Rattus exulans
(Pacific rat)

The Pacific rat, R. exulans, is an major agricultural and environmentalpest in parts of Southeast Asia and the Pacific. Thought to have spread with Polynesian colonists over the past several thousand years, it is now found through much of the Pacific basin, and is extensively distributed in the tropical Pacific. It poses a significant threat to indigenous wildlife, particularly ground-nesting birds, and has been linked to the extinction of several bird species. R. exulans may also transmit diseases to humans.

See Maps and Distribution Table. R. exulans is restricted to the Pacific basin. Although extensively distributed in the tropical Pacific, it is also found in temperate areas of New Zealand (to 48^oS latitude; Shiels and Pitt, 2014). In the Philippines, R. exulans occurs on Luzon, Mindanao, Mindoro and Palawan islands (Barbehenn et al., 1973).

R. exulans is closely associated with human settlement and is often accidentally transported to new areas by people (National Animal Pest Alert, 2011).

Aircraft and ships are vectors for repeated introductions of stowaway R. exulans. Contemporary introductions of R. exulans into areas that have never had rats or have eradicated rats are of particular concern. Formal regulations against rat transport and establishment are generally lacking, although some countries require routine inspection of ships in their ports and also require biosecurity measures for ships docking to reduce the risk of importing rodents.

There is a high risk that R. exulans could establish on mainland Australia and Australian islands, and become an agricultural, environmental and social pest. There are already established populations on some Australian islands and islands elsewhere in the Pacific. 'Agricultural crops that could be at risk [in Australia] include cereals, flowers, fruit (such as grapes, pineapples and passionfruit), legumes, nuts oil seeds, sugarcane and vegetables' (National Animal Pest Alert, 2011).

R. exulans may be found in coastal areas, grasslands, bush, shrub vegetation adjoining croplands, edges of original forest and second-growth forest. It is also found in stored grain. In the Philippines, it is more common on highlands than in the lowlands, and on interior land areas more than coastal areas (Rabor, 1977). However, R. exulans is common in lowland habitats (including coastlines) on many tropical atolls in the Pacific and on low-elevation islands off North and South Islands, New Zealand. Lindsey et al. (1999) captured R. exulans at 1650 m elevation in Hawaii.

Barbehenn et al. (1973) described the distribution of R. exulans in the Philippines. It can be found at high elevations (2250 m) in sweet potato plantations on Mount Data, in northern Luzon island. It is common in gardens and on the periphery of ricefields on Luzon and Mindoro islands, but is generally not abundant inside ricefields. It is rarely found in the lowlands of Mindanao island, even in vegetation of Imperata cylindrica and Saccharum spontaneum. It is the predominant ricefield rat in Palawan island.

It can coexist with larger rat species such as R. argentiventer and R. rattus mindanensis (National Animal Pest Alert, 2011). However, R. rattus tends to outcompete R. exulans or displace it to more remote areas (Shiels, 2010; National Animal Pest Alert, 2011; Shiels et al., 2013). For example, R. exulans was once widespread throughout all suitable habitats in New Zealand, but now (on the mainland North and South Islands) it is apparently confined to parts of Fiordland, Southland, and south Westland (Atkinson and Towns, 2005).

R. exulans is closely associated with human settlements, although on Adele Island (northern Western Australia) it lives in spinifex hummock grassland completely away from human habitation (National Animal Pest Alert, 2011).

The diet of R. exulans in agricultural areas can include rice, coconuts, maize, palms, sweet potatoes, white potatoes, cassava, sugarcane and insects (Kami, 1966; Wood, 1994). Animals can comprise a significant portion of the Pacific rat diet; arthropods are generally the most common animals consumed, yet less often seabirds, forest birds, land snails and lizards can also be consumed when available (Atkinson and Towns, 2005; Shiels and Pitt, 2014). Plant material generally dominates R. exulans diet in forests and agricultural settings (Stecker and Jackson, 1962; Williams, 1973; Shiels et al., 2013; Shiels and Pitt, 2014).  In the Tokelau Islands, northeast of New Zealand, 87.6% of R. exulans diet was coconuts found on the ground (Mosby and Wodzicki, 1973).

