Rats

Order: Rodentia
Family: Muridae
Species: Rattus rattus, R. norvegicus

Identification: The two rats most commonly found around the Puget Sound are the Norway rat (Rattus norvegicus) and the roof rat (R. rattus), also known as the black rat. The Norway rat is easily recognizable by the “oh my god, run!” response while the root rat can be distinguished by the typical “yuk, that’s gross” response when encountered. The Norway rat is a big, hefty rat and is much larger than the other rodents in the neighborhood. Norway rats can weigh 1¼ pounds while the roof rat stays at an agile ½ pound when fully grown. Roof rats have a longer tail, pointier snout and longer ears while the Norway rat’s tail is shorter than the length of the body. Both rats have naked tails but the Norway rat’s tail appears as two-tone from the partially haired skin.

Life History: Rats are perfect pests; they have a high reproductive capacity and can compete well against other pests for our rubbish. Rats can reproduce all year long depending on environment and food. They usually reach their peak reproduction in the fall. Both species average about eight offspring per litter. A single female rat can produce as many as 80 rats in her lifetime. Gestation and weaning of young can last about two months; baby rats can be reproductively mature within three to four months. If you do the math, rat populations can build quite rapidly in good conditions.

Rats will eat just about anything. Rats are very successful at feeding on garbage food items that humans throw out. Rats can also eat plant material such as grains and have been known to attack other animals such as insects, birds, fish and other mammals (including humans).

Damage: Rats have been historically associated with death and disease. Rats are the carrier of the Bubonic plague along with other nasty diseases and have been responsible for millions of human deaths throughout history. When rats are populous, they act as the perfect reservoir of disease because they are so closely associated with our lifestyles and homes.

Most of us aren’t too worried about disease but still cannot tolerate them in and around our homes. Rats get into everything and can ruin our food and gardens. Rats contaminate our living spaces and food by leaving behind feces and urine. Rats dig into garden beds and gnaw at our electrical wires. They can do physical damage to our homes and property by their foraging and nesting habits. They have also been known to bite and scratch people.

Monitoring: Be sure to visit neglected areas of your home and yard regularly. These include crawlspaces, attics, and the nooks and crannies around pipes. Frequency of your visits should increase as temperatures drop in the fall and winter. This is when rats seek better shelter and their populations increase. Look for evidence of rat activity such as nibbled wood, dry wall and paper scraps. Rats leave behind droppings that are dark, elongated pellets that are about ½ inch long. Rats also tend to run along the same trails. Preferred trails of Norway rats are next to vertical edges such as where the floor meets the wall or along foundations at ground level. Often grease marks are left from the rat’s fur rubbing along the wall. Roof rats are excellent climbers and will scale walls along gutters and pipes and will climb on wires, trees and vines.

Management: The best way to control rats is through cultural amendments to deter rats from nesting and foraging around your house and yard.

Don’t feed the rats!

• Secure stored food products in rat proof containers, such as metal bins with tight sealing lids. Be sure to securely store animal food such as dog food and bird seed in a similar manner. Glass storage jars are great for preventing rodents from feeding on stored foods. Rats can chew through some plastics and wood.
• Clean up ripe vegetables and fruits from the yard. Joyce Jimerson has great information for preventing rats in compost bins: http://whatcom.wsu.edu/ag/compost/ratsincompost.htm.
• Be sure to keep your garbage cans secure and closed. Metal cans are best to avoid rats from chewing in. A bungee cord is a great way to keep the lid closed.
• Pick up after pets. This included wasted food and droppings.

Don’t give rats a home!

• Remove dense vegetation that offers shelter. Blackberry patches and English ivy provide great protection for rats.
• Keep newspaper stacks, woodpiles and hay stacks clean and away from your house. If possible, set piles up off the ground.
• Fill in burrows and holes that rats may use to nest in. Rats are excellent burrowers.

Keep rats out of your home!

• Remove vines that attach directly to the house such as climbing ivy, or wisteria. Do not let tree branches touch the roof.
• Place rat guards on pipes and gutters leading up to the house.
• Seal up any gaps or cracks along the foundation.
• Place sturdy screen around vents and openings. The mesh size should measure at least ¼ inch.
• Repair gaps in sewer lines and other pipes leading to the house.
• Place sturdy covering on drain holes.

If populations are very high, it may be necessary to reduce the population size through trapping or poisoning. Snap traps are effective and are the most advisable mode for killing rats. Set traps in areas were rats are active such as along wall boards nest entrances but do not place them in areas that could cause potential harm to people and pets. Be sure to follow the instructions carefully. Traps can be dangerous and painful if sprung at an inopportune time. Rodenticides and poison baits should be used as a last resort. If problems are serious and pesticides are warranted, it might be worth contacting a pest control professional. If you apply a rodenticide yourself, always read and follow the directions on the label.

Although cats can cause as many problems as rats and I think are equally disgusting (I’m not a cat person), cats are effective at hunting and deterring rats in your home and yard. The presence of cat urine is known to cause predator-avoidance behavior in rats and sends them scurrying. Regardless of the management tactic that is decided upon, habitat reduction and prevention are critical for reducing rat problems.

For more information on managing rats and other vermin, visit Dave Pehling’s chapter on vertebrate pest management: http://snohomish.wsu.edu/vertchap.htm

To learn about rats and other mammals in Washington State, visit the Burke Museums online catalog: http://www.washington.edu/burkemuseum/collections/mammalogy/mamwash/mamwash.htm