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To catch a rat...

In the Chinese zodiac, rats are considered adaptable, successful and clever; rice farmers in some parts of Asia even have a grudging respect for their rodent neighbours. But pre-harvest losses to rats are increasing. Losses of between 5-10 % per year are generally expected under traditional small-holder rice systems but shorter fallow periods and greater frequency of rice cropping are giving rats a much more stable supply of food, allowing more individuals to breed within a breeding cycle, and population explosions are becoming more frequent. In Vietnam it is estimated that losses have increased sixfold in the last decade and across the region it is not unusual for farmers to report 15-30% losses, although occasionally losses are as high as 50-100%.

Trap for rice rats, Vietnam
Credit: CSIRO Rodent Research Group

However, it is hoped that a community-based approach to rodent control will prove cleverer than the rats and provide a cost-effective and environmentally friendly method of control for two species of ricefield rats (Rattus argentiventer and R. losea). The Trap Barrier System (TBS) was first devised by Lam Yuet Ming of the Malaysian Agricultural Research and Development Institute for lowland irrigated rice crops and has subsequently been modified together with a strong community focus at experimental sites in Indonesia and Vietnam. The system is based around a 20-50 metre square fence made of plastic material, surrounded by a water-filled moat enclosing an appropriate 'lure' crop. Submerged mounds in the moat lead to gaps in the fence, which are entrances to multiple-capture live rat traps. Maintenance is important: the traps need to be emptied of rats each morning, and the plastic material checked for holes. The moat also needs to be kept free of grass, which rats will otherwise use to climb over the fence. But by attracting rats from surrounding fields, the 'lured' trap can provide protection to an area of between 10-15 hectares.

One step ahead

The system works by exploiting the link between the growth of the rice crop and the breeding biology of the two ricefield rats, the most destructive of rodent pest species across Southeast Asia. Breeding in ricefield rats appears to be triggered by the maturation of the rice plant itself, with females first entering oestrous 1-2 weeks prior to maximum tillering. By the time the rice is ready for harvesting, a female may have had three litters and up to forty offspring. Thus it is crucial that females can be removed from the population before or during the breeding season. Successful CTBS systems have used an early-planted rice crop as a lure, established 2-3 weeks before the main crop. These systems catch a moderate number of rats while the surrounding crop is at its milky stage (grain formation), with fewer captures after then. If the capture rate remains high towards the end of the cropping season the system has not worked, either because of poor construction or maintenance, or because of proximity to an area where rat numbers have not been controlled.

Rats are very mobile, travelling up to half a kilometre in a night in search of food so a key to the success of the system is having a whole community approach. A network of appropriately spaced trap units is capable of offering good levels of protection to a wide area, but benefits will be much reduced if only isolated farmers set up traps. Costs for setting up and maintaining the trap units can be shared amongst farmers, but cost effectiveness needs to be considered before such a system is established. Controlled field trials in Indonesia and Vietnam have generally shown increases in rice production of 0.3 - 1 tonne per hectare within the 10 -15 hectares surrounding the units. However, if losses to rodents are anticipated to be less than 5% over the season, the units may not be cost effective.

rice rats in cage
Credit: CSIRO Rodent Research Group

The effectiveness of the trap barrier system has been investigated in a three-year study* in West Java, Indonesia and the Red River delta in Vietnam by the Rodent Research team at CSIRO and in-country colleagues. Besides trapping, other ecologically-based management methods which have proved successful include short two-week campaigns to collect rodents at key times and from source habitats, increasing general hygiene around villages and village gardens, and promoting synchronised cropping. This latter management technique is particularly important, since if farmers plant their crops at different times, the rodent breeding cycle is effectively lengthened as the animals move from field to field, and numbers can boom. To keep numbers down, farmers need to leave a fallow period of at least several weeks when food sources will be at a minimum.

Other farming practices that have been recommended include keeping bunds between fields as low and narrow as possible - less than 30cm wide - to make burrowing difficult, keeping irrigation channels free of overgrowth and cleaning up spilled grain at harvest. The researchers found that rice yields, where the new management methods were implemented, were on average 0.5 tonne per hectare higher than in areas practising traditional control methods. Total yields averaged 4 tonnes per hectare. The ecological methods were also felt to have other important advantages in comparison with use of poisons, which are not only expensive but also pose risks for non-target species, human health and the environment. In addition, captured rats can be sold or used according to local customs and demands, and non-target animals released. Fewer rats around villages after harvest also reduces the risk of rodent borne diseases such as leptospirosis, rat typhus and plague.

The three-year study was supported by ACIAR (Australian Centre for International Agricultural Research)
For further information email: Grant Singleton, CSIRO

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