Integrated Pest Management

IPM Manual for Structural Pests in British Columbia

Chapter 8: Rodents

Learning Objectives

When you have completed this lesson, you should be able to:

Identify the house mouse, the Norway rat and the black rat.
Describe a monitoring program to assess the relative numbers of rodents in a structure.
List examples of preventative measures that can be used to prevent rodent infestations.
Describe the use of snap traps, multiple catch traps and glue boards.
Describe the use of bait stations in a rodent control program.
Describe the mode of action and use of cholecalciferol and anticoagulant baits.

Identification

Three species of rodents introduced from Europe are pests in British Columbia. These are:

house mouse (Mus musculus),
Norway rat (Rattus norvegicus), and
black rat (also called the roof rat) (Rattus rattus)

House Mouse

The house mouse is small and slender, weighing less than 30 grams. It has a pointed nose, relatively large ears and a nearly hairless tail.

Unlike other structural pests, house mouse numbers are not limited by lack of water. Mice can get enough water from food, even from dry cereals. The house mouse is well adapted to stressful environmental conditions. It can become dormant to conserve energy during extreme hot or cold conditions. Females produce 7 or more litters per year, with 5-7 pups per litter.

House mice can gnaw through wood, asphalt shingles and even soft mortar and can squeeze through small cracks 1 cm in size. House mice leave small (6 mm long), dark brown droppings wherever they go. The presence of mouse droppings is often the first indication of a mouse problem.

Black Rat and Norway Rat

The black rat and Norway rat are common to the West Coast of British Columbia. They are rare in the interior of the province. What may be called a rat in the interior is usually a packrat, also known as a bushy-tailed wood rat. These and other native rodents are protected under the Wildlife Protection Act and may not be killed, except where they are damaging property (a rare occurrence).

A Norway rat has a blunt nose and ears that are relatively small for the size of the head. The tail is shorter than the combined length of the body and head. The black rat is slimmer than the Norway rat and has larger ears. The tail is longer than the combined length of the body and head. Rat droppings are dark, oval pellets, 15 to 20 mm long.

Black 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. Both rats are excellent climbers and can climb up the inside or outside of pipes. Black rats can even climb wires. Female rats have up 7 litters per year of 8-12 pups per litter.

Characteristics of Domestic Rodents

Drawing of rodents

Monitoring

The presence of rodents may be confirmed by inspections for droppings,See notes on Hantavirus gnawed food or woodwork, and by finding greasy rub marks on walls where rats travel. Often clients have seen one or more rodents in or around the structure or have heard sounds in attics and wall cavities that indicate they are present.

The relative numbers of rodents, and the areas where they are most common, can be monitored by:

using snap traps or glue boards to catch them
removing all droppings and observing where and how fast new droppings appear
setting out non-toxic baits and observing where and how quickly the baits are taken

Action Decisions

There is usually no level of tolerance for rodents in residences. In and around warehouses, it may be sufficient (and all that is possible), to keep populations at very low levels.

Because of their high reproductive rate it is important to control rodents as soon as the first signs are found. Early control prevents the build up of large populations that are difficult to eradicate.

Prevention

Installing a barrier of copper mesh to fill the gap around a plumbing pipe The following are examples of sanitation measures, habitat modification and barriers that can be used to prevent rodent infestations.

For mice:

eliminate hiding places such as stacks of firewood and dense vegetation around the foundations of structures
remove food supplies by storing food in tight-fitting metal or glass containers, and
prevent entry by filling cracks through which they could enter the building

For rats:

use rat-proof garbage cans
use rat-proof compost bins
seal holes in buildings with galvanized metal sheeting, strong wire mesh or 22-gauge aluminum sheeting, etc.
remove rat shelters, such as piles of old lumber, firewood stored on the ground close to buildings and dense, low vegetation
remove bird feeders or keep them away from buildings

Controls

Rats are often a community wide problem, therefore community involvement in their control may be required for good results.

