Rodents - least toxic control

Rodents can spread disease, cause structural damage and contaminate food. Their control is important and this article describes their biology and habits so that control strategies can be put in place to reduce their numbers without the use of pesticides.

Rats, mice and squirrels are mammals with continually growing incisor teeth which are kept at the right length by gnawing on almost anything from woodwork to water pipes and electric cables. Rats are among the most despised pests; a fear which is fuelled by the media and film industry. Individuals who spot a rat almost automatically call out a pest control operator who will inevitably use pesticides.

Biology
The main rodent species in the UK is the common or brown rat (Rattus norvegicus), believed to have been imported into this country in the 1720s from the East on Norwegian timber ships(1). The brown rat, sometimes weighing over half a kilo has a blunt nose, small ears and can reach up to 40cm long with a further 19cm on the tail. Generally brown rats stay at ground level and in the sewers in urban areas and live in burrows and hedgerows in the countryside. The litter size varies from eight to 12 and they can produce around seven litters a year.
    Black rats (Rattus rattus), rare in the UK but still found in port areas, carried the rat flea responsible for the Black Death. Black rats are distinguishable from brown rats because their tails are longer than the length of their head and body, they are generally smaller (up to 38 cm and usually weigh less than 225g), with pointed noses. They tend to nest in trees, dense vegetation, walls or attics, and often enter the upper floors of buildings.
    The house mouse (Mus domesticus) is coloured light brown to grey, reaches 9cm in length and weighs about 15g, its large ears and pointed nose distinguish it from a young rat. It nests in and around stored material, inside wall cavities, and produces up to 10 litters per year each averaging six to seven pups.
    The grey squirrel (Sciurus carolinensis), imported from eastern North America sometime in the 19th century, usually has two litters each year with an average of three young per litter and nests in dreys, holes in old trees, and attics(2). Squirrels are good climbers and will make use of nearby trees and ivy growing up the exterior walls to gain access into the home.

Non-toxic control
The following control methods relate to the common brown rat as they tend to be most prolific in UK urban areas, however these controls will also help to reduce other rodent species.
    The first step is to locate where the rats are living and feeding, how many there are and what kind of damage they are causing. Look for activity behind and under cupboards, search in attics and basements and in surrounding areas. Evidence of rat activity are droppings, urine, tracks (footprint and tail), smudge marks, gnawing marks, noises in wall cavities and runs or burrows. Recording evidence of activity will help to build up a picture of where harbourages are, feeding areas and which pathways are used most frequently (rats tend to memorise pathways and use the same routes).
    The key to integrated pest management of rats is an understanding of the capacity of a building to sustain a rat population, the 'carrying capacity'. If chemicals are used to rid a building of rats without any physical controls then the capacity of the building stays the same and nothing prevents another rat population from moving in. Rat infestations can be found virtually anywhere and favour areas where there are plenty of food sources and hiding places so action needs to be taken at the community level as well as by the individual.

Food storage
To discourage infestation, food should be stored in glass or metal containers, with any fresh fruit and vegetables, or softly packaged food kept in the refrigerator. Dry pet food, grass seed and similar materials commonly stored in sheds along with garden waste such as fallen fruit should be kept in rodent proof places. Household organic debris should be placed in rubbish or compost bins, ideally equipped with spring fasteners to keep lids on and rats out. In blocks of flats or commercial buildings that use larger bins and compactors etc. the same rules apply. Lids must be closed at all times and any rubbish that falls to the ground should be picked up to reduce the volume of accessible food for rats at dumps and landfills.

Ratproofing
Rats can fit through a half inch gap, climb the inside and outside of vertical pipes, crawl horizontally on anything and are very good swimmers. Therefore rat proofing buildings is the next step and combined with good hygiene practices should discourage infestations taking place. Any obvious harbourages like gaps under sheds, loose piles of rubbish and building materials or overgrown weed beds, should be eliminated. In landscaped areas, pathways and stretches of lawn deter rats as they dislike moving across exposed areas.
    Rodents can access buildings through holes in the foundations and walls, cracks between doors and frames, or walls and floor joists,  where pipes pass through exterior walls, by travelling inside pipes and drains. Solutions include filling holes with mortar, fixing metal collars around pipes, using metal sheet guards and covering areas that need to be open with galvanised mesh.

