Control of rodent pests

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Principles of Control

Before the various techniques, methods or strategies of controlling or managing rodents are described the general principles involved need to be discussed. An understanding of these principles by all those involved will assist in devising specific control strategies for a given situation. It will also help when explaining the need for certain activities to the staff actually executing the control work.

In tropical countries rodents pose a continuous problem because of the climatic conditions, uninterrupted food supply and relatively open structures. Therefore the control of rodent pests should be approached as a management problem much more so than a simple and single poisoning action. For a control strategy to be effective staff responsible need to be trained and informed, their activities must be co-ordinated, responsibilities confirmed, inputs and equipment readily available and the entire action must be planned.

Control strategies should aim at preventing losses and thus require a pro-active rather than the more normal reactive approach (Colvin 1990) (Figure 9.2. The philosophy behind any management strategy should be the prevention of problems.). Once a large population of rodents has established itself in a store considerable losses, that cannot be retrieved, have already occurred and subsequent control action is expensive. It should stressed that information from different sources should be incorporated into a control or management strategy and not just the techniques.

There are many more techniques and methods of controlling rats than are described here. Those given here have been selected as being the most practical for use in tropical countries. Meehan (1984) provides a comprehensive description of techniques and a complete list of available rodenticides.

(i) Monitoring

An important element of any rodent programme is monitoring. Usually it means surveillance for the presence of rodents. However it should also mean looking for features in the environment which would encourage rodents to migrate into it. Monitoring should be organised formally and regularly; that is, specie c staff should be made responsible for it and report regularly, maybe once a week to a superior on the situation. The report should include the following aspects:

Control of a rodent infestation is rarely completely successful; but if it is, it is usually only for a very short period. Therefore there is a need for continuous monitoring even after a successful control campaign regardless of the techniques and bait used.

For more ideas on monitoring techniques see Kaulkeinen (1984).

(ii) Co-operation

If an area is made rat-free due to good management and/or effective control measures, rats from near-by areas will migrate into it. It is therefore more efficient if control campaigns are conducted in several adjacent areas simultaneously. In the case of a village all households should be motivated and organised to control rats at the same time. While control in one household will still benefit the owner, benefits increase as the number of participating neighbours increases.

In the case of stores, large and small, surrounding areas including other stores should also be disinfested. This means that all the store keepers or managers involved should coordinate and synchronise their rodent control activities for maximum effect.


Preventive Measures

The maxim: 'Prevention is better than Cure' is just as true for rodents as it is for other pests and diseases. Therefore the prime objective of any rodent control campaign should be to create environmental conditions which will discourage or prevent the pests from reentering an area after its rodent population has been removed by one means or another.

(i) Sanitation

Rodents require food and shelter. Therefore it is most important to reduce the availability of these two key factors, which should be central in devising any kind of strategy. In the case of buildings the most effective method of rodent prevention is the improvement of hygiene or sanitation in and around them. Primarily this means sweeping the store and keeping both it and the surrounding area neat and tidy, i.e. free from any objects such as empty containers, idle equipment or discarded building materials, which could provide cover or nesting places for rodents. It also means removing food scraps left over from feeding pets or domestic stock (i.e. poultry farms) at the end of the day's work.

Observations have shown over and again that these simple actions, even in the tropics, are the most effective preventative measures that can be taken.

In a tidy store any infestation will be noticed at a very early stage, making other control measures far more effective. With reduced access to food and no places to hide, rats will not become established, that is live and breed, inside a building. Regular disturbance is something rats and mice avoid.

Control procedures should take the life history and behaviour of species present into account (Colvin 1990). Rats avoid clear spaces. Therefore by keeping a strip of two or more metres around a building clear of vegetation will reduce the chance of rats entering the building.

This should be augmented by keeping a strip of about one metre on the inside from the wall totally clear and swept. Branches overhanging the building should be lopped off to prevent climbing species to enter from above.

The above suggestions are enough to eliminate serious problems with rats and mice in buildings, even in stores where large quantities of food items are stored. Rats feel uneasy if their 'paths' and 'markings' are removed or cleaned daily by sweeping. They will not feel secure enough to remain in a building and damage packaging in their search for food. If they do, the damage is minimal and immediately noticeable.

