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New Agriculturist: Developments - Trapping rats in Vietnam
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Trapping rats in Vietnam

Vietnam loses at least 10% of its annual rice production to rats (© IRRI)
Vietnam loses at least 10% of its annual rice production to rats

Every year, Vietnam loses at least ten per cent of its rice production to rats; in years with serious outbreaks the loss can be as high as 30 per cent in some regions. Widely regarded in Vietnam as being cleverer than humans, rats are responsible for eating and damaging rice crops both pre and post-harvest, spreading disease to people and livestock, and causing structural damage to grainstores. But 20,000 rice farmers from Binh Luc and Kim Bang districts in the Red River Delta have successfully implemented a community action plan which has reduced damage caused by rats by 93 per cent, increased rice yields by 10-14 per cent and given them a 20 per cent income boost. And there have been similar success stories in the Mekong River Delta.

Ecologically-based management

"Farmers in many rice-growing countries see rats as one of their top three pests," says IRRI rodent expert, Grant Singleton. But, he explains, fewer than ten per cent of rodents are significant agricultural pests: "People think the only good rat is a dead rat, but 90 per cent of rodent species play a very important role in our ecosystem and so we should be trying our utmost to protect them."

Instead of controlling rats with rodenticides, which are not selective, can have long lasting impacts on the environment and to which rats can develop resistance, Singleton and his team have been working to understand the ecology of specific rat species. "Ecologically based rodent management uses knowledge about when and where rats breed, and other ecological and biological information, to control rodents effectively, without relying on rodenticides," he states.

Rat breeding, he explains, is always linked to food supply, which in Vietnam means the timing of the rice crop. By looking at the breeding ecology of rodent species that are major agricultural pests, scientists discovered that breeding started when the developing seed was forming in the rice plant - known as the booting stage. They also found that if an area produced one rice crop in a year, the rats had one breeding season, and if there were two rice crops, there would be two rat breeding seasons, and in the Mekong delta in Vietnam where there are many areas with three crops, there are three breeding seasons. With this knowledge, the scientists realised that effective timing of rat control activities was critical.

Singleton's team worked alongside village cooperatives and people's committees (© IRRI)
Singleton's team worked alongside village cooperatives and people's committees

Over a three year period, Singleton's team worked alongside village cooperatives and people's committees, giving training and support in management techniques. These actions were formulated following previous studies of the breeding ecology, habitat use and population dynamics of the main rodent pest species. In one community activity, farmers were shown dead pregnant rats, seeing the many rat embryos and realising the importance of timing in effective control. "Using a simple graphic, we show farmers that every female rat that is removed before the breeding season occurs is equivalent to removing about 60-70 rats at, or just after, harvest when the population is high," Singleton explains. For effective management of rodents, the scientists suggested conducting community control campaigns before the breeding season, within two weeks of planting the rice crop.

Community action

Correct timing was not the only key factor to success, however. In Asia, the average farm size is one hectare, and rats are highly mobile, so if only a few farmers implement good management practices, their crops would soon be re-invaded by rats from neighbouring fields. "The key to outsmarting rodents is ecologically based rodent management, but to be effective, it should be implemented strategically with community participation or collective action," says Singleton. By acting as a community, farmers could also tackle areas - such as the main irrigation channels - that no one owns and that previously were not being managed

The second 'must-do' activity was for farmers to synchronise planting. "If crops are planted at different times, there is a continuous feed source for rats which extends their breeding season, leading to exponential population growth," Singleton adds. Ecologically based management also suggests keeping irrigation banks less than 30cm wide to make it difficult for rats to build nests, cleaning up any grain spilt at harvest and practising good farm hygiene.

Trapping methods

During control campaigns, farmers were left to use their own catching and killing methods. "We leave that to the farmers because they've been doing it for hundreds of years and have some very ingenious ways of catching rats," says Singleton. "The problem is that they have been doing it individually, at the wrong time of year and not in key habitats."

Farmers were left to use their own catching and killing methods (© IRRI)
Farmers were left to use their own catching and killing methods

However, where rat populations were particularly high, communities were encouraged to plant a 20 metre square plot of rice two weeks early. Surrounded with a plastic fence, with traps set into the plastic, a community Trap Barrier System will protect up to eight hectares of rice. The challenge, Singleton reveals, is that this requires about eight farmers to decide where it will be placed and who will take responsibility for it. "This system works well in the Mekong Delta," Singleton says, "where whoever checks the traps keeps the rats and is able to sell them. But in Indonesia, for example, the rats are often just killed."

According to IRRI social science expert Florencia Palis, who is also involved in the project, it was easier for ecologically based rodent management to be adopted in northern Vietnam, where there is a history and culture of community cooperation. "We are also doing work on ecologically based rodent management in other countries such as Bangladesh, Indonesia, Laos, and the Philippines. But sometimes," she concludes, "the hardest challenge is to try to break those cultural perceptions that rats are smarter than humans."

Singleton explains that compared to Vietnam and Indonesia, the Philippines has different species of rats which not only breed during the rice growing season, but will also breed at other times of the year. So in the Philippines, farmers have to undertake two sets of community rat control campaigns during a single rice cropping season, whereas farmers in Indonesia and Vietnam are able to undertake just one. "The bottom line is that each rodent species has a different ecology and this needs to be understood before effective management strategies can be developed," Singleton concludes.

Date published: November 2012


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