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Mysterious giant earthworms and destructive rats could cost the Banaue rice terraces in the Philippines their claim to the title of 'eighth wonder of the world'. Together, the pests are destroying crops and eroding the step-like rice fields. 

The Ifugao people who live there have teamed up with scientists to try to save their contribution to the UNESCO list of world heritage sites.

In 2001, six years after the terraces joined that list, UNESCO (the UN Educational, Scientific and Cultural Organization) warned that they could vanish within ten years if the situation did not improve.

UNESCO reported that many farmers had left, abandoning 25-30 per cent of the terraces, and letting their ingenious irrigation system fall into ruin. This abandonment, together with the rat and worm pests, threatened to destroy the terraces.

The terraces, on the Philippines's largest island of Luzon, were carved out of the Cordillera mountains 2,000 years ago by the Ifugao ancestors. Once they began to grow rice there, a new society and culture emerged more than 1,300 metres above sea level.

Silent invasion

The Ifugao noticed the earthworms' 'silent invasion' in the early 1970s, but only reported it to local officials in the mid-1980s. The local people believe the worm's appearance in Banaue is linked to illegal logging near the terraces disturbing the soil.

They initially tried to solve the problem with traditional means of pest control but the dark, 20-30 centimetre long worms persisted.

In 1997, the Philippine Rice Research Institute (PhilRice) sent Indian crop scientist Ravindra Joshi to Banaue to tackle the pests. When Joshi arrived, he found that as well as eroding the terraces by burrowing in their walls, the worms were damaging the roots produced by germinated rice seeds.

The rats, meanwhile, were eating the rice flower heads and contributing to the terraces' erosion with their burrowing. Joshi says that if left unchecked, the rats could destroy about 75 per cent of rice crops.

But rice does not just represent food to the Ifugao people, says Joshi. He realised that rice is so central to Ifugao culture that any efforts to protect the rice terraces would need to mix science with tradition.

Traditional methods

After a thorough observation of the indigenous approaches to combating the pests, Joshi went into battle.

The traditional method of controlling the worms was to pound the soil with a hammer-like wooden 'compacter' to limit the their movement.

They also used a variety of local 'brews' to repel and kill the pests. These include a mixture of salt, ash and an extract of a local vine called tog-tog, a concoction made from the leaves and bark of the pojgi tree, and the seeds of the neem tree, which are reputed to be able to kill pests.

None of these methods killed all of the worms.

Some farmers tried using commercial pesticides but these also failed. Joshi says using pesticides to control earthworms in the rice terraces is impractical and environmentally unsafe, because it kills beneficial species as well as earthworms.

A female worm

Initially, the scientists struggled even to identify the worms because few local biologists were familiar with the area's animals.

Joshi now suspects that the giant worm is related to Polypheretima elongata, an Asian species that has invaded rice fields around the world.

Earthworms usually have both male and female reproductive parts but the Banaue species seems to produce no sperm. It is a 'female' worm that can multiply without mating with a male, says Joshi.

Elsewhere in the Philippines, people eat sausages and burgers made from earthworms. But for the Ifugao, eating soil-dwelling creatures is taboo.

To tackle the earthworms, Joshi's team taught the Ifugao a method of 'worm farming' that is popular with small-scale entrepreneurs in the lowlands. The Ifugao collect the worms and rear them in a mixture of soil and old newspapers. They then harvest the worms and process them into feeds used by fish farmers.

But PhilRice's executive director Leacadio Sebastian says the approach is not popular among the Ifugao, and that using it in conjunction with traditional methods might increase its acceptance.

On the recommendation of the scientists, the local government set up a quarantine system, limiting the movement of soil and plants to stop the earthworms spreading to new areas.

Eating the enemy

Another tactic PhilRice encouraged the Ifugaos to adopt was periodically flooding their fields to a depth of 2-3 centimetres to force the giant earthworms to surface, exposing them to predators such as chickens, ducks, pigs and ants.

After harvesting rice, the farmers immediately tilled the soil to kill the earthworms, then drained it to deprive the pests of moisture and expose them to predators.

Though the Ifugao do not eat the worms, when the scientists suggested they cook and eat the rats, the locals agreed. Joshi's team and younger Ifuagos convinced the elders that breaking their long-held taboo was necessary to limit damage to their crops.

To accompany the farmers' traditional method of digging rats out of their burrows, trapping them in a blanket and clubbing them to death, Joshi's group introduced a science-based, and community-run, trap system.

Working together

Adopted by PhilRice scientists from a Malaysian model, the community trap barrier system has also been used successfully on rice farms in Cambodia, Indonesia, Thailand and Vietnam.

Farmers plant a 20 by 20 metre area with an aromatic variety of rice one month before their normal rice planting time. This trap area is enclosed by a plastic fence. Rats that are lured to the area try to enter through holes in the fence only to find themselves caught in wire traps.

Working in small groups, the farming families contribute money and materials to set up the system. It costs about 7,000 pesos (US$125) but can protect at least ten farmers' rice fields - about 30 hectares.

Conchita Nanglegan, one of the farmers, says the system is effective because it is so practical. "It also teaches us the value of acting together in a crisis," she adds.

Each day, the farmers find at least five live rats in each trap.

Timing is critical. According to research Joshi and colleagues published in IRRI Notes in June 2005, about 80 per cent of Rattus tanezumi rats caught in the traps are female and 93 per cent of them are either pregnant or had recently given birth.

Trapping the female rats at this stage in their reproductive cycle is one of the best ways of reducing the rodent threat, they say.

Chrotomys whiteheadi
Photo Credit: Lawrence Heaney
Rattus tanezumi
Photo Credit: PhilRice

Making progress

While Joshi's team encourages the villagers to trap and kill Rattus tanezumi, they are urging the Ifugao to protect another rodent, and use it as an ally in the battle against the earthworms.

The 'friendly rat' is the Luzon montane striped shrew-rat (Chrotomys whiteheadi), which eats not only the giant earthworm, but also the golden apple snail – another major rice pest.

The PhilRice team taught the Banaue farmers how to tell these rats apart from the harmful ones, so they can be released from the traps.

Joshi is confident the terrace farmers are making progress in combating the pests.

But he says UN agencies such as the Food and Agriculture Organization and World Health Organization should help tackle sanitation and health hazards facing the Banaue rice farming communities.

"Pest management in the Ifugao Rice Terraces must go beyond just managing [the invasion]," says Joshi. "It should also involve developing programs that connect tourism, revenue generation, and educating young people on the values of the rice terraces and its traditions."