Introduction to Woodrats/Packrats/Traderats
The common name of woodrat originates from the fact that they typically make their nests utilizing wooden sticks. They are also referred to as packrats because they cache various man-made objects in their nests, and that of trade rats because when returning to their nest with nesting materials if they encounter something more desirable (especially anything shiny), they will exchange for the more desirable material and return with it. Nationwide, woodrats are of minor nuisance pests. However, they are becoming increasingly more important as a pest group as urban sprawl continues, especially in the southwestern United States (e.g., Arizona, Utah, and New Mexico). When woodrats move into buildings, they can cause serious gnawing damage to the structure, wiring, and other utility elements. In addition, woodrats are of medical concern because they serve as reservoirs for disease organisms that cause Chagas’ disease, Lyme disease, etc.
Woodrats occur throughout North America and range in habitats from low, hot, dry deserts to cold, rocky slopes above the timberline. Of the approximately 22 species known to occur in North America, only the 8 listed below are encountered by property owners. Only the first 4 species (treated in detail below in the Representative Species and Habits sections) of the 8 species listed below are of any high pest significance. The 8 pest species are:
- Western white-throated woodrat, Neotoma albigula Hartley.
- Southern plains woodrat, Neotoma micropus Baird.
- Mexican woodrat, Neotoma mexicana Baird.
- Dusky-footed woodrat, Neotoma fuscipes Baird.
- Eastern woodrat, Neotoma floridana Ord; also called the Florida woodrat. (Widespread: range extends from latitude of southeastern New York through the Central United States, south to the Gulf of Mexico, over about 20 states.)
- Desert woodrat, Neotoma lepida Thomas. (California, Nevada, Utah, and Arizona.)
- Bushytailed woodrat, Neotoma cinera Ord. (Northwest quadrant, occurring in 11 states and into west/southwestern Canada.)
- Stephens woodrat, Neotoma stephensi Goldman. (Northern Arizona and New Mexico, southern Utah.)
Currently (2016) only the first 3 woodrat species (numbers 1-3 above) are listed on EPA registered rodenticide labels. Number 4, the dusty-footed woodrat of California (Western California) is probably the most pestiferous. The last 4 woodrat species listed above (numbers 5-8), typically occur away from human structures and play important ecological roles, but are not structural pests. However, in areas where these 4 woodrats occur, they can be occasional invaders in unoccupied structures such as country homes, summer cabins, detached garages, storage sheds, stored campers and the like where they do gnawing damage, create messes, and possibly deposit pathogens. To a pest professional’s customers, all furry rats are the same and they will want them gone.
Recognition of Woodrats/Packrats/Traderats
In general, woodrats are about like Norway rats in size and tend to look like overgrown deer mice. Adult head and body length about 7-8″ (178-204 mm), tail about 6 1/2-7 1/2″ (165-190 mm) long. Color varies with species, from gray to brown to blackish; bellies and feet range from bright white to light brown. Ears are large, eyes are large, bulging, and black; fur is very long, fine, and soft; tails are hairy, and facial whiskers are very prominent.
Similar Groups to Woodrats/Packrats/Traderats
(1) Norway rat (Ratus norvegicus (Berkenhout) with tail almost naked, scaly (vs hairy), fur coarse, shaggy, and brown with scattered black hairs (not soft and fine); (2) Roof rat (Ratus ratus Linnaeus) with tail almost naked, scaly (vs hairy), fur brown with scattered black hair; (3) Cotton (Oryzomys sp.) and rice rats (Sigmodon sp.) with tail almost naked but annulate (containing rings); (4) White-footed mice (Peromyscus sp.) smaller, about 5-9″ (12.8-22 cm) long (vs 12-16″ or 35-45.2 cm) from tip of nose to tip of tail, tail bones longer than half body length; (5) Other common rodents with long-haired tails.
Representative Species of Woodrats/Packrats/Traderats
(1) Western white-throated woodrat. Adult head and body length about 5 3/4-8 1/4″ (144-210 mm), tail about 4 1/4-6 3/4″ (120-193 mm) long, weight about 4 1/2-10 oz (127-280 g). Color of body gray with upper back darker, belly white or grayish, hairs of throat white to their bases, feet white, tail bicolored, brownish gray above and whitish below. Occurs from extreme southwestern Colorado and southeastern Utah, west through extreme southern Nevada to southeastern California, and southward through most of Arizona and western New Mexico, and then into northwestern Mexico.
