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The Handbook: Prevention and Control of Wildlife Damage Management, Internet Center Wildlife Damage for

July 1994

Polynesian Rats

Mark E. Tobin USDA-APHIS-ADC, Hilo, Hawaii

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Tobin, Mark E., "Polynesian Rats" (1994). The Handbook: Prevention and Control of Wildlife Damage. 7. https://digitalcommons.unl.edu/icwdmhandbook/7

This Article is brought to you for free and open access by the Wildlife Damage Management, Internet Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in The Handbook: Prevention and Control of Wildlife Damage by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Mark E. Tobin Project Leader Hawaii Field Station POLYNESIAN RATS Denver Wildlife Research Center USDA-APHIS-ADC Hilo, Hawaii 96721

Fig. 1. Polynesian rat, Rattus exulans

Damage Prevention and Toxicants Identification Control Methods Zinc phosphide. The Polynesian rat (Rattus exulans) is Exclusion Fumigants smaller than either the Norway rat (R. norvegicus) or the roof rat (R. rattus). Not practical for Hawaiian sugarcane Not practical in and around sugarcane Polynesian rats have slender bodies, fields. fields. pointed snouts, large ears, and rela- Cultural Methods Trapping tively small, delicate feet. A ruddy brown back contrasts with a whitish Not practical in and around sugarcane Synchronize planting and harvesting belly. Mature individuals are 4.5 to 6 fields. of large blocks of fields. inches long (11.5 to 15.0 cm) from the Eliminate or modify noncrop vegeta- Shooting tip of the nose to the base of the tail and weigh 1.5 to 3 ounces (40 to 80 g). tion adjacent to sugarcane fields. Not practical. The tail has prominent fine scaly rings Develop potential resistant sugarcane Biological Control and is about the same length as the varieties. head and body. Female Polynesian rats Not effective. Repellents have 8 nipples, compared to 10 and 12 nipples normally found on roof rats None are registered. and Norway rats, respectively.

