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11.1 Characteristic features of rodents
11.2 Rodents as storage pests
11.3
Damage and losses
11.4
Biology of rodents
11.5 Signs of rodent infestation
11.6
Preventive measures
11.7
Control measures
11.8
Safety measures
11.9 First aid measures in case of poisoning
11.10
Equipment
11.11
Further literature
Rodents belong to the most important pests of stored produce. In a number of countries they cause as much if not more damage than insect pests. Rodents have an exceptional ability to adapt themselves to different environmental conditions and an incredible potential for reproduction One pair of rats can theoretically have 350 million offspring within the space of three years. Estimates state that over 3.5 million rats are being born daily.
11.1 Characteristic features of rodents
Rodents are characterised by their teeth. They have a pair of incisor teeth in the upper and lower jaws, separated from the molars by a large gap (diastema).
The incisors are curved inwards and have an extremely hard anterior coating The softer inside layer is worn down much more rapidly than the hard, outer layer. This means that the teeth are continually kept sharp, enabling them to damage even materials such as masonry and electric cables. l he incisors do not stop growing. This means that the animals are forced to gnaw steadily in order to wear them down.
Rats and mice (Muridae) are mainly causing damage to stored produce. Additionally some squirrels (Sciuridae) can also cause losses. The three most important rodent species are to be found all over the world:
There are also a number of species which are of great importance in specific regions:
Rats and mice cause losses in a number of ways:
- Feeding on stored produce
Rats eat an amount of food equivalent to 7% of their body weight daily, i.e. a rat with a body weight of 250 g will eat around 25 g daily, amounting to 6.5 kg of grain a year.
Mice eat a daily amount equivalent to around 15% of their body weight, i.e. a mouse weighing 25 g will eat between 3 and 4 g a day, amounting to 1.4 kg of grain a year.
It should, however, be borne in mind that the actual losses are much higher than the amount of produce eaten by the animals, as they contaminate the stored produce with urine, faeces, hair and pathogenic agents. As it is difficult or even impossible to remove filth produced by rodents from the stored produce, infested batches often have to be declared unfit for human consumption or written off as total losses.
There are around 50 diseases which can be transferred to man by rodents, including typhoid, paratyphoid, trichinosis, scabies, plague and haemorrhagic fevers like ebola. In addition, rodents may be vectors of a large number of diseases affecting domestic animals. The problems and costs resulting from these diseases are not normally taken into account when assessing infestation by rodents.
As rodents prefer food rich in proteins and vitamins and feed mainly on the embryo, they cause particular damage to the nutritional value and germination ability of seeds.
- Damage to material and equipment (e.g. tarpaulins, bags, pallets, sprayers) and to the store itself (cables, doors).
These often lead to subsequent damage:
11.4.1 Distinguishing Features
Correct identification of rodents in the store is of great importance for the success of control measures as differences in the behaviour of the individual species must be taken into account when selecting the correct strategy for treatment.
Distinguishing features of species are the size and shape of the body, the colour of the fur, the length of the head and body in relation to the tail, the size of the hind feet, the relative size of ears and eyes and the shape of the snout. The size and colour of the animals can vary greatly 50 that they arc not sufficient to provide an adequate means of identifications. It is of practical use to catch a number of animals using traps, for example, in order to be able to identify them clearly. In many countries plant protection services or institutes have departments specialized in rodents which can be consulted if necessary.
