Rodents: a gnawing problem

Rattus rattus, otherwise known as the Black rat or Ship rat, was responsible for spreading the plague in the Middle Ages. Today, it is one of only three rodent species (see box) that are found throughout the world, although they all originated from south-east Asia. Of the 1700 rodent species found worldwide, less than ten species occur in and around human habitation and only twenty or so species in total are termed as important pests. However, the economic importance of these few species cannot be ignored: rodents are prolific breeders and may have up to five litters/year under most favourable conditions, and they are capable of eating 10% of their own body weight each day. Total crop losses caused by rodents each year could, it is estimated by experts, feed 200 million people, equivalent to the population of Indonesia. With international development focussed on reducing world hunger and poverty, the impact of rodents cannot be underestimated and solutions have to be found to manage rodent populations more effectively.

A cautionary tale

Worldwide, chemical control measures continue to be used extensively and, even if they are not used exclusively, they are often incorporated into IPM rodent control programs. For smallholder farmers, who can afford chemicals, fast-acting, acute rodenticides, such as zinc phosphide, are used widely. Farmers favour these compounds because they are readily available, can be bought at relatively low cost and their effects are immediately apparent after application. However correct application can be a problem. The compounds are sold as concentrates, which must be mixed with high-quality cereal baits if they are to be effective, and many smallholder farmers have insufficient resources to be able to do this accurately and safely. Rodents are also well known for being suspicious of new objects (neophobic), a characteristic which is poorly understood or accounted for by smallholders.

Use of anticoagulants, such as warfarin, avoids the problem of bait shyness as their mode of action is chronic and not acute. These compounds are most effective when administered in small daily doses over a period of several weeks. However, a high number of important tropical rodent pests demonstrate a 'natural resistance' and only three species, of which R. norvegicus is the most susceptible, will succumb to use of warfarin in less than two weeks.

Resistance to first generation anticoagulants has led to the development of more potent compounds (e.g. bromadiolone and boradifacoum), which are effective even against 'naturally resistant' species. The use of wax-block baits containing one of these more potent compounds is regarded as the most practical and cost-effective method of rodent control using rodenticides because of the small amounts of bait and labour required for their application. Resistance to anticoagulants, which is now common in the UK for instance, is rare in tropical agriculture and has only recently become a problem in oil palm plantations in Malaysia. So it is likely that rodenticides will continue to play a role in rodent control in tropical agriculture.

However, even with an IPM strategy that uses monitoring systems to target the timing of rodenticide applications, it is felt that the emphasis of such an approach is on control rather than management. Similar concerns have arisen with insect and weed pests. Nevertheless, progress made with IPM has led to the development of an alternative approach, which is more sustainable and less environmentally harmful, known as ecologically-based pest management (ERPM). Such management strategies require a greater understanding of the basic biology and ecology of pests, particularly since killing some rodent species may reduce populations initially but the remaining animals are often found to compensate such measures with better survival and breeding performance. Progress in rodent management is being made but as one rodent zoologist observes, "There is still much to be done both to understand and to outsmart these clever mammals".

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