Traps

Snap traps: These are an effective control measure in houses if used correctly. Generally, traps should be:

placed in areas with high rodent activity
set at right angles along walls with the bait side of the trap toward the wall
placed at frequent intervals along a wall (e.g. every 0.5 to 1 m for mice, every 1 m for rats)

Good baits are peanut butter, dried fruit or anything the rodents have been feeding on at the site. Snaps traps with a well in the bait area are used with liquid baits. They are especially good for rats if there is no other source of water.

Rats are cautious about new objects in their environment, therefore it may be necessary to leave a baited, but not set, rat trap in place for several nights before setting it. Some individual rodents, especially rats, may be very hard to catch with traps. Considerable experimentation with trap placement or different baits may be necessary to catch them.

Multiple catch traps: Live traps (e.g., Ketch-All® traps) can hold from 15 to 30 mice, depending on the trap size and design. No bait is needed because mice are attracted to investigate them. Once the mice go into the entrance they cannot get back out again. These traps are very effective, but should be checked daily to destroy the captured mice.

Glue boards: These, or box traps with sticky boards inside, are effective for mouse control, especially for mice that are either too large or too small to be caught in snap traps. There is some controversy over their use because of the length of time it takes for captured rodents to die.

Rat glue boards are available, but some pest managers do not have as much success with them. Small or young rats are more likely to be caught, whereas large rats are more likely to escape from the glue.

Repellent Sound Devices

Ultrasonic repellers may be of some use when "aimed" at temporary rodent entrances such as open loading bay doors. The sound emitted by repellers does not effectively penetrate walls or travel around corners and are useless in a cluttered environment. Rodents also become accustomed to the sound from the devices, especially when it is associated with the presence of food. To be effective, good sanitation is necessary to make sure that warehouses and buildings are clean and uncluttered and are not otherwise inviting to the rodents. Other possible entry routes for rodents must also be sealed.

Other Pesticides

Tamper-proof rodent bait station Generally, rodenticides, such as cholecalciferol, multiple and single-dose anticoagulants, would be considered only where trapping has proven ineffective or cannot be used. Rodenticides may be necessary for controlling large populations of rodents or for rodent control outdoors. They can also be useful as part of an ongoing rodent control programs (e.g., for a warehouse) where baits are regularly maintained to intercept rodents from the surrounding area.

Rodenticides should only be used inside tamper-proof bait stations (see box). Once rodents have been eliminated, the bait stations should be collected and disposed of properly by the pest control company.

Bait stations should be secured to the ground to prevent the bait from being shaken out. This is particularly important when meal baits are used. Bait blocks should be fastened inside the bait stations so that rodents cannot carry them out of the stations. Pelleted baits cannot be secured inside the stations and rodents may carry the bait pellets out of the bait box. Therefore, pelleted bait is not recommended unless it is the only effective rodenticide or formulation available. Where pelleted baits must be used, throw packs of pellets could be secured inside bait stations to reduce the likelihood of pellets being spread around.

Bromethalin: Bromethalin is a potent rodenticide that provides a lethal dose to rodents in a single feeding. It causes damage to nerves, which leads to paralysis, convulsions and death. Death occurs within 24 to 36 hours after ingestion.

Cholecalciferol: Products containing cholecalciferol (Vitamin D3) present a lower risk of secondary poisoning to non-target animals that might feed on a poisoned rodent than do most anticoagulants. Cholecalciferol kills by mobilizing calcium from the bones into the blood stream, which causes the rat to die of heart failure.

A lethal dose can be consumed in a single feeding, although it is more common to see smaller multiple feedings over a period of days. After ingesting a lethal dose it takes two to four days for the rodent to die. Manufacturers claim that there is no bait shyness because toxic symptoms do not start until after a lethal dose is eaten.

Cholecalciferol baits (e.g., Quintox®) are available as bait pellets for mice or rats, or, as mouse seed. The mouse seed is a blend of canary, groat and millet seed which mice find very palatable. The bait or seed should be placed in tamper proof bait stations.

