Damage Prevention and Control Methods

Exclusion

Polynesian rat, Rattus exulans

Electric fences and physical barriers have been used to prevent rats from entering experimental farm plots. It is questionable, however, whether current 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 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 undestroy 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. Possible selection criteria include rind hardness, stalk diameter, degree and time of lodging, resistance to souring, and potential for compensatory growth.

Repellents

None are registered.

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 Polynesian 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 phosphide baits also varies. Where Norway rat populations increase during the second year of the crop cycle, zinc phosphide baits become progressively less effective.

Fumigants

None are registered for the control of Polynesian rats in Hawaii.

Trapping

Polynesian rats can be captured easily with coconut bait and standard snap traps, modified wirecage Japanese live traps, or other appropriate traps.

However, trapping in sugarcane fields is extremely labor intensive and is not practical for control purposes. Plantation personnel took an average of 141,000 rats annually from sugarcane fields on the island of Hawaii during the early 1900s, but with no apparent effect either on rat populations or on sugarcane damage (Pemberton 1925).

Shooting

This is not a practical form of population control.

Biological Control

In 1883, the Indian mongoose (Herpestes auropunctatus) was introduced into Hawaii from the West Indies to help control rats on sugar-cane plantations, and today they are common on all the major islands except Kauai. Although mongooses are diurnal and rats are nocturnal, rodents comprise the major portion of the mongoose’s diet in and around sugarcane fields. Pemberton (1925) found parts of rodents in 88% of 356 mongoose pellets collected in sugarcane fields, with 52% of all samples containing nothing but rodent parts. Kami (1964) reported that 72% of 393 mongoose scats collected along dirt roads adjacent to cane fields contained rodent pelage and bones. However, rats reproduce rapidly and continue to thrive and cause major economic damage in Hawaii. Not only has the introduction of the mongoose failed to control rat populations, but it has resulted in unforeseen ecological effects. Mongoose predation has been implicated in the decline of the Hawaiian goose (Nesochen sandvicensis), Newell’s shearwater (Puffinus newelli), and other ground-nesting birds in Hawaii. If rabies ever becomes established in Hawaii, the mongoose is likely to become a public health concern.

Between 1958 and 1961, barn owls (Tyto alba) also were introduced into the state to help control rodent agricultural pests. This species and the native short-eared owl (Asio flammeus) subsist in Hawaii in large part on rodents. Although raptors sometimes are attracted to rats fleeing recently harvested sugarcane fields, heavy thatch prevents their foraging in maturing sugarcane fields.

Dogs have also been used to control rats in harvested sugarcane fields (Pemberton 1925, Doty 1945), but controls applied after harvest are likely to have little effect on damage or yields.