Gardens can provide a safe harbor for several types of rodent, giving shelter and readily-available food sources both growing around the garden and stored in sheds. They are unwanted in the garden because of the damage they can cause to fruit, vegetables, seeds, bulbs, plants and containers, and also because they expose people and pets to various diseases and parasites.
The only rat species you are likely to find in your garden is the brown or Norway rat (Rattus norvegicus). The black rat is now rare and is mostly found near ports.
There are several mouse and vole species native throughout North America, and a small number of these can take advantage of food and lodging opportunities provided in gardens. However, their numbers are rarely high enough to cause much damage in the garden.
The house mouse (Mus musculus), field vole or short-tailed vole (Microtus agrestis), and deer mouse (Peromyscus maniculatus). These are the species most likely to be seen in gardens.
In America, wild populations of the house mouse and field vole can periodically explode to plague proportions in rural areas after mild winters when they have little die-off.
These rural species are normally found in specific habitats:
- house mouse: wooded areas, fields, croplands, yards ;
- deer mouse: old fence posts, tree hallows, log piles ;
- field vole: rough grassland;
Look for nesting areas under trash, timber, in drain pipes, under and in sheds. Rodents have a characteristic smell when in large numbers and you may also hear their activity. They are usually nocturnal feeders, so you will not normally see them in the daytime. If you do see them it usually means they are short of food and getting desperate.
- Holes or burrows: burrows are 2 inches diameter and can be located anywhere that is relatively undisturbed and near to food.
- Runs and tracks: runs are typically 2-4 inches wide near cover along walls, banks and hedges and through vegetation. Rats memorize pathways and use the same routes to and from their shelter. Smear marks may be visible where they run along stone, wood or metal, such as on steps, fencing and gate posts. They tend to travel along the ground, but they can climb and jump to get to food and shelter.
- Droppings: these are a half inch long, cylindrical, flat at one end and often pointed at the other. They are moist when fresh, but dry within hours.
- Damage: rats and mice will gnaw at food, packaging and barriers in their way, making holes or enlarging existing ones. They also gnaw objects when investigating them. Their teeth are hard enough to get through many hard materials such as wood, rubber, vinyl and low grade concrete and cement. Outdoors this can be fencing panels, sheds where food is stored, compost bins and electrical wiring in sheds, such as on power tools.
- They construct a system of tunnels to live in, which can have several chambers and exits. Voles can make a system of shallow tunnels that give a soft and uneven surface to lawns and soils.
- Small gnaw marks can be seen on fruits such as growing strawberries and stored apples and seeds. Small fruit, such as berries may be left scattered on the ground under the plants.
- Torn paper in garden sheds shows mice are gathering nesting material.
Rodents can cause a range of damage in your garden which can range anywhere from feasting on fruits and vegetables to gnawing on shed doors and wires. The types of damage caused vary depending on the species of rodent.
- Rats eat a wide range of garden vegetables, including sweet corn cobs, pumpkins and squash and various root vegetables, such as carrot, parsnip, beet root and potato tubers. They will eat the crops while they are growing and in storage. They also eat fruits in storage such as apples, and seeds.
- Rats will also eat feed put out for wild birds, poultry and pets, so feed needs to be placed on bird tables or in hanging feeders, not on the ground.
- Damaged containers and packaging materials in which foods, animal feed and seeds are stored can attract rats.
- Rats can cause structural damage to buildings by burrowing and gnawing, undermining building foundations, paving in patios and paths, causing settling, and damage earth banks.
- Rats will gnaw on electrical wires or water pipes, above or below ground.
- Rats are opportunistic, so once they are in your garden they will seek new places to live, feed and breed, such as in your house, so it is important to control them as soon as possible. You may only see one or two, but there will be many more that you don’t see! They can damage buildings further by gnawing openings through doors, window sills, walls, ceilings, and floors.
Mice and voles feed on a wide range of plants, but do little other damage in the garden:
- Eat recently sown vegetable seeds such as peas, beans and sweet corn and the foliage of seedlings.
- Eat bulbs and corms, especially recently sown ones. Tell-tale signs are holes in the soil where they have dug down to feed on them.
- Eat fruits such as strawberries, even before they are ripe and berries.
- Eat stored fruit such as apples.
- Voles can eat the bark of woody plants, especially in winter.
- Voles make a network of shallow tunnels which can give lawns an uneven surface.
Rats, mice and voles can carry a wide range of diseases caused by bacteria, viruses, protozoa and helminths (worms), including Salmonella, Leptospirosis, Weil’s disease, Listeria, Cryptosporidium and rat bite fever. They can infect both humans and pets. They can also carry ectoparasites such as ticks, mites, fleas and lice that carry another set of diseases, and can pass them on to humans and pets.
