愛媛県戸島

The Norway rat, Rattus norvegi cus norvegicus, proliferates rapidly under favorable conditions and causes great damage to human society. It is known both in Japan and in foreign countries. In Japan remarkable increases of the rat population occur when the dwarf-bamboo flowers and succeedingly fruc tifies covering a wide range. This provides the rats with abundunt food from the bamboo grains. As the bamboo grains become exhausted, the multitude of rats deminish and original constant numbers remain. Since last year, a rapid and marked proliferation of Norway , rats has been noticed on a small island called Tojima, situated on the westernmost part of Ehime Prefecture. Tojima mesures 12 kilometers in circumference and consists mostly of steep hills jutting directly up from the sea (Fig. 1). As the plains are very few and rice fields can not be made, the inhabitants cultivate stepped farms on the hill side where they raise sweet-potatoes and barley, which are their principal food. The plains where the inhabitants live, are very narrow therefore . they store their crops of sweet-potatoes in the hill-land, by digging here and there numerous pits in which they deposit their foodstuffs and cover them with simple primitive roofs thatched with straw (Fig. 2). The Norway rats on the island dwell in the hill area in abundanc e and grow semi-wild. They live in numerous tunnels made under the stepped farms. These farms are unfit for cultivation and haye been abandoned (Fig. 3). As this island is often attacked by monsoon rains and wind, the farmers make wind-breakers by planting reeds around their, farms. The base of the reed-fence provides a good dwelling place for the rats. We visited the island in the middle of March. Th e barley in the field had grown about twenty centimeters, but the grain around the area of the farm near the reed-fence had been attacked by the rats had not grown at all (Fig. 5). The multitudes of the menacing rats had moved down to the plains where the inhabitants live and not only attacked. the stored food in the kitchens but invaded even the bed rooms, and sometimes bit the hands or lips of sleeping people. On the night which we stayed there, 70 spring-traps were set in the drains within the range of 200 meters around the hotel, 31 rats were caught in approximately two hours between 8:30 to 11:00 p.m. The inhabitants are now trying to exterminate the rats, by using traps, rat-poisons and other available means. The cause of the' great increase of the Norway rats in Tojima -is probably due to the rich supply of food in the highlands. If each store-pit for sweetpotatoes described above could be examined closely, without exception, one or more openings of tunnels excavated by the rats would be found (Fig. 6). The rats seemed always, even in winter, to be able to enter and get ample food. Moreover cut pieces of. potatoes and other farm and marine produce are left scattered loosely throughout the island and these are also a favorite food. Though the menace was noticed quite recently, the invasion of the rats into highland fields should have been discovered sometime ago, for they seemed to have proliferated rapidly and remarkably in the past few years, as a -result of the rich food there, together with being favored by a mild climate in the district. The abnormal ly remarkable proliferations of the rats in the past have always been backed and supported by a rich supply of food. On those occasions, if one- would try to catch or kill a large number of the rats and still leave the rich food where it is, the rats will recover. to their former abuudance in a short time despite the fact that these large numbers were exterminated. Therefore; in such a case as Tojima's rat, it is .recommended that improvements of the storage place for the sweet-potatoes and the control of scattered pieces of farm and marine produce should be done first. Then the strutting rats should be destroyed, by the use of rat-poisons or any other means of disposal available.