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Battle continues against moth formerly known as 'gypsy'

Aerial spraying for spongy moth is planned in eastern Minnesota and western Wisconsin this summer.

spongy moth
A spongy moth.
Contributed / Iowa Department of Agriculture and Land Stewardship
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DULUTH — Entomologists have changed the name of the gypsy moth to the spongy moth, but the battle against the forest-munching invaders will continue this summer with aerial spraying in eastern Minnesota and western Wisconsin, including in Duluth and Cloquet.

With the term gypsy considered offensive to the European Romani people, the name of the moth that has been roaming across the U.S. for the past 160 years was officially changed March 2 by the Entomological Society of America.

The name "spongy," from the common French term spongieuse, refers to the moth's egg mass, which apparently has the color and texture of a sea sponge.

spongy moth caterpillar
The leaf-eating, caterpillar stage of the spongy moth, formerly called the gypsy moth. Agencies in eastern Minnesota and western Wisconsin this summer will use aircraft to spray against the insects, including in Duluth and Cloquet, to reduce damage to forest trees.
Contributed / Minnesota Department of Agriculture

Spongy moths first came to the eastern U.S. from Europe in the 1860s and have been moving west ever since, defoliating millions of acres of trees while 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.

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.


spongy moth egg mass hatching
A spongy moth egg mass on tree trunk as the eggs begin to hatch into tiny caterpillars. The eggs have the appearance of a sponge, thus the inspiration for the moth's new name.
Contributed / U.S. Forest Service

It’s when the moth is in its caterpillar stage that it does the damage, eating leaves off many species of deciduous tree and plants. 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 moths have few natural enemies here, and the caterpillars defoliate trees several times in one growing season, unlike native pests like the forest tent caterpillar which generally defoliate trees only once per season. If trees are defoliated often enough by spongy moths, the tree can succumb to disease or stress. In one Wisconsin study, up to 20% of defoliated trees died.

Spongy moths are "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 Plant Pest Regulatory and Mitigation Section of the Minnesota Department of Agriculture. “We need to slow the insect’s spread into Minnesota to protect our natural resources.”

Minnesota spongy moth outbreaks 2021
This map of where spongy moths were trapped in 2021 shows Cook and Lake counties and Duluth are the hotpots in Minnesota for outbreaks of the leaf-eating insects. It's unknown if the moths congregated in their current areas of Minnesota after moving in on vehicles, or blowing across Lake Superior from infested areas of Wisconsin, or maybe both.
Contributed / Minnesota Department of Agriculture

Aerial attack continues

As it has for the past decade, the Northland continues to be the front line in the U.S. war to slow the moth’s westward movement. In Minnesota, the state Department of Agriculture plans to use aircraft to spray four areas this summer, including 75 acres in western Duluth, nearly 500 acres in Cloquet and two areas in Lake County totaling 45,000 acres.

Minnesota’s highest concentrations of spongy moths are in Lake and Cook counties, where they may have moved in on easterly winds from Wisconsin or Michigan or may have arrived attached to the throngs of vehicles visiting the North Shore that come from already infested states.

spongy moth
A European-variety spongy moth. State agencies in Minnesota and Wisconsin this summer will again use aircraft to spray against the forest-eating insects formerly called gypsy moths.
Contributed / University of Wisconsin Extension

The treatments will be conducted in June and July, depending on insect development and weather. The two areas in Lake County will be sprayed with an organic mating disruption product with pheromones. The spraying in Duluth and Cloquet areas will see the biological insecticide Foray 48B, which contains Btk that targets spongy moth caterpillars.

In Wisconsin, the Department of Agriculture, Trade and Consumer Protection will again conduct aerial spraying in Washburn, Bayfield, Sawyer, Barron, Burnett, Barron, Buffalo, Burnett, Chippewa, Crawford, Dunn, Eau Claire, Grant, La Crosse, Lafayette, Pepin, Rusk, Trempealeau and Vernon counties.


Beginning in May and continuing through July, low-flying planes will spray select areas in western Wisconsin where a total of about 163,491 acres at 57 sites in the listed counties are scheduled for treatment.

For more information on spongy moths and treatment efforts in Minnesota, including whether or not your neighborhood may be sprayed, go to mda.state.mn.us/plants/pestmanagement/gmunit .

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

spongy moth range
Spongy moths, formerly known as gypsy moths, are considered established across much of the eastern U.S., the area colored blue on this map, but are still taking hold farther west.
Contributed / Minnesota Department of Agriculture

Concern about butterflies

Because moths are similar to butterflies, there has been concern that efforts to control spongy moths might also impact monarchs and other butterflies. But forest health officials say they are working to prevent any collateral damage when they spray for spongy moths.

One of the treatments, a pheromone flake, fools male spongy months into thinking there are females around, preventing them from mating with real female moths. The pheromone doesn’t kill anything.

The second major spray used for spongy moths is 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.

Btk breaks down quickly in the environment and becomes harmless within a few weeks. And because the Btk spraying is done several weeks before most Northland butterflies are in the caterpillar stage, agency officials say there is little concern.


Btk has to be ingested by the insect to cause harm. So it won’t harm 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 defoliation
Recent forest defoliation by spongy moths in Rhode Island.
Contributed / NASA

According to the Minnesota Department of Agriculture, Btk only works under the alkaline conditions present in the stomach of insects in the caterpillar stage of development. This alkaline condition is not present in the stomachs of humans, mammals, fish, birds or honeybees, which is why Btk does not affect them.

New spongy moth threat from the west

The Asian spongy moth, Lymantria dispar asiatica, is also a serious defoliating forest pest that can feed on several hundred different tree species, and it’s been showing up near western ports in the U.S. and Canada in recent years.

Females of this subspecies are capable of strong directed flight and are attracted to lights, which often results in egg masses being laid on either the superstructure of oceangoing vessels or their cargo. Multiple introductions of the Asian spongy moth strains have occurred when egg masses on ships from Japan and far-east Russian ports hatch after entering ports in western North America.

The Asian variety moth was first identified in North America late in 1991 near the Port of Vancouver in British Columbia, Canada. In 2020, the Hokkaido variety spongy moths from Asia were discovered in parts of Snohomish County northeast of Seattle.

An Asian variety spongy moth
An Asian variety spongy moth.
Contributed / Washington State Department of Agriculture

John Myers reports on the outdoors, environment and natural resources for the Duluth News Tribune. He can be reached at jmyers@duluthnews.com .

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