Spongy Moth Caterpillars May Be Eating MN Trees Right Now

MINNESOTA — You may know the “spongy moth” by another common name — gypsy moth, rejected last year as an ethnic slur by the Entomological Society of America — but no matter what you call these insects with voracious appetites, agriculture officials call them a severe threat to trees in Minnesota.

Clusters of counties from the Northeast to Minnesota and as far south as Virginia have imposed quarantines to stop the spongy moth’s spread — a daunting task, agriculture and forestry officials acknowledge, given that infestations in one state push them farther inland to threaten “even the West Coast.”

Essentially, that means regulations are placed on the movement of firewood, outdoor equipment and vehicles, where the invasive pest’s egg masses may be found. The insect spends most of its 10-month life cycle in the egg stage.

Established in 2014, a federal spongy moth quarantine exists for Minnesota's Lake and Cook counties, which are in the northeasternmost part of the state.

The Minnesota Department of Agriculture announced Monday it will set approximately 20,000 traps this year looking for spongy moths. The traps will be set throughout the eastern half of the state and in far northwestern Minnesota as part of a “western rotation.”

Survey staff have begun setting traps and will continue through July. Traps will remain in the field through August in the southern region of the state and through October in the northern region, the department said. The “delta” traps are small, triangle-shaped and made of cardboard. Bigger “milk carton” shaped traps will be set in select areas of Lake, Cook, and St. Louis counties to accommodate potentially higher numbers of moths. All the traps contain a pheromone to lure in male spongy moths if they are present.

Here are some things to know about spongy moths:

How Did Spongy Moths Get Here?

Spongy moths were brought to the United States by an amateur entomologist for research as a possible source of silk production in the 1860s.

After receiving a shipment of potential silk moth candidates from a colleague in France, Etienne L. Trouvelot began rearing spongy moths in his backyard in Medford, Massachusetts, and some escaped and found a variety of plants to feed on.

The destructive moths have currently been confirmed in Connecticut, Delaware, the District of Columbia, Illinois, Indiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, Vermont, Virginia, West Virginia and Wisconsin.

What Do Spongy Moths Look Like?

The best time to get spongy moths under control is during the egg stage, before they become caterpillars. That usually happens in late April through early May. The egg masses are covered with buff or yellowish hair from the female, and are usually about an inch and a half long and about three-quarters of an inch wide.

Newly hatched caterpillars you’re likely to see right now are black and hairy, but in later stages develop a mottled yellow-to-gray pattern with tufts of bristle-like hairs and a distinctive color pattern of five pairs of blue dots followed by six pairs of red dots along their backs.

As they emerge as caterpillar hatchlings, they often produce silken threads that can catch the wind and send them aloft over trees — a practice called “ballooning,” which enables their spread, according to Cornell University’s College of Agriculture and Life Sciences.

Once they’ve satisfied their appetites, the caterpillar pupates. When they emerge from the chrysalis, males are brown with a darker brown pattern on their wings, which have a span of about 1½ inches. The females are mostly white with a dark saw-toothed pattern on their wings, and are slightly larger with a 2-inch wingspan. Males can fly, but females can’t.

What Trees Are Most Vulnerable?

Spongy moths are most destructive in the caterpillar stage, when they feed on about 300 species of trees and shrubs, leaving them vulnerable to diseases and other pests, and they can eventually kill the trees, according to the U.S. Department of Agriculture.

Oaks, aspens and birches are among some of the more affected trees, along with urban trees like crabapples and lindens.

What Makes Flighted Spongy Moths Different?

Flighted spongy moths from Asian countries look very similar to the European spongy moth, the major difference being that females can fly long distances. That means they can lay their egg masses wherever they travel

Flighted spongy moths have recently been detected in Washington and Oregon, according to the USDA, which warns that “their ability to fly long distances make it probable that this moth complex could quickly spread throughout the United States.”

The United States and Canada have imposed strict import controls at ports to ensure that plants and plant products are free of flighted spongy moths. This type of spongy moth feeds on about 600 plant species in 100 botanical families.

Both the broad range of host plants coupled with the female’s ability to fly long distances could allow the species to spread rapidly into areas without infestations. If unchecked, they can wipe out large sections of forest, orchards and landscaping within two years.

Widespread introduction “would pose a major threat to the landscape of the North American continent,” according to the USDA

What Should You Do?

The best time to get after spongy moths is in early spring during the age stage — scrape them off into soapy water. But if they’ve already emerged, it’s not too late to remove the caterpillars from trees and shrubs before the females can lay another egg mass.

It’s a good idea to wear gloves, because their tiny hairs carry histamines and some people may develop an itchy, red rash.

Cornell advises several control methods, including simple burlap flap trap.

“This involves encircling a tree with a foot-wide length of burlap tied with twine right in the middle and pulling the top portion over the lower portion,” Cornell said. “This technique takes advantage of the caterpillar’s drive to crawl upwards and not turn around if they encounter the barrier of the burlap. The accumulated caterpillars can be periodically removed and destroyed.”

Insecticides and other chemical applications are also an option.

How Much Does It Cost To Stop Them?

From 1980-2020, state and federal governments spent more than $282 million on spongy moth control on more than 14 million acres. But the cost is even higher, according to the Forest Service, because it costs both the government and businesses money to comply with quarantine and other regulatory protocols.

The Forestry Service estimates that since 2000, federal and state agencies have spent about $7 million a year to monitor and manage populations in states that so far have