Rats are a major mammalian pest that damage coconut and other important crops.
Pacific or Polynesian rat (kimoa, kiore)
The three rat species can be found in most habitats: forest, urban, grasslands and scrub. They are mainly nocturnal, but at high population numbers competition can drive them to be active during the daytime also.
Rats can breed readily when food is available and weather is suitable. Females have litters of around 6-10 young, often more than twice a year.
Young rats are born without hair (pink) and blind until 2 weeks old. They will be seen with their mother up to 20-28 days. Young become sexually mature after 2-3 months. Adults live for around 12-15 months.
The Polynesian rat (R. exulans) can be identified by its pointed nose and slim body that is a red-brown to grey-brown colour with a white stomach.
They weigh around 40-80 grams and grow up to 18 cm long (not including their tail). Their tail is about the length of their body and shows noticeable rings around it that are fine and scaly.
They have large ears and small and delicate feet compared to their body. On the edge of the hind foot, near the ankle, the Polynesian rat has a dark band.
Females have 8 nipples.
The Polynesian rat nests on the ground most commonly by digging small holes. They are good climbers.
The Polynesian rat. Close up showing the dark band on the hind foot (© Gerald McCormack, Cook Islands Natural Heritage Trust)
The black rat (R. rattus) also has large (hairless) ears and a slim body but is larger than the Polynesian rat.
The black rat grows up to 20 cm long and weighing 120-160 grams (possibly exceeding 200 g). They can vary in colour from all black or a grey-brown colour with their belly the same colour or a creamy-white underneath.
Their tail is longer than their body length and one colour.
Females have 10 nipples.
Black rats are also good climbers and tend to nest in trees.
Black rats are excellent swimmers.
The large features and dark colour of the black rat (a) compared to the larger bodied brown rat (b) (© Crown Copyright 2009, GBNNSS)
|The brown rat (R. norvegicus) is the biggest of the three rats growing up to 25 cm long and weighing around 150-300 grams (sometimes 500 g). They have small features in comparison such as their scaly tail which is shorter than their body length and they have small ears.
The brown rat has a brown back with a pale grey stomach.
Females will have 12 nipples.
The brown rat makes burrows and nests underground.
The brown rat rarely climbs but tends to swim (up to 2 km) They are often seen in wet areas such as gardens, towns and ports.
The brown rat showing its short tail and small ears in comparison to its large body size (© John Hitchmough, Flickr)
Also see Guidelines on Rodent Identification to help identify each species.
The nuts/fruit are the most vulnerable plant part to rats. Fallen nuts that have large holes (5 cm in diameter) near where the stem was, are the most common sign of rat damage.
Gnawing damage by rats can often be seen on the trunk of palms.
The rats also gnaw on palm flowers. This can make the palm more susceptible to secondary infections such as diseases and insects.
Rats damage the live tissues of the plant (trunk and crown parts) and nuts.
Rats can spoil copra and other coconut products by discharging their faeces and urine on them. Coconuts stored for food can also be eaten by rats.
On the atolls of Tokelau over 87% of the diet of Polynesian rats was found to be coconut.
Damage to palm parts can greatly limit the ability of the palm to form new tissues. The increased susceptibility to disease and infestations further reduces the health of the palm.
Large holes gnawed in to fallen nuts (© TNAU)
Gnawing damage on a coconut palm trunk(© TNAU)
Globally, of the three species the black rat is considered the most detrimental to coconut, destroying around 30% of crops every year.
The Polynesian rat can be found in the Pacific basin (Asia, North America and Oceania).
The black and brown rat is widespread through Asia, Africa, America (Central, North and South and the Caribbean), Europe and Oceania.
Rats are frequently found on boats, ships and aircraft as stowaways. Rats can also swim considerable distances. Therefore, biosecurity at possible entrances must be managed.
Regulations on the movement and establishment tend to be loose in many countries. However, there are countries who perform required biosecurity for docking ships and do routine inspections on ships that are in their ports. The import of rats is prohibited into and out of most Pacific island countries, including New Zealand and Australia.
For general information on preventing pests and diseases of coconut, see the Prevention section.
We strongly recommend an integrated pest management (IPM) approach to the control of all pests, where possible. This is a combination of methods (pesticides, physical controls such as site hygiene) to minimise the use of pesticides and minimise the cost of control.
