Damage and Damage Identification

Polynesian rat, Rattus exulans

Polynesian rats are a major agricultural pest throughout Southeast Asia and the Pacific region. Crops damaged by this species include rice, maize, sugarcane, coconut, cacao, pineapple, and root crops. In the United States, sugar-cane 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 non-crop lands adjacent to sugarcane fields.

Figure 2. Sugarcane damaged by polynesian rat.

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. Damage caused by roof rats, Norway rats, and Polynesian rats is very similar. All three species chew on the internodes of growing stalks. 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. Rat depredation can be distinguished easily from that of feral pigs (Sus scrofa). Pigs chew on the entire stalk, leaving it with a shredded 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 consistent with state and federal laws and regulations.

Economics of Damage and Control

In addition to direct losses, secondary infections of stalks by insects and pathogens result in additional losses of stalks and deterioration of cane juice. The economic impact of these losses 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 (A. Ota, Hawaiian Sugar Planters’ Association, pers. commun.).

Aerially broadcasting 5 pounds of zinc phosphide-treated oats to 1 acre (5.6 kg/ha) of sugarcane costs approximately $4.99, including $3.50 for bait, $1.33 for the airplane, fuel, and pilot, and $0.16 for labor, transportation of materials, administrative overhead, and other expenses. The registration label calls for four applications during the crop cycle, which would cost about $20.00 per acre ($50.00/ha). Studies have indicated that applications of zinc phosphide reduce damage in Hawaiian sugarcane fields by as much as 30% to 45%. Thus, four applications of zinc phosphide would result in savings of $120 to $185 per acre ($296 to $475/ ha), or a return of $6.00 to $9.00 for every $1.00 spent applying bait. This assumes a potential yield of 10 tons per acre (22.5 mt/ha) without applying controls, a farm price of $368 per ton ($409/mt), and a 10% decrease in yield due to rat damage. The benefits of using zinc phosphide are less in fields with lower damage.

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