During the 2011 vole plague in Spain it was reported that 20 people caught the highly infectious bacterial disease Tularemia after coming into contact with the voles — a tiny amount of bacteria can cause this disease but it is not passed from person to person.
In the garden, contamination may not be as obvious as in the home, but there are several means of disease transmission, which include:
- direct contact with excreta (urine, feces, saliva) or inhalation of dust particles;
- handling or inhaling dust particles containing infectious microorganisms aerosolized when disturbing compost heaps, woodpiles, or other material contaminated with dried rodent urine;
- handling of infected rodents, alive or dead;
- scratches or bites from rodents;
- dogs, cats and foxes eating rodents and then catching parasites that can be passed on to humans.
There are a large number of rodent control products available for the home user, all of which need a degree of skill and care to use effectively and safely. It is illegal in many countries to use products that are not designed for the ‘target’ animal or to use methods that are considered inhumane or unsafe for other people, pets, wild animals and the environment. You should read the description and instructions on product packaging carefully.
House mice, field mice and voles rarely build up large enough populations in gardens to become serious pests, so it may not be necessary to kill them.
Rats, mice and voles need food to live on and a place to live in; therefore you can take steps to prevent them having both and reduce the chance of them invading your garden.
- Eliminate any harborage points around buildings and sheds. Seal any small gaps that allow them access. Rats need only a gap height of around half of an inch to gain entry and mice a quarter of an inch, though normally mice access holes are three quarters of an inch in diameter.
- Remove potential nesting places by keeping gardens clean and tidy. Remove piles of wood, garden clippings etc, and cut back overgrown areas.
- Cover any household food waste such as in compost heaps and garbage bins. Make sure lids are closed and garbage bags containing food are not left outside for long periods.
- Do not scatter bird feed on the ground: use a bird table or feeder basket to feed birds.
When choosing to take control measures in the home, safety is a high priority:
- All methods need care to ensure that non-target wildlife, children, other family members, pets and other people are not harmed by product use and storage.
- Appropriate protective clothing (e.g. rubber gloves, dust mask) should always be used when handling contaminated material, droppings, poison, dead or live animals, or traps contaminated with animal fluids or parts, to prevent spread of disease.
- Dead animals need to be disposed of safely by burying them deeply in the ground (to prevent them being dug up by pets or wild animals) or by placing in a sealed plastic bag and then in waste disposal. You should always check your own national and local regulations.
Rats and mice take time to get used to anything new in their environment and may avoid measures taken to control them for a while, so you may need to be patient for measures to take effect.
- Spring/snap traps: These traps are designed to kill rodents instantly with a spring release mechanism, triggered when the rodent steps on it or takes some food bait. They need to be used with some skill to position them and avoid affecting other wildlife, pets and children, especially when used outdoors.
- Electrocution traps: These discharge a high voltage shock when an animal walks on metal plates inside the containment box. They are expensive and not all can be used outdoors. They also need to be checked frequently to check for and dispose of any dead animals.
- Live capture traps: Usually a small cage with a trigger mechanism to shut the animal in when it pulls on some bait. These also need to be checked frequently to release any non-target animals caught. In some countries it is illegal to relocate and release rodent pests; therefore they must be dispatched humanely.
Ultrasound devices emit sound at frequencies beyond the range of human hearing, but audible to rats and mice. Therefore it is claimed that high volume ultrasound can repel them without affecting humans. Ultrasound, however, dissipates quickly with distance and is blocked by objects, creating shadows.
As with any unfamiliar sound or object, rats and mice are easily frightened and may show an aversion at first, but then become used to it. Ultrasound devices will not get rid of a rodent infestation and there is limited scientific evidence that they work as repellents, but some users give the devices favorable reviews.
The safe and secure option is to call a professional, who will have the right training and the best equipment to assess and deal with your infestation.
House Mouse, Habits, NPMA Field Guide to Structural Pests. page 11.10.3
Deer Mouse, Habits, NPMA Field Guide to Structural Pests. page 11.9.2
Royal Horticultural Society, Advice pages (link).
Ronert M Timm. Norway rats. The Handbook: Prevention and Control of Wildlife Damage. Paper 5. University of Nebraska — Lincoln (link).
European Biocidal Products Forum. Sustainable use of rodenticides as biocides in the EU. Cefic — EBPF, Brussels.
Rats. Integrated Pest Management Program, University of California Division of Agriculture and Natural Resources (link).
The Independent. Farmers protest after plague of voles destroys crops in Spain (link).
The Times. Ja-vole: German farms suffer plague of rodents (link).