Rodent species of post-harvest importance

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There are more than 4000 species of mammals, of which about 1700 are rodents (Anderson and Knox Jones 1967). Of the rodents the Family MURIDAE contains the most species, and of the genera the genus Rattus. However not all the 1700 rodent species are pests. About 150 species have been defined as a pest at some locality to some crop at some time or another, but only 20 could be termed important (Fall 1980). Very few species indeed are regularly described as pests in the literature. In connection with post-harvest losses, the number of species occurring in and around human habitation, drops to below ten.

Of these, three species are found throughout the world: the house mouse (Mus musculus), the house or roof rat (Rattus rattus) and the brown rat (R. norvegicus). The multimmate rat (Praomys natalensis) and the spiny mouse (Acomys cahirinus) are found in Africa; while the Pacific rat (R. exulans), the bandicoot rat (Bandicota bengalensis) and occasionally the striped squirrel (Funambulus pennant)) (Posamentier 1989) occur in Asia. Other species may enter buildings occasionally, but are of local importance only.

The brown rat, Rattus norvegicus (also known as grey, house, sewer, Norway, or wharf rat)

This species is cosmopolitan, but thought to have originated in Asia. It has spread gradually around the entire world during the last two centuries through international trade and human settlement (Meehan 1984). Its range is limited to coastal areas especially ports. In many Asian countries it is displaced by B. bengalensis (Deoras and Pradhan 1975), and it is probable that populations of R. norvegicus in these areas are replenished only by new arrivals from outside.

Many colour variations occur. In general it is brown-gray dorsally and light-gray ventrally, the tail is bi-coloured, and the feet are white. The head+body length is 180-250 mm and a fully grown adult may weigh up to 400 grams, although heavier individuals have been recorded (Niethammer 1981). The tail is shorter than the head+body length. The ears are thick, opaque, and short with fine hairs, while the snout (front of face) is characteristically blunt.

It is the most important species in Europe, because it lives in close proximity to man and has often been responsible for passing diseases on to man. Living in close social groups, it may be rated as the major post-harvest rodent pest in Europe.

The ship rat, Rattus rattus (also known as black, roof, fruit, rice field, or Alexandrine rat)

Also cosmopolitan and spread through international trade, this species originated in South East Asia (Meehan 1984). However, unlike R. norvegicus, the ship rat commonly lives well inland and has penetrated deep into continents.

There are many subspecies and forms of R. rattus and, because of this, it is difficult to give a definitive description of it. In the same country the coloration may range from almost black to red brown dorsally and dark grey to white ventrally. The head+body length is 150220 mm and the fully grown adult weight is 150-250 grams (Niethammer 1981). The tail is longer than the head+body length. The ears are thin, translucent, relatively large and hairless, while the snout is comparatively pointed.

Although it has become fairly rare in Central and Northern Europe and Asia, R. rattus has become a field pest in many countries and, because of its good climbing ability, infests fruit orchards besides entering buildings. This species was responsible for carrying the fleas which spread the plague in the Middle Ages. For more detailed information on the biology of one of the subspecies, R. rattus mindanensis, see Sumangil (1990).

The Pacific rat, Rattus exulans (also known as the Polynesian rat)

This is a relatively small species. It is coloured gray-brown dorsally and light grey ventrally. The head+body length is 110-130 mm, and a fully grown adult may weigh up to 45 grams. The tail is longer than the head+body length (Niethammer 1981).

It is common throughout the Pacific islands ranging westward to western Bangladesh (Poché 1980), and is found in agricultural fields and in villages. Due to its excellent climbing ability it is a common pest of coconut trees.

The house mouse, Mus musculus

This species is also cosmopolitan, having apparently originated in the steppes of Central Asia on the Iranian-Russian border (Schwartz and Schwartz 1943). It is now the most widespread mammal in the world (Meehan 1984).

There are many subspecies and colour variations are extreme: the fur dorsally is usually brown to brownish grey (although black and other colours occur), and grey ventrally. The head+body length is 70-110 mm, and a fully grown adult weighs 15-30 am. The tail is about as long as the head+body length. The ears are quite large in relation to the rest of the body, while the feet are comparatively small and the snout pointed.

The house mouse is a good climber and lives in social groups. It can be a serious pest in agricultural fields and buildings, but has also been recorded in native or natural vegetation.

The Egyptian spiny mouse, Acomys cahirinus

This species ranges from Mauritania to Pakistan and is usually found in semi-desert, rocky country, dry woodland, thorn scrub and savannah (Greaves 1989). However it has become commensal in some places, replacing M. musculus, causing damage to stored grain and domestic premises.

The commensal form is nearly black with a grey belly. The head+body length is 60120 mm, and the tail much shorter than this. Hairs on the back are stiff, and are the distinguishing feature of this species. While litter size may be small (2-5), a female may have up to 12 litters in one year.

Apparently this species has an unusual resistance to anticoagulant rodenticides. This means that control has to rely on strict sanitation practices and the use of acute rodenticides such as zinc phosphide.

