Rats have been estimated to damage more than 1% of the world cereal crops and, in developing countries, estimates of 3–5% have commonly been reported. There are around 50 diseases which can be transferred to humans by rodents, including typhoid, paratyphoid, and scabies. In addition, rodents may be vectors of a large number of diseases affecting domestic animals. 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.
The three most important rodent species are:
- Black rat or House rat (Rattus rattus)
- Norway rat or Common rat (Rattus norvegicus)
- House mouse (Mus musculus)
There are also a number of species that are of great importance in specific regions:
- Multi-mammate rat (Mastomys natalensis) in Africa and the Middle East;
- Bandicoot rat (Bandicota bengalensis) in Southern and South East Asia;
- Pacific rat (Rattus exulans) in South East Asia, also occurring in Southern Asia
Their teeth characterize rodents. They have a pair of incisor teeth in the upper and lower jaws. 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. The incisors do not stop growing. This means that the animals are forced to gnaw steadily in order to wear them down.
Damage caused by rodents
Rats and mice cause losses in a number of ways:
1. Feeding on stored produce
Rats consume about 25 gm of food per day and mice eat approximately 3–4 gm per day. Besides eating stored produce, rodents contaminate a lot of the stored produce with urine, feces, hair and pathogenic agents. As it is extremely difficult to remove contamination, infested batches often have to be declared unfit for human consumption.
2. Damage to material and equipment
(e.g. tarpaulins, bags, pallets, sprayers) and to the store itself (cables, doors).
These often lead to subsequent damage:
- Produce leaking out of damaged bags or storage containers
- Bag stacks collapsing due to damage to the lower layers
- Short circuits leading to sparks or fire from cables being chewed
- Silos and warehouses may subside or even collapse as a result of being undermined
- Drainage canals around a store may be damaged.
Signs of rodent infestation
When there are signs of rodent infestation, it is necessary to conduct a thorough investigation of the store, its immediate surrounding area and neighboring land. There are a large number of clear signs of rodent infestation:
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.
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 20mm in length and are found along their runs. The droppings of Black rats are around 15mm long and are shaped like a banana. Mouse droppings are between 3 and 8mm in length and irregular in shape. Droppings are soft and shiny when fresh, becoming crumbly and matt black or gray in color 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 or 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:
Back feet larger than 30mm: Black rat, Norway rat, Bandicoot rat.
Back feet smaller than 30mm: House mouse, Multi-mammate rat, and Pacific rat.
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. Damaged sacks where grain is spilled and scattered can be a further sign of rodent attack. Small heaps of grain beneath bag stacks is 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 8cm, whereas mice holes are around 2cm in diameter. These holes can be found particularly in overgrown areas or close to the foundations of a store.
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:
- Sufficient supplies of food
- Protected places in which to build burrows and nests
- Hiding places
- Access to produce
Good store management and preventive measures taken as part of an integrated control program can help to deal with these factors.
Storage Hygiene and Technical Measures
- Keep the store absolutely clean! Remove any spilt grain immediately as it attracts rodents!
- Store bags in tidy stacks set up on pallets, ensuring that there is a space of 1 meter all round the stack!
- Store any empty or old bags and fumigation sheets on pallets, and if possible in separate stores!
- Keep the store free of rubbish in order not to provide the animals with any places to hide or nest! Bum or bury it!
- Keep the area surrounding the store free of tall weeds so as not to give the animals any cover! They have an aversion to crossing open spaces.
- Keep the area in the vicinity of the store free of any stagnant water and ensure that rainwater is drained away, as it can be used as source of drinking water.
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.