Rodent Research Center, Laguna, Philippines : 1972 annual progress report

Examination of 100 hills in each of 1693 sample rice paddies during the 1971 wet season and 1972 dry season revealed that approximately 2.5 percent of the tillers (stems) had been cut by rats. More than 90 percent of the paddies surveyed had rat damage; 5.4 percent of the paddies showed more than 10 percent of the tillers cut at the time of harvest. Tiller-cutting by rats ranged from 0 to 40 percent in the sample. Preliminary field tests of sustained baiting with chronic toxicants, coupled with mid-crop exposure of zinc phosphide, were made at four small (3 to 5 hectares) rice farms. The rats readily accepted anticoagulant bait, and apparently, many rats from peripheral areas were drawn to the baited areas along with resident animals. The effect of drawing in outside animals resulted in the consumption of rather large amounts of bait from the stations and appeared to create a zone of reduced damage around the baited area. In each case, zinc phosphide was introduced at a time when early damage was noticed despite the anticoagulant baiting; in each case, this damage stopped at about the same time or shortly after the zinc phosphide treatment. However, we are still uncertain whether this noticeable early damage stopped spontaneously or as a result of the treatments with acute or chronic toxicants. Damage at each location was lower than expected when compared with damage to the adjacent untreated areas or to previous crops on the same areas.