Rodents make up the most numerous group of mammals, with 2277 species, accounting for 41% of the known mammals.
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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:
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.
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.
Mice and voles feed on a wide range of plants, but do little other damage in the garden:
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:
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.
When choosing to take control measures in the home, safety is a high priority:
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.
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).