Photo by Sergii Koviarov, Pixabay.

February 1, 2023

Nutria May Be Invading Illinois

Nutria are considered one of the planet’s most ecologically harmful invasive species. Since 2019, a handful of credible reports indicate that nutria are making their way northward into Illinois.

Why the concern? Nutria (Myocastor coypus), also known as coypu or swamp rats, are large, semi-aquatic mammals and native to subtropical and temperate regions of South America. Introduced to the United States between 1899 and 1930 by fur farmers, nutria were raised for their then valuable pelts which were popularly used in coat linings and trims. When the fur market collapsed in the 1940s, thousands of nutria escaped or were turned loose. Scattered populations now exist in 40 states and three Canadian provinces in North America (see map). Individuals from a population along the Gulf Coast have been moving up the Mississippi River Valley and reported as far north as Missouri, Arkansas, Tennessee and Kentucky.

Isolated and anecdotal reports have come from various locations in southern Illinois over the last 20 years. Reports have come from Horseshoe Lake in Alexander County, Mermet Lake in Massac County and Cypress Creek National Wildlife Refuge in Johnson County, but never led to verified evidence despite credible accounts. This changed in February 2022 when a beaver trapper in Pope County caught two individuals on private land.

Nutria typically take up residence in a variety of aquatic habitats, from farm ponds and freshwater impoundments to rivers, marshes, swamps and drainage canals. Nutria have voracious appetites and are capable of eating approximately 25 percent of their body weight daily. Although primarily herbivorous, they will consume mussels, snails and other invertebrates. Nutria not only eat the leaves and stems of plants, but also the roots, denuding an area of plants that help stabilize fragile soils.

A illustration of silhouettes of a muskrat, nutria, beaver and six foot human for comparison.
Comparisons of sizes and body shapes of muskrat, nutria and beaver (left to right).

Like beavers and muskrats, nutria are dark-colored, semi-aquatic nocturnal mammals. The incisors of these three species are wide and never stop growing. The enamel of the incisors takes on a bright orange-yellow color because of the presence of iron, which strengthens the gnawing power of these rodents.

Several characteristics can be used to differentiate between the three species.

Nutria weigh between 15 and 20 pounds, while their wetland counterparts the beaver and muskrat typically weigh 25 to 60 pounds and 1.5 to 4 pounds, respectively. Beaver are usually 3 to 3.5 feet in length and muskrats 16 to 25 inches in length, with nutria falling in the middle of the range, with a 2-foot body and additional 1- to 1.5-foot-long tail.

A chart comparing different characteristics of a nutria, beaver, and muskrat.

Speaking of tails, they are quite different. The broad, paddle-like tail of the beaver creates a resonating warning sound when slapped on the water. The laterally flattened tail of the muskrat moves in a snake-like fashion as the animal swims. In contrast, a nutria drags its heavy, round and rat-like tail behind it when swimming. As an animal native to subtropical South America, nutria living in cold regions of the world may suffer frostbite and present only a stub of a tail.

Unlike the mud-and-stick lodge of a beaver or the mounded mud and vegetation hut of a muskrat, nutria generally build platforms out of dead vegetation where they feed, loaf, groom and give birth. They do, however, occasionally use dens. A prolific breeder, female nutria can breed at about 4 months of age and have up to three litters a year, with an average of four to six young per litter. Nutria are social in nature, living in groups of two to 13, with a female’s offspring often remaining to form a family group.

Two brown semi-aquatic rodents with round ears rest on a platform of plant debris in a wetland.
Photo by USFWS.

Additional characteristics to use in identifying nutria are a white muzzle, 3- to 5-inch-long white whiskers (beaver and muskrats have black whiskers) and an arched back.

“The foraging behavior of nutria relies on lots of digging to get the roots of aquatic plants,” noted Stan McTaggart, Furbearer Biologist for the Illinois Department of Resources’ (IDNR) Division of Wildlife Resources. “This digging not only removes native wetland plants, it also disrupts wetland systems by causing excessive erosion and levee damage. In addition to the impacts on our wetlands, they are also known for extensive crop damage in southern states. Many of the most diverse and unique wetlands in Illinois are found in southern Illinois and they don’t need another stressor.”

Concern also exists for human, livestock and pet health as this invasive species can carry tuberculosis and septicemia, and may contaminate water supplies and swimming areas with parasites such as blood and liver flukes, tapeworms and nematodes.

Freshly churned soil on the bank of a wetland. A dirt path runs along the bank.
Nutria often cause bank erosion. Photo by Tess McBride, USFWS.

The ecological damage that nutria may cause creates concern shared by natural resource managers throughout North America. In the Midwest, the presence of nutria, feral swine and the nine-banded armadillo are tracked on the Midwest Invasive Species Information Network. IDNR is working with researchers at Southern Illinois University – Carbondale and the United States Department of Agriculture’s Wildlife Services to look for more and determine next steps if any populations are found.

Trappers who catch or find nutria are asked to send an email to Because of the difficulty in distinguishing nutria from beaver and muskrats at a distance, IDNR is only interested in reports that have a carcass and/or photos with obvious distinguishing features.

For further information on nutria visit Wildlife Illinois.

Kathy Andrews Wright is retired from the Illinois Department of Natural Resources where she was editor of Outdoor Illinois magazine. She is currently the editor of Outdoor Illinois Journal.

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