Trapping
Mechanical traps are the preferred method of eliminating existing rat populations. It is best to set traps before rat proofing begins as rats can get trapped inside rat proofed buildings and rot, or they will move onto adjacent properties (rats poisoned by chemicals often die and decompose in inaccessible places and can cause unpleasant smells and other pest problems).
    Traps should be placed near areas highlighted by monitoring programmes. It is best to position the traps baited but unset for two to three days as rats are very suspicious of changes in their environment. Good baits include fruits like pear, apple and tomato and meats such as sausage, bacon or liver. Care must be taken with pets and children around, and in such cases traps should be covered and inaccessible to all but the rodent.
    Smaller and younger rats that avoid spring-loaded traps can be caught using glue boards. Live rats that get stuck on these boards (and rats that have not already died caught on spring-loaded traps) should be disposed of humanely, this may be achieved by quick and positive dislocation of the neck.  Glue boards as a technique, should be left to the professionals who follow a code of practice for their use, which covers:

• Positioning boards so that non target animals are not at risk

• A schedule of frequent checks on laid boards - twice a day

• The humane despatch of trapped rodents

• The removal of rodent boards when the treatment has been completed or when there is any doubt as to whether the boards can be checked according to the plan(3).

Disposal of rats should be by incineration or by wrapping in a plastic bag and putting in the rubbish bin. Take care when handling traps and always wear gloves as rats will avoid traps with human scents on them, also they are vectors of disease. After use, all traps should be thoroughly cleaned with detergent.  
    Two other methods of rat control to be considered are ultrasonic devices and cats. Cats have performed the role of pest control officer for centuries and while they cannot guarantee a property to be rodent free they certainly act as a deterrent. Unfortunately the same cannot be said for ultrasound products which have no scientific research to show they reduce or eliminate rodent populations.

Chemical control
The control of rodent pests is essential in many environments, indeed it is a legal requirement for land owners and local authorities. The main chemical weapons in the battle against rodents are anticoagulant rodenticides, which inhibit the vitamin-K cycle, prevent blood clotting and cause death from internal bleeding. Warfarin, developed in the 1950s, is probably the best known anticoagulant, but second-generation compounds, such as difenacoum and bromadiolone, are now the most widely used. Usage of anticoagulants is most common in and around buildings, and certain active ingredients, such as flocoumafen and brodifacoum, are only licensed for indoor use. These rodenticides are usually incorporated in a bait of attractive food. Care needs to be taken not to place these chemical baits in areas where children, pets or other non-target animals can gain access as these pesticides are designed to kill mammals. If accidental poisoning occurs, immediately give an emetic (an agent that causes vomiting), keep the victim warm and seek urgent medical advice(4).
    Farmers and gamekeepers also commonly use anticoagulants away from farm buildings, to protect feed hoppers and root crop clamps. When rodenticides are used away from buildings it becomes much harder to control their impact on non-target species. Other mammals, such as rabbits, mice and voles are just as likely to ingest the cereal based bait and these species form a major part of the diet of many predators. Anticoagulants have long biological half lives, so predators may be especially vulnerable as residues from each contaminated prey item accumulate in the liver. Other predators, such as barn owls and birds of prey are also commonly exposed to rodenticides.
    In general rodenticides are extremely hazardous to all mammals. Other than anticoagulent chemicals, the fumigants aluminium phosphide, zinc-phosphide and sodium cyanide are used in burrows or enclosed spaces and the narcotic alpha-chloralose is also used in some situations for rapid knockdown.

Conclusion
On the occasion when chemical baits may be needed, such as when large infestations occur or when there are health hazards associated with their presence they should be used in conjunction with the above physical control methods or a new population of rats may move in. The need for prevention is further emphasised as evidence emerges of resistance in rat populations to the second generation of anticoagulants(5). These pesticides largely replaced warfarin, resistance to which is already widespread. (By David Allen)

The main source for this article was:

Common-Sense Pest Control: Least-toxic solutions for your home, garden, pets and community, Olkowski, W., Daar, S. and Olkowski, H., Taunton Press, 1991, Rats: 626-637., Mice: 287-294.

1. The National Rodent Report and what it means, Pest Control News Ltd, 1994, 15pp. 
2. Getting tough with Squirrel Nutkin, Pest Control News, October 1995, p6.   
3. THE BPCA Code of Practice for Operational Procedures, Humane use of Rodent Boards, July 1997. 
4. An A-Z of pests in your home, BPCA, 1999, p36. 
5. Pest control, Farmers Weekly, 15 January 1999, p45.

[This article first appeared in Pesticides News No.46, December 1999, p20-21]