Pests, found in nurseries and mature plantations, are detrimental to oil palm trees as they cause severe damage which can stunt the growth of the tree. Left untreated, it would make it difficult for the palm oil trees to survive.  In the case of non-lethal attacks, plants become subject to reduced  vegetative development which results in poor growth and vigour.

Rats are one of the most common pests found on oil palm plantations.  The common species found on Malaysian plantations are the Rattus argentivente, Ratus tiomanicus and Ratus diardil. The amount of damage caused by rats to oil palm areas is difficult to estimate. In nurseries, rats have been known to chew the stem base of young seedlings to feed on the tender bud tissues found there. Turner and Gillbanks describe such actions as tooth sharpening, a typical practice for rodents. Such incidences are prevalent in field nurseries situated near forest areas or those situated in new clearings with secondary forests nearby.  Young palms planted out in the field are not exempt from rodent attacks. If the attack is severe, the young palms die due to collapsed fronds which have been attacked at the base by the rats. Other than that, it has been estimated that severe rat damage could cause between 20% to 30% damage among the new plantings and the total fresh fruit bunch yield during the first twelve months after maturity was 20% lower than those from palms with no damage (Chung et al. 2000).  If attacks are not that severe, the fronds that have been attacked are removed to allow new fronds to grow, however the palm remains stunted for some time. In such incidences, preventive measures need to be carried out to ensure that subsequent attacks do not occur on the same young palms which have already been weakened by earlier attacks.

Mature palms with ripe and unripe fruits alike are also subject to rodent attacks.  Turner and Gillbanks again suggest that this could be due to the practice of tooth sharpening.  In the case of fruits being attacked, rodents gnaw to reach the kernel inside.  Crop losses caused by rats feeding on the mesocarp of the fruit has been estimated to be in the region of 5% of the oil yield - and in severe cases, can rise to between 7 to 10% of total oil production, about 165kg of oil per hectare per year.  Taking into account the current price of crude palm oil (CPO), that is a hefty price oil palm growers have to pay for the menace of rats.  It has even been suggested that the quality of the crude palm oil (CPO) from fresh fruit bunches decreases with rodent attacks.

Rats are found in abundance in plantations due to the environment. The thick vegetation of leguminous cover crops, ferns and grasses as well as the fronds that have been cut after pruning are ideal breeding grounds for rats. Various methods have been use to attempt to control and eliminate rodents from
plantations. One of the earliest methods involved traps with snap doors which close when rats take the bait. This method was initially successful but its effectiveness dropped as rats became wiser and probably learnt to distinguish the smell or presence of the prior rat trapped and thus avoided the cage.  Another problem that arose was that other mammals such as squirrels and even mangooses were caught in these traps. Subsequently this system was abandoned and is only used in the homes of workers to trap rats running around the workers’ quarters.

Another method that has been used is the rearing of snakes on the plantations. Some plantation companies tried to rear snakes as these creatures are a natural predator of rats. However, workers generally had a dislike or fear of the snakes and would usually kill them as they cannot differentiate between poisonous and non-poisonous species. Other than king cobras (Naja Hannah), cobras (Naja naja) and kraits (Bungarus spp), other snakes found on plantations are generally non-poisonous, but to the workers they are all the same and need to be eliminated. There were some plantations that impose a fine on workers found to have killed snakes, but the fines did not appear to have deterred them.

Attempts have been made to control the infestation of rats by destroying their habitats. The nests of the rats are removed from the palms, wooden stumps left to rot are removed and buried, and holes where rats are found are filled up.  But these are only temporary measures and do not have any long term effect in controlling the rat population. Various poisons such as sodium arsenite, calcium cyanide, barium carbonate and phosphorus have been used, mixed with bait such as dried fish, prawns, burnt coconut flesh and maize.

One of the most successful methods of rodent control used for more than two decades is the usage of anti-coagulant poisons. Anticoagulants such as warfarin and coumachlor, mixed with food substances like maize, palm oil and wax are left at the planting rows and at the foot of the palms to be consumed by the rats.  This is also successfully used in rice fields. However, the main drawback to this method is that other creatures may accidentally consume the baits on occasion.  

However, the most well-tested and environmentally friendly method of rodent population control is the rearing of barn owls (Tyto alba) on plantations.  Nature has created the barn owl to be the most apt of rat predators.  Several researchers have written about the rearing and breeding of barn owls on plantations.  One team, Genister, Wells and Lenton (Duckett, 1980) found that a pair of owls and its offsprings consume around 2000 rats per year and also 98% of their diet consists of rats.  It thus became logical to breed barn owls on plantations as a way to control the rat population.

The first recorded breeding of the barn owl was conducted by Wells who did so at Fraser Estate in Kulai, Johore. Barn owls are normally bred in specific wooden boxes that are placed in various places in the plantations.  Location is crucial and the boxes should be placed in open isolated areas before being placed in oil palm areas.  As a normal breeding process, a pair of owls take up a box and are initially fed with rodents for about two months before they become independent and are able to hunt for their food. The average hatching of the barn owl eggs in the boxes was found to be more than 90%.

Barn owls are swift in attacking their prey and normally swallow them whole and use their digestive juices to dissolve the nutrients of the fleshy parts. The tougher undigestable parts such as the bones are coughed out. They normally prefer environments where they could hunt their prey and in this case, in palm oil plantations, the rats are in abundant supply.  The results have been encouraging.  For instance, damages to the palms that were recorded in the Golden Hope Plantations after introducing barn owls showed that they fluctuated between 1.8% and 12.6% within the first two years but remained below the baiting standard of 5% from the 3rd to the 7th year which has eliminated the need to carry out baiting simultaneously. The cost of putting up the boxes were also economical. (Tuck, 1997). The findings from these trials have concluded that using barn owls as a natural control of rats is feasible. This fact is supported by reports from Heru et.al. who have also confirmed that barn owls once established in the field can completely replace chemical control altogether.

References:
Chung G. F and Balasubramaniam R. (2000) Rat Baiting Techniques in Oil Palm Estates Facing Labour Shortage, Incorporated Society of Planters, Kuala Lumpur
Duckett J. E. (1980) Barn Owls (Tyto alba) a Proven Natural Predator of Rats in Oil Palm, The Oil Palm in Agriculture in the Eighties, The Incorporated Society of Planters, Kuala Lumpur.
Heru S. B., Siburan J., Wanasuria, S. Chong K.C. and Tiagarajan S. (2000) Large Scale Use of Barn Owl (Tyto alba) for Controlling Rat Population in Oil Palm Plantations in Riau, Sumatera. Incorporated Society of Planters, Kuala Lumpur.
Tuck H.C. and Lay T.C. (1997) Integrated Pest Management in Plantation Crops in Malaysia: Challenges and Realities, Incorporated Society of Planters, Kuala Lumpur.
Turner P.D and Gillbanks R.A. (1974) Oil Palm Cultivation and Management, Incorporated Society of Planters, Kuala Lumpur