Friday, April 6, 2012


Rats have been a destructive pest to mankind and agriculture since antiquity. They are especially important as disease carriers including plague, murine typhus, leptospirosis, and salmonellosis. The Hamakua coast area was a former plague epidemic area, but the last reported human case occurred in 1949.  Rats and other rodents consume and contaminate the foods in the homes and in the field. Their control is not easy due to their ability to adapt to changes and their capacity to reproduce.

There are four rodents of economic importance in Hawaii: the roof or black rat, the Norway or brown rat, the Polynesian or Hawaiian rat, and the house or field mouse.
Roof rat (Rattus rattus)- is a medium to large rat, 5 to 7 inches long.  Body color varies from grey to jet black; the underside grey, grey-white, or white.  They are expert climbers and can easily walk along electric wires. They frequent cane fields, macadamia nut, coffee, papaya, and banana groves and, in other part of the country, are commonly found damaging citrus groves.  They will gnaw on the bark of limbs and hallow out mature citrus fruit. They also feed on avocado fruit and have been found nibbling on a snail or two. They nest in attics, trees, palms, and dense vines.

Norway rat (Rattus norvegicus)- Largest of the rats in Hawaii, weighs 10 to 18 ounces and measurers 8 to 10 inches long. Body color reddish brown to grey to black; underside whitish color.  It is commonly found around chicken coops, in sewers, rubbish heaps and ships; also found in houses, farm buildings and warehouses, usually within one story of ground level. They are well adapted for swimming and burrowing. They are also aggressive and will drive other rats from the lower floors of buildings and will not hesitate, at times, to attack children.  

 Polynesian rat (Rattus exulans)- is the smallest of the three,  weighs 2 to 3 ounces and measures 4 to 5 inches long.  The body color is cinnamon-brown to grey, stiff black guard hairs on the back and sides, the underside light buff or grey.  This rat is basically a field rat.  Rarely found near buildings in Hawaii, it nests in gulches, rock piles, rock walls, wastelands, fields, and embankments. It can cause great damage to sugarcane, pineapple, macadamia nut, coconut, coffee, and other fruit and vegetable crops.

Rats are nocturnal. They have excellent memories and very repetitious habits.   They are easily frightened of new things placed in their environment. The roof rat is most pronounced in this tendency. Rats have a keen sense of smell and hearing, and only a fair sense of sight with the ability to see in the dark.

Common signs that rats are nearby are droppings, rub marks,  gnawings, nests, and rodent odors.  In controlling these pests, the first step is to clean up the environment by removing accesses to food and shelter.  Physical barriers such as screens may need to be installed.  When only a few rats are involved trapping can be effective.  For field problems, snap traps can be secured to limbs and baited with raisins, citrus or other fruits. Traps should be baited and left unset until bait is readily consumed. 

Paraffin-type bait blocks containing anticoagulants are very effective in controlling rats.  They appeal to the rats gnawing instinct; especially those blocks with numerous ridges.  They should be replaced immediately as they are eaten, since only a single feeding on the first generation anticoagulants (warfarin, diphacinone) will not control rats.  Baits must be eaten over a period of several successive days.  For safety, baits other than in block form, should be placed in bait stations.   Read and follow label instructions.

Some poisons have a secondary effect which will affect animals which consume dead or nearly dead rodents. Thus, it is imperative that strict safety precautions be used in the placement and disposal of poison baits for rodents.