(2) Southern plains woodrat. Adult head and body length about 7-9 1/2″ (180-245 mm), tail short and heavy, about 5-7″ (130-175 mm) long, weight about 7-11 1/4 oz (180-317 g). Color of body steel-gray above, belly pale gray, hairs on throat, breast, and feet white to bases, tail dark grayish above and white below. Range extends south-ward from southwestern Kansas, through western Oklahoma, western Texas, and most of New Mexico, and southward through northeastern Mexico.
(3) Mexican woodrat. Adult head and body length about 5 3/4-8″ (147-204 mm), tail about 4 3/4-6 1/4″ (123-157 mm) long, weight about 4-6 3/4 oz (112-194 g). Color of body grayish to brownish or rusty with darker gray mixed in (approaching black in some lava areas), belly whitish or yellowish, feet white, tail bicolored, blackish above and whitish below. Occurs primarily in the montane regions from northern Colorado and the 4-corners area southward to Honduras.
(4) Dusky-footed woodrat. Adult head and body length about 7-9 1/2″ (180-240 mm), tail about 5-9 1/2″ (148-240 mm) long, weight about 8-15 1/4 oz (200-430 g). Color of body buff-brown above, belly grayish to whitish, tail brown above, slightly paler under-neath, and feet and ankles dusky with toes and claws white. Occurs in western Oregon, southward through northern California and then southward down the western California coastal area to the Santa Barbara area.
Biology of Woodrats/Packrats/Traderats
Unlike the commensal rodents, populations of woodrats do not increase quickly. In the north, they have 1 litter each year. The breeding season is from February to March with birthing occurring during March to May. The gestation period is 33-43 days. In the south, they have 2-5 litters each year, with the breeding and birthing occurring year-round. They usually have 2-4 young per litter (range 1-7 young/litter), and the young are weaned when about 4-7 weeks old. Adult life span is up to 3 years, average about 6-12 months due to predation. They tend to be solitary animals except during mating and rearing times.
Woodrats are of medical importance because they have been found to be reservoirs of a wide range of pathogenic viruses, bacteria, and protozoans that may present disease risks to humans. Below is a listing of important diseases associated with the four pest species. If further information is needed, consult (www.cdc.gov) for the profiles of rodent-borne diseases associated with woodrats.
Western white-throated woodrat:
- Borrelia burgdorferi (Lyme disease) in Colorado;
- Arenaviruses (e.g., hemorrhagic fevers, Lymphocytic choriomeningitis (LCMV) others) primarily in the southwestern United States.
Southern plains woodrat:
- Typanosoma cruzi (Chagas’ disease; 6+ species identified as reservoirs; parasitic blood-infesting protozoans).
- Arenaviruses (See above).
- Arenaviruses (See above).
- Borrelia burgdorferi (Lyme disease) in Colorado.
- Anaplasma (formerly Ehrlichia) phagocytophilum (human granulocytic anaplasmosis/HGA; formerly HGE).
- Borrelia burgdorferi (Lyme disease) in California and Oregon.
Habits of Woodrats/Packrats/Traderats
Woodrats are nocturnal in habit. Their nests are typically built out of sticks and are called “houses.” The different species occupy specific habitats or ecological areas. The range of woodrats is about 0.5-1.5+ acres (2,024-6,070 sq m), or about 200 feet (61 m) along well-marked trails from nest to feeding area. They drink very little water, but during the dry season they may make use of water-laden plants such as the fleshy stems of cacti in desert areas.
The foods of woodrats include plant tissues (e.g., roots, stems, and leaves), seeds, nuts, acorns, and fruits. For protein, they eat insects, snails, birds, small mammals, and carrion. They also eat easily accessible human and/or pet food.
As is typical of rodents, woodrats constantly gnaw (the genus name Neotoma means “new teeth”). Woodrats gnaw on electrical lines in wall voids and attics posing serious dangers to human safety from the resultant shorts and fires. In vacant summer cabins, they can cause serious damage to the furniture and stored items. Given their habit of caching, various combustible materials can accumulate in wall, ceiling, and floor voids creating fire hazards. Around landscaped yards and in orchards, woodrats damage vegetation by clipping young limbs (1/4″ or 6 mm in diameter) of fruit trees, or rarely girdling small trees and shrubs.