The multunammate rat, Praomys (Mastomys) natalensis

This species is economically the most important rodent pest in Africa, and a true indigenous commensal (Fiedler 1988). In many areas it may be replaced by the much larger R. rattus.

The fur is soft, brownish on the back and greyish underneath. The head+body length is up to 150 mm, and the fully grown adult weight is 50-100 g. The tail, which is uniformly dark, is about the same length as the head+body.

Most distinctively, the female has up to 24 nipples on her belly (other rat species rarely have more than 10) and the reproductive potential is high, particularly since this species lives in large social groups. Consequently, very large population explosions occur from time to time, causing huge losses.

The lesser bandicoot rat, Bandicota bengalensis

This is a common species in Asia, which ranges from Pakistan eastwards and, according to some reports, has reached Indonesia. Otherwise it does not seem to have left the mainland continent of Asia. It seems to be replacing R. rattus and R. norvegicus in India (Prakash 1975) and probably other Asian mainland countries also.

The fur is dark or (rarely) pale brown dorsally, occasionally blackish, and light to dark grey ventrally. The head+body length is around 250 mm, and the uniformly dark tail is shorter than the head+body length.

In addition to consuming or spoiling much stored produce, the lesser bandicoot rat is a very active burrower and is responsible for much structural damage to the storage buildings as well. It is also a very good swimmer able to live in deep water rice fields, where it can cause much damage to the crop. In Bangladesh and Myanmar (India) it is the most important rodent pest in both urban and rural areas. It is certainly also important in other Asian countries.

B. bengalensis is very aggressive even against individuals of the same species (Posamentier 1989); consequently, the large burrow systems made by these rats are normally occupied by only one adult each.

This species is very susceptible to most rodenticides (Brooks et al 1980, Poché et al 1979), although findings by Greaves (1985) indicate that some individuals seem very tolerant to some rodenticides. However in practical field trials in Bangladesh no problems were encountered with zinc phosphide or coumatetralyl (Posamentier 1989).


Notes on the Biology, Behaviour, and Habits of Rodent Pests relevant to their control.

(i) Reproduction

Although most rodents live for only about one year they are prolific breeders, multiplying rapidly under most favourable conditions. A female rat may have up to to five litters in her lifetime, R. norvegicus and R. rattus averaging 7 or 8 young in each litter. The multimammate rat can have up to 20 young in a litter, the average being about 11. A female bandicoot rat may share a burrow with a weaned litter, have a litter suckling and be pregnant all at the same time. The house mouse can have a new litter every four weeks (Meehan 1984).

(ii) Senses

Rodents have well developed senses of smell and touch, but poorly developed eyesight. They have excellent light sensitivity but poor acuity and are colour blind (Meehan 1984). This allows poison baits to be coloured, for safety reasons, without modifying their acceptability by the target species (assuming that the colouring agent does not have an adverse taste or odour).

They possess a good sense of hearing including frequencies in the ultrasonic range up to 100 Khz. This has led to the development of ultrasonic deterrent devices, of variable effectiveness.

(iii) Physical capabilities

The following facts need to be remembered when it is intended to rat-proof a building.

R. rattus R. exulans, and the house mice are very good climbers, R. norvegicus less so and the bandicoot rats almost not at all. However all are able to use very small openings for their size or move up cracks and pipes to gain access to buildings. They are also good swimmers and readily take to the water. They are also good jumpers: R. norvegicus can jump vertically 77 cm and horizontally more than 120 cm, house mice can jump to a height of 24 cm (Meehan 1984).

The burrowing activity of rodents (especially the bandicoot rats, R. norvegicus and the multimammate rat) is a particular nuisance to store owners in tropical countries. Floors subside, easing the entry of other individuals, providing hiding places, causing a loss of stored produce and even leading to a partial collapse of buildings.

Rodents make burrows to breed in, for storing large amounts of food, and for protection against predation and extreme climatic conditions. In the case of bandicoot rats these burrow systems may be 100 cm deep and very extensive. Additional small 'escape burrows' are made by some species to minimise travelling to food and water.

R. rattus and house mice do not always make burrows but construct well hidden nests on the ground, in trees, or the upper parts of buildings.

Rodents derive their name from their gnawing behaviour (Latin: rodere = gnaw). Their incisor teeth grow continuously and need to be used, otherwise they will grow back into the cheek disabling proper feeding. The ability to gnaw through even soft metals is not only a nuisance but can also be hazardous, as mentioned in the opening paragraph of this Chapter.

(iv) Eating Habits

Rodent pest species are omnivorous, an additional reason why they are successful pests. In spite of this there may be some preferences in the field if a choice is available. Overall, rats and mice in the wild will take a balanced diet.

The quantity of food taken may also vary. Under laboratory conditions, rodents have been observed to consume about 10% of their body weight per day (shitty 1954, Meehan 1984, Spillet, 1968, Brooks et al 1981, Posamentier and Alam 1981). Enclosure studies indicate that under near field conditions the amount consumed or destroyed is about five times the amount eaten in the laboratory (Haque et al 1980), although the proportion actually consumed is uncertain. What is certain, however, is that the actual losses caused are a multiple of their dietary requirements.