The characteristic distinguishing features of the most important species of rodents are listed in the following tables and illustration
Colour and general characteristics of stored product rodents
| Rodent species | Description |
| Rattus rattus | Back black or grey, ventral side lighter coloured; smooth fur. Long and thin tail, sparsely haired. Large eyes; thin, translucent and hairless ears; pointed snout 5 pairs of teats. |
| Rattus norvegicus | Back brown-grey, ventral side light grey; harsh and shaggy fur. Thick, bi-coloured tail. Small eyes; small and thick ears with tine hairs; blunt snout. 6 pairs of teats |
| Rattus exulans | Colour similar to R. norvegicus; tail uniformly dark. 4 pairs of teats. |
| Mus musculus | Colour variable. Large ears; small eyes; pointed snout. 5 pairs of teats. Back brown grey, ventral side grey or white, soft fur. |
| Mastomys natalensis | Large ears; small eyes; pointed snout. 7 to 12 pairs of teats. |
| Bandicota bengalensis | Back dark brown, ventral side grey. Feet and tail black Relatively small ears and eyes, 6 pairs of teats |
Measurements of rodents harmful to stored products
| Rodent species | Length of head and body (mm) | Length of tail (mm) | Length of hind foot (mm) | Length of ear (mm) weight (g) | Adult |
| R. rattus | 150-220 | 180- 240 | 32 - 40 | 22 - 27 | 150 - 250 |
| R. norvegicus | 180 - 250 | 150 - 220 | 35 - 45 | 17 - 23 | 150 - 400 |
| R. exulans | 110 - 130 | 120 - 150 | 22 - 26 | 15 - 17 | |
| M. musculus | 70 - 110 | 60 - 110 | 16 - 21 | 12 - 15 | 15 - 30 |
| M natalensis | 90 - 150 | 90 - 150 | 20 - 30 | 16 - 24 | 25 - 70 |
| B. bengalensis | - 250 | - 44 |
11.4.2 Reproduction
Rodents have an important reproduction ability. They reach full sexual maturity very soon after birth, have a number of litters a year and a large number of offspring per litter:
| Rodent species | Number of litters per year | Number of offspring per litter | Gestation period (days) |
| R. rattus | 6 - 8 | 4 - 12 | 21 - 23 |
| R norvegicus | 3 - 7 | 6 - 10 | 20 - 24 |
| R. exulans | 2 - 6 | 2 - 5 | 20 - 21 |
| N. musculus | 7 | 4 - 8 | 20 - 22 |
| M. natalensis | up to 12 | 9- 13 | 23 |
| B. bengalensis | up to 11 | 6 - 8 | 22 - 26 |
Mastomys natalensis has by far the highest reproduction rate of the six species listed here.
11.4.3 Senses
Rats and mice are animals which are most active at night or at dusk. Their vision is thus quite poor. They are very sensitive to light, but they cannot see clearly and are colour-blind.
Their poor vision is compensated for by their excellent senses of hearing, feeling, smell and taste. Rodents are able to sense ultrasound. Their sense of smell enables them to find food, identify other beings and recognize runs and territorial limits. Their excellent sense of taste makes them fastidious. This is an important fact to be borne in mind when selecting bait. Their whiskers serve as feelers.
11.4.4 Behaviour
Rats are exceptionally cautious and intelligent creatures. Changes in their environment, such as newly laid bait, are initially regarded with suspicion and only accepted hesitatingly after several days of becoming accustomed to their presence. This is known as "new object reaction". it has important consequences for control measures.
11.4.4.1 The Black Rat (Rattus rattus)
The Black rat lives in loose colonies and Usually in the roof area of stores, where it generally builds its nest. Therefore it is often called "Roof rat". it is an excellent climber, can jump heights of up to 1 m and squeeze through an opening of only 12 mm in diameter. Black rats rarely use established runs as Norway rats do.
11.4.4.2 The Norway Rat (Rattus norvegicus)
The Norway rat lives in colonies or groups outdoors, only entering stores for food. It lives in burrows which it digs near stores or beneath foundations. Norway rats are good runners and swimmers, but cannot climb as well as Black rats. They can jump about 60 cm and squeeze through openings only 12 mm wide. They tend to use established runs and normally return to a food source once they have accepted it.
11.4.4.3 The House Mouse (Mus musculus)
The House mouse lives in fixed families, primarily in buildings or stores, and its preferred source of food is grain. It can survive for long periods without water, being able to make use of the grain moisture and water produced by its own metabolism.
Its radius of activity is very small, not exceeding a space of 10 x 10 m. It is capable of spending its entire life within the confines of a single pile of stacks.
This makes its detection particularly difficult. House mice are good runners and climbers, and can jump heights of up to 30 cm. Their small size enables them to squeeze through openings only 6 mm wide. House mice are very curious animals; the paths they follow are irregular, nibbling here and there, and they do not regularly return to specific food sources.
