Integrated Pest Management

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Rats and Mice

There are over 20 species of rodents native to B.C., most of which are not pests. The problem rodents are three introduced species that came with the Europeans settling in North America. These are the house mouse, Norway rat and black rat. These rodents have spread wherever humans provide them a good living, but most people do not come into conflict with them until the animals take up residence in a house or outbuilding. Removing an occasional invading rat or house mouse is relatively easy if the problem is tackled immediately. Established populations, however, can be difficult to eliminate, especially if they are in barns or in older houses that offer many entry points and hiding places. Because these pest species breed prolifically, ignoring a problem soon results in a much larger and more damaging infestation.

Mice

The house mouse is the most common animal infesting human dwellings; it also thrives in agricultural areas around barns and grain fields. It is a small, slender animal, weighing less than 30 g (1 oz.), with a pointed nose, relatively large ears and a nearly hairless tail. Although they prefer grains and seeds, mice will nibble on almost anything. Females produce 7 or more litters per year, with 5-7 pups per litter. They make their nests in hidden, enclosed spaces, using shredded paper, insulation, string or other soft materials. Mice can gnaw through wood, asphalt shingles and even soft mortar and can squeeze through cracks little more than 1 cm (1/2 inch) wide. Their droppings are dark brown pellets, about 6 mm long (1/4 inch), left wherever mice have travelled.

Rats

The black rat and the Norway rat are both common in coastal areas, whereas there are virtually none in the interior of B.C. What people in the interior of the province call a rat is usually a packrat, also known as the bushy-tailed wood rat because of its distinctly furry tail. Wood rats and other native rodents are protected under the Wildlife Protection Act and may not be killed, except where they are damaging property.

A Norway rat weighs up to 0.5 kg. (about 1 lb.) and has a blunt nose and ears that are relatively small for the size of its head. Its tail is shorter than the length of its body plus head. The females have up to 7 litters per year of 8-12 pups per litter. The black rat is slimmer than the Norway rat, with larger ears and a tail longer than its body and head put together. Black rats (also called roof rats) nest in ceilings and attics; Norway rats prefer moist conditions and generally live at ground level, in crawl spaces and burrows around building perimeters. They are both excellent climbers and can climb up inside or outside of pipes; black rats can even climb up wires. Although usually only one species seems to infest a building at a time, either species can be found in buildings in B.C. Rat droppings are dark, oval pellets, 15-20 mm (1/2 to 3/4 inch) long.

Damage

Rats and mice damage buildings by chewing on insulation, siding, wallboard, etc. They can start fires by gnawing on wiring, causing electrical short circuits. They enter buildings by enlarging gaps in the walls and foundation, around plumbing pipes or through other holes in the structure. Although a pair of house mice can eat up to 2 kg. of food in 6 months, they ruin much more by contaminating it with droppings and urine. Rats swiftly consume and contaminate large amounts of stored food. All three pest species can transmit diseases carried on their feet or in their urine. In some areas, the fleas found on rodents are carriers of disease, which is a compelling reason for preventing them from entering buildings (see Prevention).

Detection

If a rodent has not actually been sighted, its presence can be detected from other signs. They leave a trail of droppings and it is easy to tell by the size whether the droppings belong to mice or rats. Gnawed food and chewing marks on woodwork and around plumbing pipes are indicators of these rodents. They also make noises in walls and leave dirty rub marks along frequently used routes.

Prevention

Exclude them:

• Make sure that the building is in good repair. Fill or block all openings with durable materials, such as concrete and lumber, and use heavy, welded wire mesh to cover any vents, conduits, drains or other openings that cannot be blocked.
• Be persistent in inspecting and repairing entry points that can be reopened by gnawing rats.
• Remove potential hiding places near the house: don't stack firewood, garden supplies, equipment, etc. beside the house; thin or remove dense vegetation around the foundation.

Remove food and water sources:

• Store cereals and dry food in glass or metal containers and keep pet food and bird seed in sturdy, covered bins.
• Stored fresh food, such as fruit that is often kept in the garden shed or back porch, is very attractive to rodents. If possible, store produce in a refrigerator or a secure room that has heavy wire screen on any vents open to the outdoors.
• Compost kitchen waste in closed bins, such as the thick, black plastic compost bins available commercially, or in other heavy, closed containers. To be rat-proof, a home made bin must be built of wooden planks and heavy 1/2" welded wire mesh (not chicken wire, which rats easily chew and which mice easily pass through). Do not put meat scraps or bones in the compost bin.
• Store outdoor garbage in tightly closed containers, preferably made of metal.
• Make sure bird feeders are well away from the house and that the feeder prevents excessive seed from spilling onto the ground. This is very attractive to rats, who will also gnaw into a bird feeder if they can reach it. Stop them from climbing bird feeder poles by placing a wide metal collar on the pole.
• Repair any leaking plumbing, indoors or out, to remove a water supply. Norway rats, in particular, need a great deal of water and sometimes learn to chew into irrigation lines and garden hoses.

