Ministry of Agriculture, Food and Rural Affairs

Does Your Farm Have a Rodent Problem ?

Producers should not be embarrassed to admit they have a rodent problem. Surveys in Ontario indicate that 80% of poultry producers and 89% of swine producers have rodent problems. The probability is that mice or rats currently exist on your farm. The embarrassment and costs occur if something is not done to confront the problem.

Why Control Them?

Rodents are the first and second most destructive vertebrates in the world to man. It is thought that they damage/spoil/consume 1/5th of the world's food. It has been estimated that rats can eat/spoil more than $35 worth of feed/rat/year. They can be vectors for over 200 diseases, are responsible for greater than 25% of fires in rural farm houses and barns because of their propensity to chew on electrical wire (Figure 1).

They also have an enormous ability to reproduce. A pair of rats (or mice) can generate a population of 15 000 rodents in one year and 20 000 000 in three years.

Figure 1. Signs of chewing on electrical wire

Figure 2. Droppings are a clear sign of rodents.

The following are signs of rodent infestation:

• Sounds: Gnawing, climbing noises in walls, squeaks.
• Droppings: Found along walls, behind objects and near food supplies.
• Burrows: Rat burrows are indicated by fresh diggings along foundations, through floorboards into wall spaces.
• Runs: Look for dust-free areas along walls and behind storage material.
• Gnawing marks: Look for wood chips around boards, bins and crates. Fresh gnawing marks will be pale in colour.
• Rodent odours: Persistent musky odours are a positive sign of infestation.
• Visual sighting: Daylight sighting of mice is common. Rats are seen in daylight only if populations are high. Quietly enter your barn at night, wait in silence for 5 minutes and listen for the sound of rodent activity. Look around with a powerful flashlight; rat eyes will reflect the light.
• Smudge marks: These may be found on pipes or rafters where dirt and oil from their fur leave a greasy film.

It is a generally accepted rule of thumb that there are approximately 25 mice or rats for every one that is seen.

Is It a Rat or a Mouse Problem?

Since rats and mice require different control strategies, determine whether the problem is rats or mice (Table 1). The simplest way to differentiate between the types of infestation is by examining the droppings. Mouse droppings are black and rice-kernel size (Figure 2), whereas rat droppings are black and bean-sized.

What Do Mice and Rats Like to Eat?

Rats and mice can be considered to be omnivorous. Given a choice, they prefer cereal grains. Rats eat meat when available. However, when food supplies are scarce, they will eat almost anything, including plaster and even soap or animal carcasses. Mice have been known to nest over winter inside the carcass of a deer stricken with rabies, consume the meat and become infected. They then become vectors of this disease. Rats and mice eat every day and prefer a water supply. Rats usually drink every day, but mice can survive several days without water.

Characteristic	Norway Rat	Mouse
Size (including tail)	16 cm (6 in.)	42 cm (16.5 in.)
Average weight (adult)	500 gm (18 oz)	20 gm (0.7 oz)
When active	nocturnal	nocturnal
Sight	poor (1.5 m) (4.9 ft)	poor (1 m) (3 ft)
Smell, touch, taste	excellent	excellent
Hearing	highly accurate	highly accurate
Range from nest	45 m (148 ft)	9 m (30 ft)
Fear of new objects	3-7 days	3 min-5 hr
Water requirements	daily	2-4 days without
Food per day	28 gm (1 oz)	3 gm (0.1 oz)
Water	57 gm (2 oz)	3 gm (0.1 oz)
Favourite foods	rolled oats, meat, fish, vegetable oil	grains, rolled oats, sugar, raisins
Droppings	bean size	rice size
Minimum width for entrance (hole diameter)	12 mm (0.5 in.)	6 mm (0.2 in.)
Can enter building through a hole the size of:	your thumb	your pinkie finger
Can chew through (given edge to gnaw on)	rubber, aluminum, cinder blocks, plastic, wool	same as rats

Table 1. Physical and behavioural characteristics of adult rats and mice

Rodent Control (The Principles)

Rodent control requires an integrated pest-management strategy involving many techniques. The producer's first objective should be to prevent, or at least greatly reduce, rodent numbers through management programs that eliminate entrance to the facility, nesting sites for the rodents, food supplies and water. Populations build when food, water and nesting sites are readily available.

Habits and Biology

To control mice and rats, we have to understand their habits and biology first. Mice and rats are similar in their habits and biology, although there are some differences between the two (Table 1).

Both are highly reproductive and extremely capable of surviving in all kinds of conditions. Theoretically, if there is one pair of mice (1 male and 1 female) in your barn at the beginning of a year, under fair living conditions, by the end of the year, you may have thousands of them on your farm.

On farms, mice and rats will be near a food source such as barns, granaries, livestock buildings and silos.

Mice live in the barn and rats live outside the barn and travel in for food.

Rats and mice can climb and jump. Rats can jump vertically as high as 91 cm (36 in.) and horizontally as far as 122 cm (48 in.).

Mice and rats can climb brick and other rough walls, and travel along utility wires.

Rats can cross (sneak in) through openings as small as 1 cm (½ in.) and mice can squeeze through openings of 0.6 cm (¼ in.), or less, in diameter.