Anticoagulants: Anticoagulant rodenticides prevent blood clotting and cause the poisoned rodent to bleed to death internally. Warfarin, diphacinone and chlorophacinone are three examples of multiple dose anticoagulants. All are toxic to non-target animals, such as dogs and cats. A looming problem with overuse of these products is the development of resistant rodent populations. For example, there have been reported cases of rodents becoming resistant to warfarin in British Columbia.

Hantavirus Pulmonary Syndrome

Hantavirus Pulmonary Syndrome, or HPS, is a severe illness that is caused by a virus. This rare disease was first described in the southwestern United States in 1993. It is believed that the virus has been present for a long time, but was only just recently recognized. The first time HPS was found in Canada was in 1994, when three cases were reported in British Columbia. Since then only seven other cases have been found in Canada, one of them also in British Columbia. The disease is considered to be extremely rare - as of June 1995 only 120 cases had been reported in all of the United States and Canada. HPS begins as a flu-like illness. In the early stage of the disease, a person may have a fever, sore muscles, headaches, feel sick to their stomach, throw up, and have shortness of breath. As the disease gets worse, fluid builds up in the lungs, making it harder to breathe. In North America, about five out of 10 people with HPS have died. Most patients need to go to the hospital and get intensive care. Some patients may be given anti-viral drugs.

The virus is normally found only in rodents, especially deer mice, but also rats and other kinds of mice. People get the disease (HPS) when they breathe in the virus that is found in the urine, saliva or droppings of infected rodents. It may also be possible to catch the virus by eating or drinking food or water that has been soiled by rodents. The disease does not seem to spread from one person to another. Always wash your hands after touching any rodents or their droppings.

People who live in areas where the virus is present, and who come in close contact with rodent burrows, or are exposed to the saliva, urine or droppings of rodents, are at some risk of catching the virus, although the chances of this happening are extremely low. Pest control professionals should take special precautions during their work, including wearing protective clothing and using any one of several respiratory protection devices. These range from disposable dust & mist masks to high-level, air filtration devices. Only buy disposable, or single-use respirators that are clearly marked as NIOSH* approved and have an approval number (e.g. TC-21C-XXX). They are available at safety supply houses and at some building/hardware firms. You can get further information on how to take these special precautions, and about the safe and correct operation and use, and the limitations of these devices, from your local health unit or department offices or from the Workers Compensation Board. The special breathing devices used for high-level protection must be properly fitted, tested, maintained and operated in order to work properly. They may not be suitable for a person with heart and/or lung problems. Use gloves when handling dead rodents. Put dead rodents into a bucket of household disinfectant (e.g., one part bleach to 10 parts water) for 30 minutes before burning or burying the bodies. If you can't bury or burn the bodies, triple-bag them after disinfection and place in domestic garbage. Disinfect the traps after dead animals have been removed.

*(National Institute of Occupational Safety and Health, United States Centre for Disease Control)

Excerpted by permission from: Health Files #36, British Columbia Ministry of Health

The single dose anticoagulants include brodifacoum and bromadiolone. Rats and mice generally require only one feeding to obtain a lethal dose. Single dose anticoagulants are generally more toxic than multiple-dose anticoagulants, therefore are more dangerous if eaten by children, pets and wildlife. The hazard of secondary poisoning to animals that eat the poisoned rodent is less, however, with these single-dose rodenticides because so little active ingredient is consumed by the rodent.

Tracking powders: These contain a very toxic ingredient, zinc phosphide. They are placed in areas where rodents travel so that rodents get the powder on their fur. They then ingest the poison while grooming themselves. The use of tracking powders is not recommended because rodents will track the powders from areas they have been applied into other areas, where the powder may present a hazard.

Evaluation

Rodent control programs should be followed up by visual inspections for droppings and damage, and by monitoring, using tools such as glue boards, snap traps or multiple catch traps to determine whether any rodents remain or are continuing to invade the site.