Let's talk about nutria. Nutria (Myocastor coypus) are large, semi-aquatic rodents that are native to South America. The species is invasive in the United States and currently established in 17 states, including California. Nutria inhabit both freshwater and brackish coastal water areas and can be found near permanent water sources, including rivers, streams, lakes, ponds, wetlands, and continual rice production. However, they can still thrive in urban conditions; in cities, they can be found under buildings, in overgrown lots, on golf courses, and in storm drains. Nutria thrive in warmer climates, such as the southeastern region of the U.S., and their reproductive success is reduced by severe winter conditions. They can grow in size up to 20 lbs and have partially webbed feet. Often mistaken for small beaver or large muskrats, nutria can be differentiated by large front teeth that are yellow to orange in color, a heavy, rat-like tail, and prominent white whiskers that protrude from either side of their nose.
Incidentally, nutria have actually been present in California for over a century. Introduced in 1899 to stoke the fur trade, the first members of the species were spectacularly unsuccessful. Subsequent introductions of nutria followed in the 1940s and 50s, but once again failed (as did the nascent nutria fur market), and the species was declared eradicated from California in the 1970s. This remained true until the spring of 2017, when CA Department of Fish and Wildlife (CDFW) trapped and necropsied a pregnant female nutria in Merced County (CDFW). This triggered monitoring and eradication efforts across the state, which have indicated that nutria is spreading further north every year.
(Photo of a nutria on the left courtesy of Tony Northrup, and photo of a muskrat on the right courtesy of Joyce Gross, UC Berkeley)
The Problem with Nutria
Nutria create havoc through 1) the damage they wreck and 2) the abundance of their offspring.
1) Nutria cause various kinds of damage through burrowing, intense herbivory, and carrying pathogens and parasites.
a) Nutria do not construct dens; rather, they burrow, frequently causing water-retention or flood control levees to breach, weakening structural foundations, and eroding banks.
b) They can consume up to 25% of their body weight in above- and below-ground vegetation each day, but they waste and destroy up to 10 times as much, causing extensive damage to the native plant community and soil structure, as well as significant losses to nearby agricultural crops (CDFW). The loss of plant cover and soil organic matter results in severe erosion of soils, in some cases destroying marshlands. The destructive feeding habits of nutria threaten populations of rare, threatened, or endangered species that rely on critical wetland habitats.
c) Nutria also serve as hosts for tuberculosis and septicemia, which are threats to humans, livestock, and pets. Additionally, nutria carry tapeworms, a nematode that causes a rash known as “nutria itch”, and blood and liver flukes, which can contaminate swimming areas and drinking water supplies (CDFW).
2) Nutria are such prolific breeders that one female can lead to 200 offspring in just a year.
a) Nutria reach sexual maturity at about four to six months. Females have anywhere from five to seven babies in a litter, and they have several litters every year.
b) Additionally, nutria have a high rate of migration and can move up to 50 miles from their original colonies.
The CDFW map image below shows the location and density of nutria taken in each area in red, with yellow circles indicating hot spots of habitation and blue halos indicating areas of likely infestation. As of January 8, 2025, a total of 5,448 nutria have been taken in California, with additional animals confirmed present, across Merced, Stanislaus, Fresno, Solano, San Joaquin, Fresno, Mariposa, Sacramento, Madera, Contra Costa, and Tuolumne Counties.
What Does This Mean for CA Rice?
So, why are we talking about this semi-aquatic mammal in the rice newsletter?
The current geographic distribution of nutria in California concerns those of us involved in rice production. As the preferred habitat of these rodents is identical to that of a rice field, the potential for damages is high. Additionally, identifying the rice damage caused specifically by nutria can be challenging, as it is easy to confuse it with damage caused by muskrats: both rodents clip the stems of the rice plants at the water line.
At high densities and under the right environmental conditions, the foraging of nutria can substantially impact plant communities. In the U.S., rice is one of the primary crops damaged by nutria, which can reduce yields through grazing and other crop destruction. However, nutria also favor crops and plants that can neighbor rice fields, including corn, grain sorghum, beets, alfalfa, wheat, barley, oats, peanuts, melons, and a variety of vegetables from home gardens and farms.
What is Being Done To Address Nutria?
CDFW is collaborating with other agencies and local partners to develop the most effective strategy for eradicating nutria from California. The organization has created an “Invasion Curve” (below) that represents a hypothetical population increase from an invasive species infestation. The infestations typically experience a lag phase, while populations and area infested are relatively small and successful eradication has the most potential for success. As time progresses, the population size, area infested, and costs required for control increase exponentially, and the probability of successful eradication is lost.
CDFW believes that Stage 1 represents the current state of the nutria population in California, indicating that eradication is possible if rapid response is taken. This is good news, especially compared to Stage 3, which is conceptually represented by the nutria population in Louisiana, where population control costs up to $2 million per year for bounty harvests alone.
In California, nutria are classified as a nongame mammal. Fish and Game Code §4152 specifies property owners or their agents (who possess written permission from the owner or tenant) may take nutria at any time by any legal means to address damage to crops or property. Restrictions apply to the use of traps and types of traps. Nutria are a Restricted Species in California under the California Code of Regulations, Title 14, Section 671, and cannot be imported, transported, or possessed live in the state of California.
In other states, bounty programs are underway to encourage taking of nutria. However, in California, it is illegal to offer a bounty for nutria (https://www.redding.com/story/life/2019/06/14/cdfw-why-bounty-program-get-rid-nutria-wont-work-california/1416370001/). Under California Fish and Game Code, section 2019 clearly states: “It is unlawful for any person, including state, federal, county and city officials or their agents, to authorize, offer or pay a bounty for any bird or mammal.” State legislation changes would have to take place to alter the code and provide an exception for nutria.
Given their very similar appearances, particularly in overlapping size classes, citizens should take extra caution to distinguish nutria from other aquatic mammals. The majority of nutria reports received by CDFW have been muskrats, as have been some "nutria" featured in the media. Any nutria taken on private or public land should be reported to CDFW as soon as possible for purposes of delineating the extent of the infestation. At minimum, CDFW needs photos to confirm identification; preferably, CDFW needs the carcass to determine sex, age, and reproductive status. Suspected observations or potential signs of nutria in California should be photographed and immediately reported to CDFW ONLINE, by email to Invasives@wildlife.ca.gov, or by calling (866) 440-9530. Observations on state or federal lands should be immediately reported to local agency staff. If this species is captured, do not release it; immediately contact your local CDFW office or County Agricultural Commissioner.