(ii) Proofing

Since it is not practical to remove all food from stores and households, it is necessary to restrict access by rats. This is accomplished by proofing buildings or keeping food in -rat proof containers.

When rodent-proofing a building only materials which they cannot gnaw through should be used. Also, it should be remembered that some rodent species are good climbers and jumpers, and most can squeeze through surprisingly small holes and cracks (young mice need no more than a 0.5 cm wide crack to gain access).

Hard metal strips should be fitted to the bottom edges of all wooden doors and their frames, and vulnerable windows should be protected with tight wire netting screens in hard metal frames. Steel rat guards fitted to drainpipes and other attachments to the building should be at least one metre above ground level. Door hinges and similar fittings should be so placed or protected that rats cannot use them for climbing.

Floors and walls should be kept in good repair. New holes dug by rats should be filled in immediately, with cement reinforced with pieces of crumpled chicken wire. If cement is not immediately available a temporary seal can be effected with tightly packed earth between the wire mesh. The important point is that repairs should be carried out as soon as the damage is noticed, which should be within a few hours of it being done if the building is inspected daily.

Although rats are active mainly after dark, they will move about during day as well when there is no human activity. Therefore doors of stores should stay tightly shut during the day as well, when the store is not in use.

If the building itself cannot be made rat proof, then foods and other valuables should be kept in earthenware containers or metal drums with good lids.

Jenson (1965) provides further detailed information on rodent proofing.

(iii) Natural Prevention (Predation)

Normally predation will not keep rats and mice at economic population levels. One exception is the keeping of cats. Cats do not directly control rats and mice by feeding on them. It is their presence, which keeps most rats and mice away. A survey conducted in a Myanmar village has clearly shown that households with cats had no rats while those without cats in the same village were visited by rats.

Examples where predation may have an effect on field rodents and its limitations are described by Prakash (1990) and Wood (1984).

While work done in Australia on controlling house mice with a nematode has shown promise (Singleton and Redhead 1990), there is no practical parasitic control method for rats and mice available at present.


Mechanical Control

Mechanical rodent control as a rule is not very practical. It is cumbersome, labour intensive, and often not very efficient. Mechanical techniques are more appropriate in households, and can be used if the owner has no access to poisons or is averse to their application

The method most commonly used in buildings is trapping. Often local traps are available and in some cultures people are very good at using them. They should be placed where rats move regularly. If placed along a wall, the trap should be perpendicular to it and the treadle with the bait should face the wall.

Sticky or glue traps are another way of catching rats and mice (Prakash 1990, Meehan 1984). They are boards made of wood, hard- or cardboard covered with very sticky material. There are different types of glue available and they should be checked for suitability (stickiness, and usability in humid or dusty conditions) before large quantities are ordered. The boards are placed in the same way as traps, and normally there is no need for bait to attract rats. These traps should be checked daily, but are not regarded as very 'humane'.

Flushing rodents out of their burrows, with smoke or by flooding them with water, can be very effective and suitable in some situations. Ultrasonic devices are mentioned regularly, particularly by manufacturers of these devices, as a good repellent of rats and mice in buildings. However there is no scientific evidence of their effectiveness. It appears that rats become habituated to the sound or stay in 'sound shadows'. The subject is discussed by Meehan (1984).


Chemical Control

In large stores, particularly if situated in the city, it may be necessary to complement hygienic practices with chemical control. Because acute poisons invariably cause bait shyness, especially if applied over longer periods, it is strongly recommended that only anticoagulant rodenticides are used in buildings. Therefore acute rodenticides will not be discussed here.

It should be remembered that rats living in and around buildings are particularly suspicious of new objects, such as bait, bait stations and traps. Therefore it may take some time before these are accepted by rats. For this reason it is important that once these objects are placed they are not touched or removed again. If the bait or trap has not been touched after, say, a week rats are probably not nearby and it should be moved to another location. However chemical control is only useful in connection with strict hygienic practices.