Both mice and rats are active at night, particularly right after dusk.

Rats are smart and tend to avoid new objects. Therefore, it may take a few days for traps and baits to work.

Rodents need to chew. Their incisors never stop growing, up to 12.5 cm (5 in.) per year. They damage the structure of the building itself, but they love chewing on electrical wire, hence the fire risk.

They are excellent diggers/excavators.

Rodent-Proofing Farm Buildings

Proper construction and maintenance of buildings helps prevent rodents from entering your barn. Initial construction footings should extend 0.5 m (19 in.) into the ground. A gravel perimeter that is 30 cm (1 ft.) deep and at least 1 metre (3 ft.) wide will prevent rodents from burrowing into your building. To prevent frost damage, footings may have to be deeper. Examine your building at least once a year for possible entry ways for rodents. Remember, a mouse needs only a 0.6-cm (¼-in.) opening to gain access (the size of your pinkie finger). Rats need a 1-cm (½-in.) opening (the size of your thumb). Cracks around door frames, under doors, broken windows, water and utility hook-ups, vents and holes surrounding feed augers are all potential points of entry. Use coarse steel wool, hardware cloth or sheet metal to cover any entrances. Do not use plastic, wood or insulation, as rodents simply gnaw their way through.

When constructing walls, ensure that sheeting lies flush to the wall studs rather than on strapping. This keeps nesting sites confined to a single section between studs rather than allowing complete access to all wall spaces. For further information, see Plan No. M 9451 of the Canada Plan Service Series, Rodent and Bird Control in Farm Buildings. A well-maintained structure is your first defence against rodents. Most rodents enter your barn directly from the fields, and then the population builds. It is important to maintain good sanitation outside the barn. Eliminate vegetation for 1 m (3 ft) around buildings, clean up spilled feed, remove loose wood, garbage, etc. Do not attract rodents from fields to your operation. The more open space you have around the building the better.

Eliminating Hiding Places and Nesting Sites

Rodents do not like to be exposed. Maintain sound housekeeping, eliminate loosely piled building materials, old feed bags or anything else that a rodent can hide in or under. Keep piles of lumber, miscellaneous equipment 24-30 cm (9-12 in.) off the floor and at least 24 cm (9 in.) out from a wall. Look for entrances into double wall construction. Most rodents nest in the insulation of double walls. Block off all entrances into walls and destroy all nesting material.

Remove Food and Water

Eliminate water sources such as leaky taps, open water troughs, sweating pipes and open drains. Keep all feeds in rodent-proof bins, covered cans or metal hoppers. Reduce feed spillage and immediately dispose of dead animals. Without readily available food and water, populations cannot build.

Control of Existing Population

If there is already a rodent problem inside the barns, prevention alone won't solve the problem. In this case, consider a population-reduction program. For this article we will not be discussing rodenticides based on whether they are (not) allowed for certification.

Snap Traps

For small populations, snap traps or box traps are very useful for eliminating rodents. Rats prefer fresh bacon, fish and meat, while mice favour cheese, peanut butter or seeds. Try several baits to find out which your rodents prefer. Rats are distrustful of anything new in their environment, so leave baited non-set traps out for 4-5 days to allow them to get used to the traps. Ensure that previous baits have been taken before actually setting the traps. If rats are the problem, use rat traps. After you catch a rat you have to remove the carcass immediately otherwise other rats will stay away from that trap location. You may need to move the trap to another location and start baiting non set traps again till you know they are taking the bait before you set them again.

If mice are the problem, use mouse traps. Mice are naturally curious so there is no need to put on baited, non-set traps before-hand. Locate traps close to walls, behind objects, in dark corners, where you see droppings or gnaw marks. When trapping next to a wall, set the trap at right angles to the wall with the trigger and bait closest to the wall. Orient multiple-catch traps with the entrance hole parallel to the wall.

Figure 3. Live trap

Figure 4. Live trap

Live traps

Live traps can work very well near runways used by mice and rats. When setting and baiting for any traps wear rubber gloves so that your scent does not get on the baits or the traps as deterrence to the rodents.

Glue Boards

Glue boards are very effective against mice and are the method of choice in locations where toxic baits are a concern. Glue boards will not work well if there is too much dust. They are only recommended where dust can be kept away from them. Check glue boards and traps daily and remove and dispose of dead mice and rats. Wear rubber gloves when handling them to prevent any chance of disease infection.

Abundant food supplies make baited traps less effective. Eliminate as many sources of food as possible before starting the program. For barns and poultry houses with moderate infestations, set 50-100 traps. The trapping program should be short and decisive to prevent trap shyness. Odours from humans or previously caught rodents do not cause trap shyness. When disposing of dead rodents use plastic gloves and place the rodents in tightly sealed plastic bags.

Predators

Cats may limit low-level mouse or rat populations (Figure 5). However, if conditions are ideal for rodents, cats cannot eliminate a problem. Cats may introduce disease into a facility by bringing in rodents caught in fields. Cats will not be able to catch mice as quickly as they multiply. The same can be said for terriers.

Figure 5. Cats may limit low-level mouse or rat

Sound and Ultrasound Devices

These two methods may not be effective. Rodents may be frightened by strange noises in the first few days but then quickly become used to them.