Controls

Take great care to ensure that native species are not harmed by control methods. Most native species cause little or no damage, therefore it is essential to identify problem rodents before deciding on controls. If in doubt, use a live trap, which allows you to identify the animal and release it if desired. Most people should be able to control an occasional infestation of house mice, black or Norway rats. A serious rat problem, however, may require assistance from a professional pest control operator to ensure that all entries are sealed and that traps and baits are set correctly.

Traps: Snap traps are sold in every hardware store. Small traps are available for mice, larger ones for rats. When baited and set properly, these are very effective and kill the offending animal instantly. Although it is unpleasant to deal with a dead mouse, it is a swift, humane end compared with the fate of a poisoned rodent, which may take several days to die.

Set snap traps at right angles along walls where the rodents are known to travel, with the bait side of the trap toward the wall. A couple of well-placed traps will suffice for a mouse or two, but for larger populations, you may need to place traps every metre or so along walls and mouse routes. An excellent bait is a piece of dried fruit, which should be tied to the pedal with thread to make sure that the rodent trips the trap when it moves the bait. Other baits: peanut butter smeared on the top and bottom of the bait pedal, a mixture of peanut butter and oats, cheese, marshmallows, even onions, or any other food that the animals have been nibbling. Be sure to secure the bait to the trap, for if it is easily removed, the pedal may not be disturbed sufficiently to trip the spring action of the bail. Rats are cautious about new things in their environment so it is a good idea to leave the baited, but not set, trap in place for several nights before setting it.

Wear gloves or use a plastic bag as a glove to handle the trap and the dead animal, which should be wrapped in plastic and put in the garbage. If the captured animal isn't dead (an extremely rare occurrence) use a shovel to pick up the trap and drop it into a bucket of soapy water to drown the animal. The traps are reusable and used traps are more attractive to mice than new traps.

Live traps are also available and very effective. They are expensive, but do not need to be reset to continue catching mice. If using a live trap appeals to you because it seems more humane, consider what you will do with the live mice you have trapped. Mice quickly die of stress and exposure if they are held in live traps for long without food or water. If you intend to release them, check the trap at least daily. Mice in a live trap can also be killed by submerging the trap in soapy water.

Poison: Although the most common rodent control is to use poisoned baits, this should be considered only as a last resort, if other methods fail. Putting out poison baits exposes children, pets and wild animals to the risk of poisoning from most compounds, whether they eat the bait or just handle it. A poisoned rodent often crawls away to die in an inaccessible place in the house. As it decomposes, it may smell and the decaying carcass frequently hosts an infestation of fly maggots, carpet beetles or other pests. Even if the bait is correctly placed in inaccessible locations, there is also the risk of secondary poisoning to pets or wild animals that may catch an ill and dying rodent after it has eaten the bait.

The most common baits contain anticoagulants, which means they prevent blood from clotting. The rodent dies from internal bleeding. Some anticoagulants, such as brodifacoum and bromadiolone, act after a single dose. Others, such as warfarin, require multiple doses. In areas where warfarin has been used extensively, particularly in cities, it is useless because rat populations have become resistant to it. These baits present a moderate to high risk of secondary poisoning to other animals that might eat the poisoned rat. Products containing cholecalciferol (Vitamin D3), however, present a lower risk to other animals. This compound kills by a vitamin D overdose after 3-4 days, which mobilizes excessive amounts of calcium in the blood. It is the least hazardous of the rodent poisons for domestic use and least risky to other animals if they catch a poisoned rodent.

No matter what poison is used, it must be placed in areas absolutely inaccessible to children, pets or other animals, preferably in tamper-proof bait stations. Bait stations are plastic or metal boxes with a compartment inside for the bait and lids that are secured with a screw or key to prevent children from opening them. The rodents can get to the bait compartment through small holes in the ends of the box, but larger animals are excluded. Never scatter poison baits over the ground or inside a building (this is dangerous and illegal). When using any bait, always read the label and follow the directions. For a larger scale baiting program, consult a licensed pest control operator for assistance. Once the pests have been eliminated, dispose of the bait stations, preferably at a household hazardous waste collection depot. If this is not possible, double wrap them in plastic and put them in the garbage. Be sure to repair or seal any access points to prevent a new invasion of pests.

Ultrasound Repellers: There are several devices on the market that emit ultrasound waves to repel rodents. Although rodents will avoid the area initially, they apparently get used to the sound and learn that there is no harm associated with it. The repellers are expensive and seem to be effective only over small areas for a limited time. They are usually not cost-effective compared with methods listed above.

1994