent years.

For more information on spongy moths and treatment efforts in Wisconsin, go to .

spongy moth
An adult spongy moth.
Contributed / U.S. Department of Agorculture

Pheromone, not insecticide

Because moth caterpillars are similar to butterfly caterpillars, there has been concern that efforts to control spongy moths might also impact monarchs and other butterflies. But officials say the pheromones that they will use this summer, which target male moths after they hatch, won’t impact butterflies at all.

The product, SPLAT GM-O, is completely non-toxic and harmless to humans and animals. All the ingredients in SPLAT GM-O are listed by the EPA as safe and have been approved for food use. Should you, your children and/or pets come in contact with SPLAT GM-O during aerial spraying, the Department of Agriculture suggests washing the affected area with soap and water. Clothing can be cleaned with hot water and laundry detergent.

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In some years, including in 2022 in the Duluth and Cloquet areas, officials instead opted to spray an insecticide called Btk, an organic caterpillar-killing compound that occurs naturally in soil. The product is sprayed onto treetops where spongy moth caterpillars are feeding. When ingested, the bacterium is toxic to certain susceptible caterpillars like the spongy moth. Caterpillars stop feeding and die within a couple days.

“We use Btk in areas where we have a localized heavy population in a relatively small area,’’ DeVito said.


Btk breaks down quickly in the environment and becomes harmless within a few weeks. It must be ingested by the insect to cause harm. So it won't harm butterfly eggs and it is not effective on adults since they don't spend much time eating leaves coated in the bacteria.

Still, some critics of widespread Btk spraying say that there are some species of butterfly that are in the caterpillar stage at the same time and that Btk shouldn't be used at all, that the damage caused to unintended insect species outweighs the benefits of reduced forest defoliation.

Spongy moth meeting Tuesday in Duluth

The Minnesota Department of Agriculture will host an in-person public meeting on proposed aerial spraying in Duluth for spongy moths will be held from 6-8 p.m. Tuesday in Room 330 at Duluth City Hall.

Online meetings will be held Thursday at 10 a.m. and 6:30 p.m.

To register for an online meeting, or for more information on spongy moth infestations and treatments, including maps of proposed treatment areas, go to .

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John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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