As a rule operators should be supplied only with ready-to-use rodenticide baits. Firstly, mixing can be dangerous to the operator. Secondly, a wrong concentration can lead to bait shyness if too high, or to sub-lethal dosing if too low. Normally ready-to-use baits do not increase costs substantially.

ALL rodenticides can also harm other animals including man. Therefore great caution should be observed at all times when they are used.

(i) Anticoagulant Rodenticides

Anticoagulant rodenticides interfere with the blood clotting mechanism of the body - the animal gradually dies because of loss of blood through external and internal wounds, that is haemorrhage. Very small internal wounds (breaking of small capillaries) are constantly caused by normal movements. The rat feels almost nothing, it simply feels more and more tired and eventually dies. Therefore bait shyness with anticoagulants is unusual even with higher concentrations of active ingredient.

Resistance to some anticoagulants has been observed in industrialised countries, where they have been used very extensively over long periods. So far, resistance has been of no serious consequence in tropical countries, particularly in view of the fact that new compounds (e.g. difenacoum, brodifacoum, bromadiolone) are now available in most countries. The antidote to anticoagulant rodenticides is Vitamin K.

Multiple Dose Anticoagulant Rodenticides

Multiple dose anticagulant rodenticides, such as coumachlor or coumatetralyl, need to be available at all times because rats have to feed on the bait several times over a period of up to seven days (depending on the species) before death is caused.

The poison bait should be placed inside bait-stations. To save resources each bait station should contain 50 to 100 gm of bait at all times. Bait availability should be checked each morning and bait taken by rats or which has become mouldy should be replaced. The quantity of bait used depends on the level of infestation and should be adapted to local conditions; and the number of bait stations necessary depends on the size of the building. As a rule of thumb a station should be placed every five to ten meters along the wall. Additional bait stations should be located in positions where rats are likely to enter a building, along obvious rat runways, or in places where rats may hide. The distance between bait-stations depends on the species involved: house mice, for example, normally have a smaller feeding range than rats. However an operator will quickly learn how and where to place bait-stations, if the situation is regularly monitored and the operator is not changed.

If no more bait is taken and it appears that no more rats are present, baiting can be discontinued. However surveillance should continue and baiting must be re-started at the first signs of rats.

Single Dose Anticoagulant Rodenticides

Single dose anticoagulant rodenticides, such as brodifacoum, flocoumafen or bromadiolone act in the same way as described above but rats normally have to feed on the poisoned bait once only (1.5-2 0 g for rats in the case of the most potent compounds such as brodifacoum). In some situations and for some species it appears that more than a single feed is necessary (see Meehan 1984). Overall though these anticoagulants are by far the most poisonous to rats and very useful to practical rodent control.

If loose bait is available the use of bait-stations is recommended. These prevent spillage and spoilage. If blocks are used they can be laid down at regular intervals and in places frequented by rats, but should not be accessible to other animals or children.

(iii) Pulsed Baiting

The technique of pulsed baiting was introduced with the new single-dose anticoagulants, such as bromadiolone and brodifacoum (Dubock 1979, 1984). This contrasts with saturation baiting, in which bait is available to rats continuously over long periods until the population has declined to near zero. Pulse baiting is not necessarily more effective, but it is certainly cheaper, because the amount of labour and the quantity of bait required is much lower than in saturation baiting.

As mentioned earlier, death is delayed by three or more days after ingestion (depending on the species of rat and the type of rodenticide). This means that rats will continue to feed on bait even though they have received a lethal dose, which would be a waste of bait. In addition in some species (e.g. R. norvegicus), animals of lower hierarchal ranking cannot feed until 'higher' animals are removed from the population.

This behavioural characteristic is exploited by baiting in pulses. Poisoned bait is laid for 13 days (depending on the rodenticide) and discontinued for about a week, allowing the first batch of animals to die and thus be removed from the population. The next baiting pulse will remove another batch of rats. Normally three baiting pulses are sufficient to remove almost the entire population. The intensity of baiting periods (pulses) depends on the rat population in and around the building and the rate of immigration from neighbouring areas. In spite of the above the intervals between and number of pulses has to be decided each time based on the results of monitoring.

The positive experience with the use of pulsed baiting in different countries and crops is summarised in Dubock (1984).