Strecker and Jackson (1962) reported that field rats on the Pacific island of Pohnpei, including R. exulans, fed on cereals, groundnuts, sugarcane, candy, cheese, fish, meat, ripening bananas, tapioca, tomatoes, beans, pineapples, pawpaws, soursop (Annona muricata), cocoa, passion fruits, seeds on spikes of Bermuda grass (Cynodon dactylon), fruits of rainforest trees and snails.

Reproductive Biology

Like many other rodents, the length of the oestrus cycle of adult female R. exulans is 4-6 days. R. exulans can breed throughout the year. In Hawaii, the peak in reproduction was from October to December (Tamarin and Malecha, 1971). In New Zealand, breeding is from September to March (National Animal Pest Alert, 2011).

The gestation period for R. exulans is 19-23 days. Young rats are born blind, pinkish and without hair. Young can see and hear at 2 weeks old (Atkinson and Towns, 2005). They are weaned from their mother at 20-28 days old and generally mature at 8-12 weeks old (though this may occur sooner). Individuals can be sexually mature at less than 50 g (Atkinson and Towns, 2005). Rats may exhibit postpartum heat or oestrus and can be impregnated and fertilized while nursing their young. Average life expectancy in the wild is about one year.

Sumangil (1990) reported that in 1958, samples of R. exulans from Mindoro island in the Philippines had 3.83 pregnancies per year, with an average litter size of 4.07 and an annual productivity of 16. Litter size can vary from one to nine (National Animal Pest Alert, 2011), and in New Zealand they averaged four to seven (Atkinson and Towns, 2005). Williams (1973) reported that in Ponape (Pohnpei) litter size ranged from one to 10 (average 3.8), and that researchers failed to increase that rate of reproduction with R. exulans. Williams (1973) also reported that 1-13 (average 5.2) litters were born each year and most females had a reproductive lifespan of less than one year in tropical Ponape.

Many factors (such as rainfall) affect rat reproduction. Temme (1981) reported that R. exulans has two to six litters per year, with two to five young in each litter (average litter size: 3.5). See also Wood (1994).

Activity

R. exulans is very agile; it is good at running and jumping and at climbing trees. It does not burrow extensively and constructs its nest mainly in leaf litter, under forest debris, beneath brush, and in crevices (Temme, 1981). R. exulans is mainly nocturnal, like other Rattus species, but it becomes increasingly diurnal at high population densities.

Diet

R. exulans is omnivorous, eating seeds, flowers, fruits and other plant parts, snails, insects and larvae, earthworms, lizards and birds and their eggs and chicks (Shiels and Pitt, 2014). R. exulans often carriesy food to small husking stations that provide shelter from predators, competitors and the elements during feeding (Cambell et al., 1984).

R. exulans is commonly hunted by owls, hawks, snakes, cats and dogs.

Parasites that affect rats are mentioned by Sandosham (1953), Audy and Harrison (1954), Lim and Heyneman (1965) and Atkinson and Towns (2005). Some of these include various mites, lice, fleas and nematodes (Atkinson and Towns, 2005).

In ricefields, the presence of rats is indicated by cut tillers, scattered grains and leaves on the ground, foot prints and runways. In coconut groves, fallen nuts with holes are seen. In fruits and root crops, gnawed holes made by the rats’ sharp incisors are the common signs of damage. It is difficult to differentiate rodent species merely by the damage they cause. Faeces are not reliable indicators of species because of their sizes overlap with other rodent species (Atkinson and Towns, 2005; Shiels et al., 2014). R. exulans faeces generally range from 6.4-9.0 mm in length (Atkinson and Towns, 2005).

R. exulans often has dark-coloured upper sides of hind feet which is unlike R. rattus, R. norvegicus, and Mus musculus, which all have uniformly colouring over whole feet (Atkinson and Towns, 2005).

