A pest may be defined as a creature which is undesirable in a particular situation, for reasons of health and hygiene, comfort and acceptability. The number of individual pests which renders them undesirable will depend on a particular situation.

Therefore, 20 or 30 cockroaches in a hotel boiler room may be acceptable, whereas a single insect in a hospital operating theatre clearly is not. A critical customer might object to one fly in a restaurant, we would all object to one fly in our bowl of soup. It is the location of pests rather than their number that is important.

The Problem.
Rodents which are usually found associated with people, are the brown rat (Rattus norvegicus), the black rat (Rattus rattus) and the house mouse (Mus musculus musculus and Mus musculus domesticus). It should be noted, however, that the black rat is no longer a pest in the UK, being confined to probably one dockside area only. They have been pests since time immemorial and have resisted all attempts to eradicate them. This is hardly surprising, given that they are highly adaptable, omnivorous creatures with tremendous reproductive potential, acute senses and a high degree of athleticism. Moreover, as recent studies such as the 2003 Rodent Survey in the UK demonstrate, their numbers are not diminishing.

The costs attributable to rodents in both financial terms and in human suffering are enormous and commensal rodent control is rightly given a high priority by local authorities, businesses and householders.

Description.
Rats and mice are found wherever there is food and shelter. The availability of such resources dictates population density. In ideal conditions, a pair of mice can produce more than 2000 offspring and a pair of rats 200 per year. Their gestation period is only 21 days and therefore, population explosions can occur when effective control is not implemented.

Life Cycle.
Rats, Rattus rattus and the black rat (also known as the ship rat or the roof rat) are grey to black in colour, weighing some 300gm and between 150 and 220mm long with a tail that measures 180-250mm. They have large prominent eyes and a pointed snout. The black rat is very agile and is often found living in the upper parts of buildings and in trees.

The black rat is omnivorous, eating around 30gm of food per day. Its diet consists mainly of cereals and fruits. Living for 9-12 months, it has an average of 6 litters, each producing 6-10 young. It has become rare in parts of northern Europe, where the changes in habitat have favoured the brown rat - a bigger and more aggressive species.

Rattus norvegicus, common rat or Norway rat has a larger body than the black rat - weighing some 500gm and measuring 200-250mm, the length of the tail being 150-200mm. It has small eyes, a rounded snout and is truly omnivorous, eating 50gm of food a day. Found living near water, in buildings and in sewers, it nests in burrows underground. Whilst not as agile as the black rat, the brown rat is nevertheless a good climber. It lives for 9-18 months and produces around 7 litters a year with 8-10 young in each.

Both species also need to drink daily, with a typical intake of about 20ml. This can be exploited during control measures.

Physical Abilities and Senses of Rodents.
A thorough understanding of the physical abilities of rodents is very useful when designing a control programme. For instance, rats are excellent swimmers. They can swim up to half a mile in open water, travel through sewer lines against substantial currents and tread water for up to three days. Climbing is also easy for them. Roof rats and house mice are excellent climbers. Norway rats, although somewhat less agile, can climb effectively. If they can't climb, they just jump. From a standing position rats can jump vertically up to three feet - and getting down is easy. If necessary, rodents can drop from heights of 50 feet without injury.

If rodents can't get around an object, they go through it. Rodents are capable of gnawing through a variety of materials including lead sheathing, cinderblock, aluminium siding, glass and improperly cured concrete. Rodents can also squeeze through very small openings - one inch (2.6cm) for rats and one inch (2.6cm) for mice.

All of these physical abilities have allowed rats and mice to survive hundreds of years in man's environment. The following are some of the other sensory abilities that make rodents so remarkably adaptable.

Hearing.
Rodents use hearing to locate objects to within a few inches. Rats and mice have a frequency range of 50 kilohertz or more, which is much higher than humans who have a range of about 20 kilohertz. Rodents make high frequency noises in various situations such as in mating, but the function of these sounds is poorly understood.

Vision.
Rats and mice have poor vision beyond three or four feet, but they are very sensitive to motion up to 30-50 feet away. For the most part, rodents are colour blind but very light-coloured or reflective objects may stand out in their environment and cause initial avoidance among sensitive rodents.

Taste.
Rodents have a highly developed sense of taste, which allows them to detect some chemicals at parts-per-million concentrations. This taste sensitivity may lead to bait rejection if the baits are contaminated with insecticide odours or other chemicals. Use of fresh, food-quality grain ingredients is the best guarantee of good bait attractability and acceptance.

Smell.
Odour is one of the rodent's most important senses. Rodents mark objects and pathways to and from food sources. Members of the opposite sex, who are ready to mate differentiate between members of their own colonies and strangers and to tell if a stranger is a strong or weak individual.

Touch.
Rodents have a highly developed sense of touch due to very sensitive body hairs and whiskers (vibrissae), which they use to explore their environment. Most of a rodent's movement in a familiar area relies heavily on the senses of touch and smell to direct it through time -tested movements learned by exploration and knowledge of its home range. Rodents prefer a stationary object on at least one side of them as they travel and thus commonly move along walls, a fact which is very useful when designing a control programme. In captivity, rodents will hide quite contentedly in a clear glass jar since it feels enclosed and secure to them.