The habits of woodrats vary according to the species. Some occupy deserts to densely wooded and cavernous areas, while others are ground dwellers or live in trees. Specifically:
- Western white-throated woodrats. Occur in brush lands and in rocky cliffs with shallow caves. Most common in the Sonoran and Chihuahuan desert grassland and desert shrub habitats, and will utilize riparian areas when available. Houses usually constructed among cacti, brush, or in cliff caves and measure 3-10 ft (1-3m) in diameter and up to 3 ft (1 m) high, usually. White-footed woodrats feed on cactus, mesquite beans, and seeds, fruits, flowers, and occasionally on beetles, ants, and small reptiles; some may store food in their houses.
- Southern plains woodrats. Occur in semi-arid brush lands and areas of thorny vegetation located in low valleys and plains. Constructs houses of brush, cactus, and debris 3-5 ft (0.9-1.5 m) in diameter and almost as tall, typically positioned among thorny vegetation such as mesquite and cactus. It feeds on seeds, acorns, and cacti; stores food.
- Mexican woodrats. Occur among rocks and cliffs, often in mountainous areas containing conifer trees. This woodrat rarely constructs houses. Sticks and debris are simply dropped into crevices among rocks and cliffs, or under logs or tree roots, and in deserted buildings. Mexican woodrats feeds on nuts, acorns, seeds, fruits, mushrooms and cacti, but mainly on foliage. Occasionally stores its food.
- Dusky-footed woodrats. Occur in the heavy chaparral, streamside thickets of deciduous or mixed hardwoods. It constructs 1-3 large houses of sticks on the ground or up in trees. Establishes colonies of 3-15 or more multiple homes within their habitats. Dusky-footed woodrats feed on nuts, acorns, seeds, fruits, vegetation, and fungi; stores food in its houses. The dusky-footed woodrat can be a pest in homes and summer cabins located in forested/mountainous areas. It is an excellent climber and often gains access via rooftops or attics by climbing, or from overhanging tree branches. They may locate their nests in the attic or in wall voids. Nests are commonly located in attics, overhanging soffits, and wall void spaces.
Control of Woodrats/Packrats/Traderats
The presence of woodrats may be indicated by damage to the interior contents of cabins or homes located in conducive areas. Finding large stick nests in a shed, beneath a porch or deck, in a crawl space, or in an attic would indicate their presence.
Several methods of control are available. Select the one or combination that best suits the situation at hand:
- Exclusion. This is the most permanent way to keep woodrats out of a structure. It is done in the same manner that would be used to exclude Norway and roof rats. Because access is usually at the roof or attic level, pay particular attention to the removal of all over-hanging tree limbs and cut back all tree limbs 6 ft (2 m) from the roof. Also, look for openings around roof dormers, gables, skylights, and attic vents, eaves, broken roof shingles or tiles, etc. Before closing such openings, be sure that the woodrats have not been unknowingly trapped inside the structure.
- Live-trapping. Live-catch wire cage traps 16x5x5″ (40.6×12.7×12.7 cm) are elective for woodrats. Soft nesting materials and/or small shiny objects (e.g., ball of aluminum foil are good attractants.
- Lethal trapping. The traditional rat-size snap traps work well on woodrats. Bait with a prune, raisin, or shelled nut tied on to the trigger with non-flavored/scented dental floss. Place the traps across the rat’s runways. Woodrats are not fearful of new objects found in their environment and tend to investigate these traps.
- Toxic baits. Anticoagulant rodenticide baits are registered for ONLY 3 woodrat species (see above). Only block bait formulations are recommended for woodrats because the blocks facilitate the bait being consumed where it is encountered and it is thus less likely to be cached in areas that can harm non-targets. Baits are most effective when carefully placed near the woodrat’s major runways, feeding sites, or nests (i.e., inspection is important). Always secure bait blocks with wire or via nailing the bait to a structural element. Bait stations are NOT recommended for woodrats because woodrats fill and jamb the stations with sticks and other debris.
: Before using rodenticides for woodrats, ALWAYS CONFIRM that the pest woodrat is on the rodenticide label and is not a threatened or endangered species. Otherwise, use exclusion and live trapping techniques given above.
What do Woodrats/Packrats/Traderats look like?