PREVENTION AND CONTROL OF WILDLIFE DAMAGE — 1994 Cooperative Extension Division Institute of Agriculture and Natural Resources University of Nebraska - Lincoln United States Department of Agriculture Animal and Plant Health Inspection Service Animal Damage Control B-121 Great Plains Agricultural Council Wildlife Committee Range General Biology, home ranges of both sexes decrease as Reproduction, and the sugarcane matures. Individuals Polynesian rats are native to Southeast typically stray less than 100 to 165 feet Asia but have dispersed with humans Behavior (30 to 50 m) from their burrows. across the central and western Pacific. Today, these rodents inhabit almost Reproduction varies among geo- Population Changes o graphic areas and is influenced by every Pacific island within 30 of the Roof rats, Norway rats, and weather, availability of food, and other equator. They occur from the Asiatic Polynesian rats coexist throughout factors. Reproductive activity of Poly- mainland south to New Guinea and much of the Pacific basin. It is not nesian rats on Oahu reaches a peak in New Zealand, and east to the known how much, if any, interspecific late summer and ceases in mid to late Hawaiian Islands and Easter Island. competition exists. After the arrival of winter. Polynesian rats on Kure Atoll Polynesian rats accompanied early Norway rats, roof rats, and house mice in northwestern Hawaii produce most Polynesian immigrants to Hawaii and (Mus musculus) in New Zealand, popu- litters from May through August. On today occur on every major island of lations of Polynesian rats declined. the windward side of the island of Ha- the archipelago. The Polynesian rat is Today, they are very rare on the two waii, Polynesian rats breed throughout not present in the mainland United main islands. It is not clear whether a the year, with peak reproduction States. similar decline occurred in Hawaii, but occurring in the summer and early fall. if so, Polynesian rats have adjusted. Habitat Females have an average of 4 litters Today, they are the most abundant per year, with a range of 3 to 6 and an lowland rat in many parts of the state. In Hawaii, Polynesian rats are most average of 4 young per litter. The mini- common below 2,500 feet (750 m) mum gestation period for captive rats In Hawaii, roof rats, Norway rats, and elevation, although individuals have is 23 days, with lactation prolonging Polynesian rats often occur in the same been captured at an elevation of 4,900 gestation by 3 to 7 days. In captivity, sugarcane fields. Only the latter two feet (1,500 m) on Mauna Loa on the newborns open their eyes about 2 are major pests in sugarcane, with roof island of Hawaii and 9,700 feet (2,950 weeks after birth and are weaned rats occurring mostly near field edges. m) on the rim of Haleakala Crater on when about 3 weeks old. Captive-bred Since the late 1960s Norway rats have Maui. Polynesian rats prefer areas individuals reach reproductive matu- increased their abundance relative to with good ground cover on well- rity when they are 60 to 70 days old the other two species in Hawaiian sug- drained soil. Throughout much of their and weigh about 1.5 ounces (40 g). The arcane fields and are now the species range, Polynesian rats live in close life expectancy of wild rats is less than of primary concern to the Hawaiian association with humans. In Hawaii, 1 year. sugarcane industry. Polynesian rats, however, Polynesian rats are not a however, are still locally abundant in commensal pest, but rather favor wild Hawaii is one of the few areas in the many fields. lowland habitats such as wooded and world where sugarcane is grown as a grassy gulches, fields, and waste areas. 2- to 3-year crop. Most rats living in They reach their highest densities on cane fields either die or migrate to sur- Damage and Damage agricultural lands such as sugarcane rounding areas during harvest, and Identification fields and abandoned pineapple fields. populations do not rebuild until the second half of the crop cycle. During Polynesian rats are a major agricul- Food Habits much of the first year, the sugarcane tural pest throughout Southeast Asia stalks stand erect, the crop canopy is and the Pacific region. Crops damaged Polynesian rats eat a wide variety of open, and most fields have little by this species include rice, maize, sug- foods, including broadleaf plants, ground cover. Some rats from adjacent arcane, coconut, cacao, pineapple, and grasses, fruits, seeds, and animal mat- waste areas forage along the periphery root crops. In the United States, sugar- ter. They prefer fleshy fruits such as of young sugarcane fields, but few cane is the only crop of economic con- melastoma (Melastoma malabathricum), venture into the interior until the sug- cern damaged by Polynesian rats. The passion fruit (Passiflora spp.), guava arcane is 8 to 12 months of age. At this most severe damage is to unirrigated (Psidium spp.), thimbleberry (Rubus time the sugarcane stalks fall over and sugarcane on the windward side of the rosaefolius), and popolo (Solanum dead leaves accumulate. The resulting islands of Hawaii and Kauai. Here, nodiflorum). In sugarcane fields, sugar- thatch layer is rich in invertebrate food rats find excellent habitat in the lush cane comprises about 70% of their diet and provides protective cover in fields vegetation of noncrop lands adjacent by volume, while in surrounding where rats establish dens. to sugarcane fields. noncrop gulches, it comprises about 20% to 50%. Rats cannot subsist on Movements and home ranges in sugar- Rat damage to Hawaiian sugarcane is sugarcane alone. They need additional cane fields vary depending on popula- negligible until the crop is 14 to 15 protein, such as earthworms, spiders, tion density, crop age, and other months old, after which it increases amphipods, insects, and eggs and factors. Polynesian rats are nocturnal substantially and progressively until young of ground-nesting birds. and are relatively sedentary. Males harvest. Damage caused by roof rats, travel farther than females, but the Norway rats, and Polynesian rats is

