INTRODUCTION

The roof rat is the smaller of the 2 commensal rats (Norway rat is larger) and the more common commensal rat in the subtropical and tropical regions of the world. It not only damages/destroys materials by gnawing, eats and contaminates stored food, but it is also of human health importance as a vector or carrier of disease organisms. Roof rats are usually thought to be of southeast Asian origin, and are now worldwide in distribution. In the United States, it is more common in the coastal states, seaports, and the southern third of the country.

RECOGNITION

Adult with combined head and body length 6-8″; (16-20 cm), tail length 7-10″ (19-25 cm), usual weight 5-9 oz. (150-250 g) but up to 12 oz. (340 g). Fur soft, smooth, color usually brown with black intermixed, to gray to black above with underside white, gray, or black. With muzzle pointed, eyes large, ears large (can be pulled over eyes) and almost naked. Tail scaly, uniformly dark, /longer than head and body combined. Adult droppings up to 1/2″ (12.5 mm) long, spindle-shaped, with pointed ends.

SIGNS OF INFESTATION

1. Gnaw marks. New gnawings or holes tend to be rough whereas, old gnawings are smooth from wear.

2. Droppings. Fresh droppings are soft and moist whereas, old droppings are dried and hard; adult roof’s about 1/2″ (12-13 mm) with pointed ends vs. Norway’s about 3/4″ (18-20 mm) with blunt ends.

3. Tracks/footprints. Front foot 4-toed and print is in front of usually longer hind print with 5 toes. Fresh tracks are clear and sharp

4. Rub marks or dark, greasy markings on vertical surfaces. Fresh marks are soft, greasy, and easily smeared whereas, old marks are with the grease dry and flaky. Swing marks often present around rafters.

5. Burrows. Not common, but if present they are shallow. They usually nest in or under vegetation.

6. Runways. Travel routes may not be apparent outside because they may travel along fences or on overhead power or telephone lines. Indoors, they usually move along walls, stacked merchandise, etc. Active runways with greasy appearance, free of dust and cobwebs, with fresh tracks and/or droppings.

7. Damaged goods. Roof rats prefer fruits, vegetables, and cereal whereas, Norway rats prefer meat fish and cereal.

SIMILAR GROUPS

(1) Norway rat (Rattus norvegicus) with blunt muzzle, small eyes, ears small and hairy, tail bicolored and shorter than head plus body, droppings rod-shaped with blunt ends. (2) Hispid cotton rat (Sigmodon hispidus) with tail about half head-body length combined and less heavy (weight 2 3/4-7 oz/80-198 g), and fur coarse and grizzled, grayish above with mixed buff, black, and whitish below. (3) House mouse (Mus musculus) with tail about as long as head plus body, smaller (about 1/2-1 oz/14-28 g), shorter (head, body, and tail 5.25-7.5-/6.5-10.2 cm), droppings 1/8-1/4″ (3-6 mm) long, rod-shaped, with pointed ends. (4) Most native rats and mice with tail hairy, hairs short or long, or if tail almost naked, it is also annulate (appears to be of ringlike segments).

BIOLOGY. Roof rats reach sexual maturity in 2-5 months. Pregnancy lasts an average of 22 days. The young are blind and naked at birth, with hair appearing in about 7 days and eyes opening in 12-14 days. They are weaned at about 3-4 weeks. The average number of litters is 4-6 per year, each containing an average of 6-8 young. Adults on an average live 9-12 months.

They have rather poor vision and are color blind, but their senses of hearing, smell, touch, and taste are keenly developed. Touch is via their vibrissae or long whiskers. They are good runners, excellent

A roof rat requires 1/2-1 oz (14-28 g) of food and 1 oz (30 ml) of water each day, with the water often coming from its food. This results in about 30-180 droppings and 1/2 oz/3 teaspoons (16 cc) of urine per day.

Historically, bubonic plague has been associated with the roof rat and its fleas, which move from infested rats to man. Fortunately, plague has not been found in rats in the United States for many years. Other transmitted disease organisms include murine typhus via fleas (also probably via droppings and urine), infectious jaundice/leptospirosis/Weil’s Disease via urine in water or food, rat-bite fever via bites, trichinosis via undercooked pork, and food poisoning or Salmonellosis via droppings. Another problem is tropical rat mite dermatitis which is caused by these mites when they feed on humans.

HABITS. Roof rats are primarily nocturnal in habit and they are very cautious. Although they constantly explore their surroundings, they shy away from new objects and changes. Roof rats prefer to nest in the upper parts of structures but may be found under buildings as well as occasionally in basements and sewers. Outdoors, they prefer to nest in higher places such as in trees but may occasionally be found in burrows in or under vegetation around the structure. These are social animals but less so than Norway rats. Several nests may be located within a given area. An opening of greater than 1/2″ (12 mm) is required for entry into buildings.

Although they will eat practically anything, roof rats prefer fruits, vegetables, and cereals. If the eaten food material proves disagreeable, they are quick to develop food/bait shyness. Once they find an acceptable/preferred food, rats tend to eat their fill at one sitting/place and will return time after time.

Once established indoors, roof rats tend to follow the same route or pathway between their harborage and food and/or water sources. Runways along vertical surfaces will usually include dark rub or swing marks on the vertical surface where their fur makes contact. Their runways will be free of debris, and outdoors, the grass will be worn away to the bare soil.

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