If more than one rodent species is suspected to be in the habitat or area, traps (snap traps and live traps) can be used to catch rats for proper identification. In the Philippines, the trapping period may be extended (more than three nights) to catch the dominant species (R. rattus mindanensis, R. argentiventer or R. norvegicus) during the first nights of trapping, before catching R. exulans. A similar pattern of R. rattus dominance during the first trap nights, and R. exulans only captured on nights after the first two nights, has been also found in Hawaii (Shiels, 2010).

For regular monitoring of R. exulans populations, use of tracking tiles (made of vinyl or other material) are common, where mimeographing ink is painted on half of the area of each tile and laid along the path of the rats (Sanchez and Benigno, 1985) or otherwise in the habitat of interest (Shiels, 2010).

R. exulans adults are often mistaken for the young of medium-sized rats such as R. rattus and R. norvegicus, and in some cases they can be mistaken for Mus musculus (Atkinson and Towns, 2005).

R. exulans often has dark-coloured upper sides of hind feet, unlike R. rattus, R. norvegicus and Mus musculus, which all have uniform colouring over the whole foot (Atkinson and Towns, 2005). Further comparisons among the three most common invasive rats in the Pacific are detailed in Atkinson and Towns (2005) and Shiels and Pitt (2014). For comparisons with M. musculus, the feet of a juvenile R. exulans are proportionally longer and broader than those of a same-sized mouse. R. exulans also has an elongated fleshy pad on the under surface of the feet, whereas the pad on mice is round-shaped (National Animal Pest Alert, 2011).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Introduction

Control methods have often been directed against the rat species that cause most damage. For example, in the Philippines, in mixed populations, problems with R. exulans surface once the major species (e.g. R. rattus mindanensis, R. argentiventer, R. norvegicus or R. tiomanicus) are controlled. Fortunately, the same control methods generally work for all species (for further details see the data sheets on these species).

Prevention

Research has shown that it can often be difficult to eradicate rats from islands in the early stages of invasion; therefore it is better to prevent rodents arriving on islands in the first place. R. exulans is prohibited from import into Australia and many Pacific islands. As an example of such restrictions, the following is taken from National Animal Pest Alert (2011):

'It is very important that those travelling into Australian waters maintain quarantine measures against rats and immediately report R. exulans found here [Australia]. Similarly, it is vital that people visiting Australian islands that currently support R. exulans populations do not transport them to other islands or the mainland.'

Chemical Control

Various modifications of sustained baiting and a modified form of continuous baiting in crops, using principally the first-generation multiple-dose anti-coagulants, have been recommended for use in the Philippines (PCARRD, 1985) and elsewhere. Second-generation anticoagulant poisons are used widely for invasive rat control in the Pacific (including eradication attempts involving R. exulans), but possible consequences of any ongoing control should always be considered. These consequences include primary or secondary poisoning of species that are targeted for protection or other non-target species. Secondary poisoning of other vertebrate pests such as cats, and the development of resistance to these poisons by Pacific rats, should also be considered.

Cultural Control

Rat-proofing has been done with varying success in storage, on coconut trees (trunk banding with metal or plastic sheets), and using lethal and non-lethal electric fences or metal and plastic enclosures in experimental ricefields.

Field Monitoring/Economic Threshold Levels (ETL)

Benigno (1979) placed a 3% tiller cut as the ETL for rat damage, on the basis of mechanically simulated rat damage-yield loss studies, statistical differences of yield in control and damaged plots, and farmers' perceptions of damage. Sumangil (1990) made a more conservative estimate of 1% yield loss (about 1.46% tiller cut at 14 weeks after transplanting). In Malaysia, ETL was estimated at 15% hill damage (Rennison and Buckle, 1988).

Benigno (1979) devised a sequential sampling plan, with three ETLs for different damage levels, for use in individual fields: light <3% tiller cut), medium (3-5% tiller cut) and severe (>23% tiller cut). Rennison and Buckle (1988) described a more general sequential sampling plan across fields with the upper line at 15% hill damage and the lower line at 5% hill damage.

However, there is no evidence that such damage in rice is caused specifically by R. exulans and there is a need for more information.

15/05/14 Datasheet updated by:

Aaron Shiels, USDA, USA