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Rice field rats
Rattus argentiventer Robinson and Kloss, R. exulans Peale, R. rattus spp., R. tanezumi
Tillers are cut and bent (GR Singleton)
Tillers are cut at 45° and retillering is observed (Van Vreden and Ahmadzabidi)
Often the runways, active burrows and footprints of the rice field rats are visible in the muddy areas, which are near the damage they have created.
Cut tillers and active holes on the bunds that surround the fields can be closely examined for the presence of the rice field rats. To catch rats to identify the species, traps are best placed along runways, or the rats can be dug from their burrows.
The feeding damage on the stem caused by the rice field rats may resemble insect damage although rat damage is usually distinguished by the clean cut at 45° of the tiller. The damage on the grains is similar to bird damage.
In lowland irrigated rice crops both the wet and dry seasons are favourable for rat reproduction and crop damage. In rainfed rice crops rodents have their greatest impact in the wet season. The availability of food, water, and shelter are factors, which provide optimum breeding conditions. The presence of grassy weeds also triggers their development.
Rice field rats feed at night with high activity at dusk and dawn. At daytime, they are found among vegetation, weeds, or maturing fields. During fallow period, they utilize major channels and village gardens as prime habitats. At tillering, 75% of time they are in burrows along the banks and after maximum tillering, 65% of time they are in rice paddies.
Rice field rats are black to brown in color. They have scaly, thinly furred tails and distinctive chisel-like incisors. The rice field rat, R. argentiventer, is the major rodent pest in SE Asia and is distinguished by a tuft of red hair at the base of its ears, fur on back orange-brown flecked with black, and a silvery white ventrum.
In some countries, such as Indonesia, the rice field rats are the most important pre-harvest pests that reduce crop production. Rodents are also noted to consume and contaminate significant amounts of stored grain. Although the rodents that cause post-harvest losses are often a different suite of species (e.g. Rattus norvegicus, Bandicota indica).
Aside from the rice plant, rice field rats also feed on grasses and invertebrates living in and around the rice field.
Breeding in ricefield rats appears to be triggered by the maturation of the rice plant itself, with females first entering estrous 1-2 weeks prior to maximum tillering. Breeding extends through until harvest. After a short pregnancy lasting 3 weeks, female rats produce litters of up to 18 pups (average of 11-12 pups). The pups grow rapidly and are ready to breed at 6 weeks in age. Adult females are able to fall pregnant again within a few days of giving birth, and can therefore produce three litters during the generative phase of growth of a rice crop – a total of 30-40 extra rats per female by harvest time.
Rice field rats feed on seeds directly. They pull up germinating seeds. They either cut or pull up transplanted plants. Tillers are usually cut and then chewed.
The breeding season of the rice field rats and their relative amount of damage are closely linked to the crop growth and development. If there is one crop per year then there is one breeding season. If there are two rice crops per year then there are two breeding seasons. Where harvest is staggered by more than one or two weeks within a single cropping area, the rat population will move from field to field, causing increasingly severe damage in the later-harvested crops. Even more critically, rats born during the early part of the cropping season will themselves be old enough to start breeding. This can produce a sudden explosion in the rat population, with densities peaking at many thousands of animals per hectare.
In Asia, an estimated rodent damage from 5% to 10% prior to harvest was recorded in 1999.
Effective management of rodents will involve strategic actions that limit population growth so that damage is kept below the threshold of economic concern of farmers. Unfortunately, most of the rodent management in Asian countries is reactive – only occurring once a problem has been noticed. Generally this is too late to provide effective management. Many of these reactive actions, such as organising a bounty or application of an acute rodenticide provide farmers with a feeling that they have achieved some measure of success in their fight against rats because they can see dead bodies. However, in reality, once rodent numbers are high the management actions would have to remove at least 70% of the population to have a marked effect on reduction in yield loss to rice crops.
Strategic actions for management are most effective if they are developed on the basis of a sound knowledge of the ecology of the species to be controlled.
There are a range of physical methods available to farmers, ranging from simple woven or plastic barriers designed to deflect rats from growing crops, through to complete enclosures, most often erected around stored grain. These ‘barrier systems’ sometimes incorporate traps or snares set across gaps or ‘doorways’ – hence the term Trap Barrier System or TBS.
In recent years the TBS system has been modified as follows:
Incorporation of a ‘trap’ or ‘lure’ crop to draw rats to the TBS;
Development of minimum specifications for construction and maintenance of a TBS;
Use of the TBS technology to develop both a community and an integrated approach to rodent pest management.
The result is what we now call the Community Trap Barrier System or CTBS method. The word ‘community’ in the name gives emphasis to the fact that the method works best, and is most cost effective, when it is adopted by an entire farming community.
Other methods of physical control includes hunting, rat drives, digging, and exclusion.
Cultural management actions such as hygiene around villages, keeping the cover low along the banks of main irrigation canals, maintaining smaller bunds or banks with height and width of less than 300 mm to prevent rats from utilizing these as nesting sites, synchrony of cropping, and rat campaigns at key times are recommended at the community level. Other actions include shortening the harvest period and having a 1 to 2 month fallow over the dry season.
In Vietnam and Lao PDR, the use of bounty systems failed because they considered bounties as a source of income rather than as a control measure.
Lethal control can be implemented through the use of rodenticides like acute poisons (e.g. Zinc phosphide), chronic poisons or anti-coagulants.
Among the biological control, fertility control or the use of immunocontraception is being studied in Australia. In Malaysia, the use of the barn owl was reported to reduce rat damage. Wildcats, snakes, and birds are also predators of rice field rats.
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GR Singleton, JLA Catindig, KL Heong, and RC Joshi