Many workers have studied food preferences in attempts to find the 'perfect' bait. The results of these studies are very variable depending on genetics, learning ('food imprinting'), weather and other complicating factors. A review of the subject can be found in Meehan (1984) and Posamentier (1989). In terms of bait acceptance the most important variable is to lay the bait at a time when little food is available. In buildings this means making food as inaccessible as possible, which will be discussed later.

Most rats return to a fixed place of feeding. House mice on the other hand are haphazard, inquisitive feeders (Crowcroft 1966).

(v) Activity

Most activity takes place during the hours of darkness, which is also when they do most of their feeding. There are two peaks of activity, the major peak occurring just after sunset and a minor peak just before sunrise. This has been observed for M. musculus (Dewsbury 1980), R. rattus (Barrett et al 1975), R. norvegicus (Calhoun 1962) and B. bengalensis (Parrack 1966). When hungry, or under crowded conditions, they may also be active during daylight hours.

(vi) New Object Reaction and Bait Shyness

It is probably their ability to rapidly adapt their behaviour to new or changing situations, above all else, that has caused some rodent species to become major pests. This is most apparent in their reaction to 'new objects' placed in their environment by man.

R. norvegicus is naturally very suspicious and tends to avoid any object that is new to it. It may take several days before an individual will enter a trap or take bait. Even then, if the new object appears to be food, only a small amount is taken. If the food contains an acute poison causing symptoms after a short while, rats may not touch the bait again. This is commonly called bait shyness.

R. rattus behaves similarly but not to the same extent, while M. musculus tends to explore rather than avoid new objects.

The New Object Reaction wears off in time, but has to be taken into account when a rodent control programme is planned.

(vii) Movement

Many rodent pests are characteristically mobile and able to disperse rapidly. This allows them to move quickly into and take advantage of new areas with favourable conditions (Fiedler 1988, Meehan 1984). However once individuals have established a territory or home range, they will not move very far, as long as conditions remain favourable.

It is often believed that rodent pests invade areas from several kilometres away. This is not exactly true. If large numbers of rodents suddenly appear in an area, it is probably because environmental conditions have become favourable for them, and indigenous populations are able to increase at several places in the area at about the same time. This then gives the impression that the animals are on the move. It should be realised that for such a small ground living animal like a rat it is far too risky to move long distances because of predation and exhaustion.

Nevertheless it is known that bandicoot rats, and others, will move from surrounding fields into villages at harvest time, that is when fields suddenly no longer provide enough food (Posamentier 1989). In built up areas containing food stores B. bengalensis moves within an area 30 to 146 meters in diameter (Spillet 1968, Chakraborty 1975, Frantz 1984), depending on the location of the warehouses, when they are emptied, structural conditions and the availability of water.

Under experimental conditions and in certain environments R. norvegicus will move about three kilometres in one night (Meehan 1984). It is therefore not surprising that disinfested areas are quickly invaded by new animals from neighbouring areas or buildings. Increasing the area in which a rodent control programme is to be carried out will therefore help to reduce the rate of reinvasion.

(viii) Habitat

Rodents prefer buildings with good cover in surrounding areas; where vegetation reaches right up to the walls of the building, which ideally (for them) should have soft floors, broken brickwork and the like, and be untidy (Figure 9.1). Under such conditions control, particularly with rodenticides, is virtually impossible.



Animals compete for food and shelter. Such competition may be either inter- or intraspecific; that is between different species or among members of the same species.

For example it has been shown that B. bengalensis, M. musculus and R. rattus compete with one another for space; the bandicoot being the dominant species, and the house mouse the least competitive. In the context of control this means that if B. bengalensis is successfully eliminated from a store, R. rattus may move in. If R. rattus is then removed, M. musculus will move in. This situation needs to be considered when devising a control strategy, because the control techniques for these different species vary.

Several species live in loose or tight social groups (i.e. R. rattus and R. norvegicus) with a fairly fixed hierarchy. More dominant animals will have first access to food and shelter. In a control programme this means that the more dominant animals are removed first, because they are first to feed on the poisoned bait. The parts of the population lower in the hierarchy will be controlled with the second or subsequent bait applications. The technique of 'pulsed baiting' is based on this behaviour and is discussed later in this Chapter.


Indicators of the presence of Rodents

There are several ways by which rodents may signal their presence. The most easily noticed are damage and burrows. In stores footprints may be noted in dusty places and, of course, rodents will leave their droppings scattered about. Often the species can be identified by the size and shape of droppings.

Less obvious are the 'smears' found in places regularly visited by rats. They are caused by rats brushing their bodies against objects or when they slide around rafters and corners. Smears are indicators of heavy usage and infestation, and good places for laying down tracking powders'.

However these signs are normally apparent only after a substantial population has become well established, when the point in time for economical control has already passed. The acute observer should try to find the more subtle signs of rats just passing through to investigate the store. This can be done by searching for foot prints in fine sand or tracking powder placed at strategic points. Areas around and outside the store should also be checked frequently for the presence of rodents.

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