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Ecological rodent control approaches in East Africa

Traditional farm storage systems have evolved to provide good protection against mould deterioration, provide a barrier to insect and rodent pest attack and deter theft. Legs of storage structures, for instance, will be fitted with metal halos to prevent rats from shinning up and more modern maize cribs may be protected with metal mesh. However, for many maize farmers, rats not only cause serious damage after harvest but they are also responsible for causing serious damage to crops whilst they stand in the fields. Farmers are well aware of the losses resulting from rodents, particularly at the planting stage when maize seeds and seedlings are at their most vulnerable. Household maize store on stilts with rat protectorsBut local understanding of rodent population dynamics is limited and control is mostly attempted through intensive use of zinc phosphide poison, a practice which is neither cost-effective, nor environmentally sensitive. Other techniques such as hunting, trapping or field sanitation are used in some areas but are not common.

These were some of the initial findings of a rodent management project working in four countries in East Africa. The collaborative project, known as 'STAPLERAT', is targeting rodent damage to staple crops in Ethiopia, Kenya, Tanzania and Zambia, and aims to identify rodent control methods that are both economically and ecologically sound.

However, rodent control is only viable if it results in a net economic gain for the farmers. To determine which control methods will be most cost-effective and the optimal timing for control, a 'bio-economic' modelling system has been developed. The model has already been used to analyse poisoning as a control technique, and results suggest that there is a strong relationship between when poisons are used, and their cost-effectiveness. While applying the chemicals throughout the year is most effective in reducing animal numbers, the net benefits to farmers are small because of the high cost. But applying rodenticide only in February, just before the planting season is projected to have a significant impact on yield losses despite the lack of impact on rodent population size over the course of the year. If the model proves correct, farmers who confine their use of poison to this crucial period will get the greatest net benefit from their expenditure.

A testing time

Further modelling will be used to analyse the likely benefits of other control methods and their optimal timing. Three such methods are currently being researched under the project. The first involves the use of seed dressings, which protect the maize seed and thereby reduce the need for other physical forms of rodent control. Nine different repellents have been tested, and of these, methiocarb, has been rejected as it is too toxic, killing rather than deterring the rats and, most seriously, requires users to wear protective clothing. Of the others, three (mota, a mixture of vegetable oils, thiram and cinnamamide) have proved to be good repellents, whilst not having a detrimental effect on germination rates and are currently undergoing further testing.

Agroforestry for rodent control is also being put to the test. Tephrosia vogelii bushes are believed to act as a deterrent to molerats, a pest of cassava and enset crops. Research teams in Zambia and Ethiopia are testing this hypothesis by planting the bushes both around and among the crops and monitoring numbers of molehills. Baby barn owl feasting on a ratEarly results from Zambia, where the bushes have become quickly established, indicate that this plant indeed offers a high level of protection.

The third method under investigation is the use of perches and nest boxes to encourage birds-of-prey, such as buzzards, owls and kites, which are natural predators of rodents. At a 100-hectare test site in the Rongoi rift valley in Kenya, perches have been set up at 50 metre intervals, and twenty nest boxes installed. In the first year, seven of the boxes have been occupied, two with brooding pairs. Further research will analyse what effect the perches and boxes have on bird numbers, and whether higher numbers have a noticeable impact on rodent population and crop yield. Similar work is also planned for a test site in Tanzania. However in both areas there is a traditional belief that some birds-of-prey, particularly owls, are unlucky, and local people were initially reluctant to encourage them. In Kenya, however, the situation has been resolved through discussion meetings with project staff explaining the aims and possible benefits of the work. By giving information on owl biology and ecology and by explaining that the owls are already probably present in farmers' barns, fears have been allayed and farmers are now hoping that the nestboxes and perches will be successful.

Success of future rodent management in the region will depend on an integrated approach that is initiated by the communities themselves. In Tanzania, for instance, an over-reliance on external intervention has developed from previous government control campaigns through use of rodenticides. Farmers are aware of the problems caused by these pests and they need to be encouraged to control them on a routine basis using a number of cost-effective and ecologically sound techniques. Monitoring of rain patterns and rodent densities in fields in order to forecast likely population explosions will also result in control approaches only having to be implemented at strategic times during the year or cropping season.

For further information see STAPLERAT website

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