B-122 very similar. All three species chew on the internodes of growing stalks. In- jury ranges from barely perceptible nicks in the outer rind to neatly chis- eled canoe-shaped cavities. Small chips usually are evident on the ground where rats have fed. Rat depredation can be distinguished easily from that of feral pigs (Sus scrofa). Pigs chew on the entire stalk, leaving it with a shred- ded appearance. Trampled vegetation is further evidence of pig activity. Legal Status Rats are an exotic species in Hawaii and are not protected by law. They may be controlled by any method con- sistent with state and federal laws and regulations. Damage Prevention and Control Methods Exclusion Electric fences and physical barriers have been used to prevent rats from entering experimental farm plots. It is questionable, however, whether cur- rent fencing designs and exclusion techniques are practical for Hawaiian sugarcane fields. Cultural Methods Advancing harvest from the usual 22- to 24-month schedule would reduce losses. Adoption of a shorter crop Fig. 2. Rat-damaged sugarcane cycle, however, would increase plant- ing and harvesting costs and probably include rind hardness, stalk diameter, and least effective against Norway would not be feasible considering cur- degree and time of lodging, resistance rats. Because the relative abundances rent economic conditions. Synchro- to souring, and potential for compen- of the two species vary substantially nized planting and harvesting of satory growth. from field to field and may shift as the adjacent fields might reduce move- crop matures, the efficacy of zinc phos- ments of rats from recently harvested Repellents phide baits also varies. Where Norway fields into younger fields. Modification None are registered. rat populations increase during the or elimination of noncrop vegetation second year of the crop cycle, zinc adjacent to sugarcane fields would Toxicants phosphide baits become progressively help reduce invasion from surround- Zinc phosphide is the only toxicant less effective. ing areas. Cattle grazing or commer- registered in the United States for rat cial production of trees for energy or Fumigants control in sugarcane. Baits are formu- timber might reduce the vegetative un- lated either as pellets or on oats and None are registered for the control of derstory in such areas. Herbicide use usually are broadcast by fixed-wing Polynesian rats in Hawaii. probably is not economical or environ- aircraft at the rate of 5 pounds per acre mentally desirable. Trapping (5.6 kg/ha). A maximum of four appli- Development of sugarcane varieties cations and 20 pounds per acre (22.4 Polynesian rats can be captured easily that are less susceptible to damage by kg/ha) may be applied per crop cycle. with coconut bait and standard snap rats is a promising avenue for traps, modified wire-cage Japanese Zinc phosphide baits in Hawaii are research. Possible selection criteria live traps, or other appropriate traps. most effective against Polynesian rats

