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   Rattus exulans (mammal)
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    Details of this species in Hawaii
    Status: Alien
    Invasiveness: Invasive
    Occurrence: Reported
    Source: SPREP, 2000
    Arrival Date: Arrived with early polynesian settlers
    Species Notes for this Location:
    In Hawaii, Pacific rats (Rattus exulans) are most common below 2,500 feet (750 m) elevation, although individuals have been captured at an elevation of 4,900 feet (1,500 m) on Mauna Loa on the island of Hawaii and 9,700 feet (2,950 m) on the rim of Haleakala Crater on Maui. Pacific rats prefer areas with good ground cover on well-drained soil. Throughout much of their range, Pacific rats live in close association with humans. In Hawaii, however, Pacific rats are not a commensal pest, but rather favor wild lowland habitats such as wooded and grassy gulches, fields, and waste areas. They reach their highest densities on agricultural lands such as sugarcane fields and abandoned pineapple fields (Prevention And Control Of Wildlife Damage,1994).
    Hawaii is one of the few areas in the world where sugarcane is grown as a 2- to 3-year crop. Most rats living in cane fields either die or migrate to surrounding areas during harvest, and populations do not rebuild until the second half of the crop cycle. During much of the first year, the sugarcane stalks stand erect, the crop canopy is open, and most fields have little ground cover. Some rats from adjacent waste areas forage along the periphery of young sugarcane fields, but few venture into the interior until the sugarcane is 8 to 12 months of age. At this time the sugarcane stalks fall over and dead leaves accumulate. The resulting thatch layer is rich in invertebrate food and provides protective cover in fields where rats establish dens.
    Movements and home ranges in sugar-cane fields vary depending on population density, crop age, and other factors. Pacific rats are nocturnal and are relatively sedentary. Males travel farther than females, but the home ranges of both sexes decrease as the sugarcane matures. Individuals typically stray less than 100 to 165 feet (30 to 50 m) from their burrows (Prevention And Control Of Wildlife Damage,1994).
    Management Notes for this Location:
    Management in sugarcane fields:
    Cultural Methods: Advancing harvest from the usual 22 to 24-month schedule would reduce losses. Adoption of a shorter crop cycle, however, would increase planting and harvesting costs and probably would not be feasible considering current economic conditions. Synchronized planting and harvesting of adjacent fields might reduce movements of rats from recently harvested fields into younger fields. Modification or elimination of noncrop vegetation adjacent to sugarcane fields would help reduce invasion from surrounding areas. Cattle grazing or commercial production of trees for energy or timber might reduce the vegetative understory in such areas. Herbicide use probably is not economical or environmentally desirable. Development of sugarcane varieties that are less susceptible to damage by rats is a promising avenue for research.
    Toxicants: Zinc phosphide is the only toxicant registered in the United States for rat control in sugarcane. Baits are formulated either as pellets or on oats and usually are broadcast by fixed-wing aircraft at the rate of 5 pounds per acre (5.6 kg/ha). A maximum of four applications and 20 pounds per acre (22.4 kg/ha) may be applied per crop cycle. Zinc phosphide baits in Hawaii are most effective against Pacific rats and least effective against Norway rats. Because the relative abundances of the two species vary substantially from field to field and may shift as the crop matures, the efficacy of zinc phos-phide baits also varies. Where Norway rat populations increase during the second year of the crop cycle, zinc phosphide baits become progressively less effective.
    Location Notes:
    Pacific rats eat a wide variety of foods, including broadleaf plants, grasses, fruits, seeds, and animal mat-ter. They prefer fleshy fruits such as melastoma (Melastoma malabathricum), passion fruit (Passiflora spp.), guava (Psidium spp.), thimbleberry (Rubus rosaefolius), and popolo (Solanum nodiflorum). In sugarcane fields, sugar-cane comprises about 70% of their diet by volume, while in surrounding noncrop gulches, it comprises about 20% to 50%. Rats cannot subsist on sugarcane alone. They need additional protein, such as earthworms, spiders, amphipods, insects, and eggs and young of ground-nesting birds (Prevention And Control Of Wildlife Damage,1994).
    Agricultural: Pacific rats (Rattus exulans) are a major agricultural pest throughout Southeast Asia and the Pacific region. Crops damaged by this species include rice, maize, sug-arcane, coconut, cacao, pineapple, and root crops. In the United States, sugarcane is the only crop of economic concern damaged by Polynesian rats. The most severe damage is to unirrigated sugarcane on the windward side of the islands of Hawaii and Kauai. Here, rats find excellent habitat in the lush vegetation of noncrop lands adjacent to sugarcane fields.
    Rat damage to Hawaiian sugarcane is negligible until the crop is 14 to 15 months old, after which it increases substantially and progressively until harvest.Injury ranges from barely perceptible nicks in the outer rind to neatly chiseled canoe shaped cavities. Small chips usually are evident on the ground where rats have fed, (Prevention And Control Of Wildlife Damage,1994).

    Economic/Livelihoods: The economic impact of damage of sugarcane by Pacific rats (Rattus exulans) fluctuates from year to year, largely dependent on the prevailing price of sugar. In 1980, when the average price of raw sugar was at a 50-year high, the Hawaiian sugarcane industry may have lost $20 million. Current losses are conservatively estimated to be greater than $6 million annually (Prevention And Control Of Wildlife Damage,1994).
    Last Modified: 24/03/2005 12:17:40 p.m.

ISSG Landcare Research NBII IUCN University of Auckland