Integrated Pest Management
The goal of IPM is to keep pest populations to a level below which they cause economic harm. IPM involves using multiple control options together for the economic control of pests (i.e. cultural, natural and chemical).
In an agricultural context the Food and Agriculture Organization defines IPM as "the careful consideration of all available pest control techniques and subsequent integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment. IPM emphasizes the growth of a healthy crop with the least possible disruption to agro-ecosystems and encourages natural pest control mechanisms".
Removing one species of rat can make areas more vulnerable to other species as it reduces their competition. Therefore, all three rat species should be focused on equally.
Rats are commonly introduced from and found around human populations. Therefore, keeping environments clean and tidy and storing food resources tightly can help reduce rat populations.
Preventing rats from invading new areas is very important as it can be difficult to eradicate them later.
Rat-proofing storage places can help reduce products being destroyed. Putting metal trunk banding or plastic sheets around the coconut palm trunk can help deter rats and stop them from reaching the crown. Electric fences (lethal and non-lethal) and other enclosures have been successful, but is expensive.
Rats are preyed on by cats, dogs, ferrets and weasels, owls, hawks, eagles and snakes. However, none of these have been shown to be effective at controlling rat populations unless combined with other techniques. Predators should never be introduced to control rats, as these cause even more problems.
Trapping is a great option to catch all species of rats and help with identifying what species is present. Snap traps and live traps can be used. More than 3 nights of trapping should be completed as the brown and black rat (dominant species) are generally caught first then the Polynesian rat the remaining nights.
Monitoring populations is also recommended. This can be done by rat traps, tracking tunnels or tiles and rodenticide bait blocks. They are a relatively cheap method for surveillance of populations in known areas and possible areas of concern.
Be aware that generally trapping does not catchall individuals. Rats can become 'trap-shy' (i.e., they avoid traps), thus they are not caught and then able to repopulate areas.
Some countries, such as New Zealand, use dogs specifically trained to detect rats. Especially in conservation and transport areas. Predator-proof fencing has also been successfully but is costly.
Experiments are currently underway involving population control by contraceptive methods which may be highly effective.
Baiting with pesticides is commonly used as a control or eradication tool. It includes intermittent and permanent treatment with anti-coagulants rodenticides.
Commercial baits can be brought from most stores to eradicate rats within small areas (homes and storage). For wide ranges, bait stations (spaced at 50 to 200 m) are used or spread along the ground. Aerial distribution is also used to dispense poisons in large and hard to reach areas.
Rodenticides should be used with caution. Poisoning of non-target species may occur including vertebrates and invertebrates.
Rats may also become 'bait shy' to these poisons. So, even though baits are used, monitoring and surveillance continue to be important. Always follow guidelines and instructions on the use of pesticides. There may be restrictions on their use within your country.
Plant-derived pesticides are used within the Pacific and are less toxic than commercial pesticides (more needs to be consumed). The young leaves of legume shade tree, Gliricidia sepium are pounded and mixed with cooked rice, maize or other food as a lure. The bark can be used as well. Chemicals in the leaves, once converted with bacteria, are similar to the brodifacoum in commercial baits.
In the Solomon Islands Barringtonia asiatica fruits' white inner flesh has been used as part of a rat poison. The scraped fruit is added to cooked rice or shredded coconut. However, this is a very toxic approach, with the concentration of the toxin unknown.
Plant-derived baits should be changed daily and kept away from pets.
A list of possible places to purchases rat control products can be found here.
Sightings of rats in new areas should be reported immediately to the nearest wildlife authority or relevant government department.
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CABI. 2018. Rattus norvegicus (brown rat). [ONLINE]
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EPA. 2017. Restrictions on Rodenticide Products. [ONLINE]
FAO. 1994. Grain storage techniques; Evolution and trends in developing countries: Rodent control. [ONLINE]
Global Invasive Species Database. 2018. Species profile: Rattus exulans. [ONLINE]
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Global Invasive Species Database. 2018. Species profile: Rattus norvegicus. [ONLINE]
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Pacific Invasives Initiative (PII). Resource Kit for Rodent and Cat Eradication. [ONLINE]
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content reviewed by Bob Macfarlane, Solomon Islands, September 2018