11.4.4.4 The Multi-mammate Rat (Mastomys natalensis)
These animals live in colonies in burrows outdoors, but enter stores to obtain food. They live largely from plant substances, but do also eat insects and meat. They are excellent climbers, jumpers and swimmers. Due to the small size of these animals, they are referred to in many countries as mice.
11.4.4.5 The Bandicoot Rat (Bandicota bengalensis)
The Bandicoot rats often live on their own outdoors where they dig elaborate burrows. The entrances are marked by noticeable piles of earth. The burrows comprise a number of chambers, where the rats store up to 10 kg of grain. They use established runs, which may well be sprinkled with odd grains they have dropped in transporting food. They are excellent swimmers.
11.4.4.6 The Pacific Rat (Rattus exulans)
These animals live outdoors, building their nests above ground in branches, bushes or niches in rocks. They feed largely on plant substances. They are very active and are good runners, jumpers and climbers. Occasionally they enter houses and stores.
11.5 Signs of rodent infestation
When signs of rodent infestation are seen, it is absolutely necessary to conduct a thorough investigation of the store, its immediate surrounding area and possibly even neighbouring land. In order to be able to perform measures to effectively control rodents, it is necessary to know what rodent species you are dealing with, where the nests are, how they enter the store and which runs they take.
There are a large number of clear signs of rodent infestation:
- Live animals
Rodents are mainly active at night. If animals are nonetheless seen during the daytime, this is a sign of an already advanced stage of infestation.
- Droppings
The shape, size and appearance of droppings can provide information as to the species of rodent and the degree of infestation.
The droppings of Norway rats are around 20 mm in length and are found along their runs.
The droppings of Black rats are around 15 mm long and are shaped like a banana.
Mouse droppings are between 3 and 8 mm in length and irregular in shape.
Droppings are soft and shiny when fresh, becoming crumbly and matt black or grey in colour after 2 - 3 days.
- Runs and tracks
Runs, such as those of Norway rats, are to be found along the foot of walls, fences or across rubble. They virtually never cross open areas of land, but always pass through overgrown territory, often being concealed by long grass.
Runs inside buildings can be recognized by the fact that they are free of dust. The animal's fur coming into contact with the wall leaves dark, greasy stains. Even Black rats, which do not have any fixed runs, can leave similar greasy stains at points which they pass regularly, e.g. when climbing over roof beams.
- Footprints and tail marks
Rats and mice leave footprints and tail marks in the dust. If you suspect there might be rodent infestation, scatter some sort of powder (talcum powder, flour) on the door at several places in the store and later check for traces. The size of the back feet serves as an indication of the species of rodent:
- Tell-tale damage
Rats leave relatively large fragments of grain they have nibbled at (gnaw marks). They generally only eat the embryo of maize. Sharp and small leftovers are typical for mice.
Rodent attack can further be detected by damaged sacks where grain is spilled and scattered. Small heaps of grain beneath bag stacks are a clear sign. These should be checked for using a torch on regular controls.
Attention should be paid to damaged doors, cables and other material.
- Burrows and nests
Depending on their habits, rodents either build nests inside the store in corners as well as in the roof area or in burrows outside the store. Rat holes have a diameter of between 6 and 8 cm, whereas mice holes are around 2 cm in diameter. These holes can be found particularly in overgrown areas or close to the foundations of a store.
- Urine
Urine traces are fluorescent in ultraviolet light. Where available, ultraviolet lamps can be used to look for traces of urine.
The most essential factors for the occurrence of rodents are:
Good store management and preventive measures taken as part of an integrated control programme can help to deal with these factors.
The information given in sections 5.2.3.2 (Activities to Prevent Losses in Storage), 5.2.4 (Storage Techniques) and 5.1.1 (Constructional Features of Stores) applies here.
11.6.1 Storage Hygiene and Technical Measures
11.6.2 Keeping Rodents Out
The requirements of preventive rodent control must be taken into account whenever new stores are being built. Particular attention should be paid to doors, ventilation openings, brickwork and the junctions between the roof and the walls. Repair any damage to the store immediately! This applies especially to the doors.
