Pack Rat (woodrat) Issues as Winter Nears

By Mark J. Platten

CSU Extension Director, Teller County, Colorado

After several conversations relating to woodrats (also known as pack rats, or trade rats) over the past few weeks, I thought it would be a good focus for a blog article.  There are six species of woodrats in Colorado, with desert woodrats (12 inches long, 4 ½ ounces) the smallest, and the bushy-tailed woodrat of mountain talus slides (to over 16 inches long and 11 ounces) being the largest. 

 

Courtesy of Terrell P. Salmon and
W. Paul Gorenzel

 As the name implies, they have a tendency to pack away small objects such as jewelry, utensils, can tabs, and other items.  If a woodrat finds something desirable, they will drop what they are currently carrying, and "trade" it for the new item. They are particularly fond of shiny objects, leading to tales of rats swapping jewelry for a stone. 

Color differs from gray (gray woodrat) to blackish brown (Mexican woodrat), to rich reddish tan (bushy-tailed woodrat), with most having white, to grayish, bellies.

Bushy-tailed woodrat Photo by Thomas Haney

The cold and snow may drive them from their more exposed homes, to under our porches, abandoned vehicles, or in sparsely used cabins and campers.  Obviously this can lead to damage and nuisance issues.   

The breeding period occurs from January to August, peaking between March and June. Generally they have one litter per year, although two have been observed in longer periods of warmth. Litter size ranging from 1 to 6, with an average of four.

Woodrats are primarily nocturnal and are most active during the half hour after sunset and at dawn, year round. Shelter and topography are important determinants of habitat suitability. The availability of rock shelters may be a more important than the associated plant communities.  Hollow trees, logs, dwarf mistletoe brooms, and coarse woody debris may also be used for denning, foraging, and shelter.  And, as previously mentioned, human structures and vehicles may be used as well. 

Photograph by Kennan Ward/Corbis

Woodrats are herbivores with a broad, flexible diet including cones and needles of coniferous trees, berries, leaves, shrubs, forbs, and mushrooms during our wet years.

They are classified as nongame animals in Colorado, which means they are protected from harassment, killing, or possession except when they are creating a nuisance or creating property damage.

Woodrats may transmit certain diseases, including Colorado Tick Fever, but are rarely, if ever, associated with plague or Hantavirus. Dead or dying woodrats should not be handled.

Damage Prevention and Control Methods

Exclusion

Since they are agile climbers, all entrances to buildings, including those at the attic level, must be closed.  Also check for openings in attic vents, broken roof shingles, or other gaps next to the eaves. No hole larger than 1/2 inch (1.3 cm) should be left unsealed. As long as you’re in the process of preventing rodents from entering, why not prepare for potential wildfires and ensure the screens are no larger than 1/16 inch in size?     

Anticoagulants (blood thinners)

Anticoagulants are effective for woodrat control and are especially suited for use around structures because of their low hazard to pets and children. Most baits formulated for rats and house mice give effective woodrat control. Finely ground, or meal-type, anticoagulant baits are recommended. Since woodrats have a tendency to pack away items, pellet bait should be avoided because it will be stored at the nest site.

Anticoagulants are usually put out in bait boxes, but woodrats tend to fill boxes with sticks and other debris. Therefore, use open bait containers. Bait exposed in this manner must be placed so non-target species, pets, and children do not have ready access to it. Access to the bait by pets can be minimized by inverting a wooden crate over the bait tray. Baiting sites should be located near existing woodrat runways, feeding sites, or nests.

Trapping

The majority of woodrat problems in structures can be dealt with by using traps because they show little fear of new objects in their environment. The standard rat snap trap is quite effective. Trap bait should be wedged into, or tied, to the treadle. Good baits include nut meats, bacon rind, peanut butter and oatmeal, prunes, raisins and other dried fruit, and biscuits.

Cage traps for woodrats are against Department of Parks and Wildlife regulations.  Also, many studies have shown that animals released into new areas often die from exposure, predation, or competition with resident animals. 

References: University of California, Dept. of Wildlife, Fisheries and Conservation Biology, Colorado Division of Parks and Wildlife, Montana Field Guide, and USFS Rocky Mountain Research Station.