(iv) Perimeter Baiting

The idea of perimeter baiting is to place bait in a circle around and outside the immediate area of interest and hopefully prevent rats from immigrating. However it is very difficult to give exact guidelines on the diameter of the circle, the distance between baiting points and the quantity of bait to use. The idea has its merits and an operator should experiment with this technique; for example, by placing baiting points between the store and places through which rodents might reasonably be expected to immigrate.

(v) Fumigation

Individual rodents may be killed incidentally during the fumigation of grains and buildings for the control of insects (see Chapter 8). This section deals specifically with fumigation for rodent control.

The control of rodents by fumigation can be very effective, but it may be expensive and dangerous. It should be remembered though that the gas must have access to burrows, if these are present in the building. That is the burrows should be open and the fumigant used must be heavier than air.

If the species concerned makes burrows which are easy to spot (e.g. R. norvegicus, B. bengalensis or P. natalensis), they can be fumigated directly. The simplest method is to use a powder which releases hydrogen cyanide, or aluminium (magnesium) phosphide tablets which release phosphine when placed into the burrows. The gases are generated when the powder or tablets come into contact with moisture in the soil. Alternatively, methyl bromide gas may be pumped into the burrow system.

As soon as the fumigant has been applied all burrows must be closed, by filling the entrance holes with earth. However, fumigants cannot be used in loose or sandy soils as too much gas escapes, and the treatment may not be effective. Occasionally, rats have been known to block tunnels and prevent complete distribution of the gas, so that some individuals survive.

It is important therefore, always to check for reopened burrows or other signs of survival a few days after the prescribed fumigation period.

Fumigation gases used for rodent control are also dangerous to man and other animals. Therefore, strict safety precautions must be observed Meehan 1984). Only trained and properly qualified operators should be employed to use fumigants. They should be seen to be observing the following basic principles of fumigation:

  1. two operators must always be present at a fumigation;
  2. suitable respirators must be worn;
  3. operators should stand upwind when gassing;
  4. fumigants should not be used in strong winds or when it is raining;
  5. fumigants must not be used near buildings inhabited by man or animals as open burrows may be inside; and
  6. operators should know about first aid and have appropriate first aid equipment with them.


Contact dusts

Contact dusts are dusts containing rodenticide which are placed on runways and other places frequented by the house mouse, for example near burrow entrances. Such dusts, while serving also as tracking powders (see earlier), are favoured in the control of house mice which, because of their erratic feeding behaviour, are not easily controlled by baiting. The dust is picked up on the fur and feet and, since mice groom themselves often and regularly, it is automatically ingested.

However care must be taken when using such dusts, as they may easily contaminate stored products like grains and may be undetectable.



Rodenticides, whether chronic (i.e. anticoagulants) or acute, are poisons and should be treated as such and at all times. Some may be more toxic to humans or non-target animals than others, some non-targets may be less affected by certain rodenticides than others.

Nevertheless, it is important that safety procedures are rigidly enforced wherever they are used. Meehan (1984) discusses the toxicity of various rodenticides in some detail.

Standard safety precautions when handling poisons include:


Unwanted poisoning

Much has been written about the potential danger to non-targets of feeding on bait (primary poisoning) or animals feeding on poisoned rats (secondary poisoning). For a short overview the reader is referred to Kaukeinen (1984), Godfrey (1984), Hoppe and Krambias (1984) and Hegdal et al (1984).

It is difficult to assess the exact effect under practical conditions. Unwanted poisoning reported by large scale operations or through accidents, usually involve domestic animals. To date no significant effects on non-rodent wildlife have been associated with the use of conventional anticoagulants (Kaukeinen 1982). The danger of unwanted poisoning can be virtually eliminated in buildings if some simple rules are adhered to:

Stray dogs and cats (and crows and vultures in some countries) may be at risk through feeding on dying or dead rats (secondary poisoning). Normally these animals, because of their size, would need to feed on several rats before they would be affected and more to receive a lethal dose. The chance is very low with most anticoagulants and even with zinc phosphide, because most of the poison is broken down in the stomach. Nevertheless, operators should be aware of these potential dangers at all times.

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