B-123 However, trapping in sugarcane fields prevents their foraging in maturing Acknowledgments is extremely labor intensive and is not sugarcane fields. practical for control purposes. Planta- D. Fellows, L. Fiedler, A. Koehler, and R. Dogs have also been used to control Sugihara reviewed earlier drafts of this chapter. tion personnel took an average of rats in harvested sugarcane fields D. Steffen sketched the line drawings. 141,000 rats annually from sugarcane (Pemberton 1925, Doty 1945), but con- fields on the island of Hawaii during trols applied after harvest are likely to For Additional the early 1900s, but with no apparent have little effect on damage or yields. effect either on rat populations or on Information sugarcane damage (Pemberton 1925). Economics of Damage and Doty, R. E. 1945. Rat control on Hawaiian Shooting sugarcane plantations. Hawaiian Planters’ Control Rec. 49:72-241. This is not a practical form of popula- Fellows, D. P. and R. T. Sugihara. 1977. Food tion control. In addition to direct losses, secondary habits of Norway and Polynesian rats in Hawaiian sugarcane fields. Hawaiian Biological Control infections of stalks by insects and pathogens result in additional losses of Planters’ Rec. 59:67-86. In 1883, the Indian mongoose stalks and deterioration of cane juice. Hirata, D. N. 1977. Species composition of rats (Herpestes auropunctatus) was intro- The economic impact of these losses on Mauna Kea sugar company from 1967 through 1976. Hawaiian Sugar Technol. 1977 duced into Hawaii from the West fluctuates from year to year, largely Rep. pp. 91-94. Indies to help control rats on sugar- dependent on the prevailing price of cane plantations, and today they are Hood, G. A., R. D. Nass, and G. D. Lindsey. sugar. In 1980, when the average price 1970. The rat in Hawaiian sugarcane. Proc. common on all the major islands ex- of raw sugar was at a 50-year high, the Vertebr. Pest Conf. 4:34-37. cept Kauai. Although mongooses are Hawaiian sugarcane industry may Hood, G. A., R. D. Nass, G. D. Lindsey, and D. diurnal and rats are nocturnal, rodents have lost $20 million. Current losses N. Hirata. 1971. Distribution and comprise the major portion of the are conservatively estimated to be accumulation of rat damage in Hawaiian mongoose’s diet in and around sugar- greater than $6 million annually (A. sugarcane. J. Wildl. Manage. 35:613-618. cane fields. Pemberton (1925) found Ota, Hawaiian Sugar Planters’ Asso- Kami, H. T. 1964. Foods of the mongoose in the parts of rodents in 88% of 356 mon- ciation, pers. commun.). Hamakua District, Hawaii. Zoonoses Res. goose pellets collected in sugarcane 3:165-170. Aerially broadcasting 5 pounds of zinc fields, with 52% of all samples contain- phosphide-treated oats to 1 acre (5.6 Nass, R. D., G. A. Hod, and G. D. Lindsey. 1971. ing nothing but rodent parts. Kami Fate of Polynesian rats in Hawaiian kg/ha) of sugarcane costs approxi- (1964) reported that 72% of 393 mon- sugarcane fields during harvest. J. Wildl. mately $4.99, including $3.50 for bait, Manage. 35:353-356. goose scats collected along dirt roads $1.33 for the airplane, fuel, and pilot, adjacent to cane fields contained ro- Pemberton, C. E. 1925. The field rat in Hawaii and $0.16 for labor, transportation of dent pelage and bones. However, rats and its control. Hawaiian Sugar Planters’ materials, administrative overhead, Assoc. Exp. Stn. Bull. No. 17. 46 pp. reproduce rapidly and continue to and other expenses. The registration thrive and cause major economic dam- Sugihara, R. T., L. F. Pank, D. P. Fellows, D. N. label calls for four applications during Hirata, R. S. Stott, H. W. Hilton, and H. Kaya. age in Hawaii. Not only has the intro- the crop cycle, which would cost about 1977. Noncrop habitat manipulation as a duction of the mongoose failed to $20.00 per acre ($50.00/ha). Studies means of controlling rats and reducing control rat populations, but it has re- damage to sugarcane. Hawaiian Sugar have indicated that applications of zinc sulted in unforeseen ecological effects. Technol. Rep. pp. 83-90. phosphide reduce damage in Hawai- Mongoose predation has been impli- Tobin, M. E., and R. T. Sugihara. 1992. ian sugarcane fields by as much as 30% cated in the decline of the Hawaiian Abundance and habitat relationships of rats to 45%. Thus, four applications of zinc goose (Nesochen sandvicensis), Newell’s in Hawaiian sugarcane fields. J. Wildl. phosphide would result in savings of Manage. 56:815-821. shearwater (Puffinus newelli), and other $120 to $185 per acre ($296 to $475/ ground-nesting birds in Hawaii. If ra- Tobin, M. E., and R. T. Sugihara, and A. K. Ota. ha), or a return of $6.00 to $9.00 for 1990. Rodent damage to Hawaiian sugarcane. bies ever becomes established in Ha- every $1.00 spent applying bait. This Proc. Vertebr. Pest Conf. 14:120-123. waii, the mongoose is likely to become assumes a potential yield of 10 tons a public health concern. Tomich, P. Q. 1970. Movement patterns of field per acre (22.5 mt/ha) without applying rodents in Hawaii. Pacific Sci. 24:195-234. Between 1958 and 1961, barn owls controls, a farm price of $368 per ton Tomich, P. Q. 1986. Mammals in Hawaii, rev. (Tyto alba) also were introduced into ($409/mt), and a 10% decrease in yield ed. Bishop Museum Press, Honolulu. 375 pp. the state to help control rodent agricul- due to rat damage. The benefits of van Riper, S. G., and C. van Riper III. 1982. A tural pests. This species and the native using zinc phosphide are less in fields field guide to the mammals in Hawaii. The short-eared owl (Asio flammeus) subsist with lower damage. Oriental Publ. Co. Honolulu. 68 pp. in Hawaii in large part on rodents. Although raptors sometimes are Editors attracted to rats fleeing recently har- Scott E. Hygnstrom vested sugarcane fields, heavy thatch Robert M. Timm Gary E. Larson

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