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Report on a "ratada" occurring in 2011 in western Argentina

posted Jul 20, 2011, 6:24 PM by IRRC Coordination Unit   [ updated Sep 7, 2014, 7:30 PM by Gerardo Tomasito Laviña ‎(IRRI)‎ ]

In western Argentina, the bamboo, Chusquea culeou, flowered massively last spring (October-November, 2010), from San Carlos de Bariloche to Lago Puelo (41.3° to 42.2°S). It also flowered in a separate area further to the south in the Aysen-Coihaique area of Chile (≈45°S). The bamboo is the dominant understory plant on the Península Llao Llao (25 km W of San Carlos de Bariloche) (see photos at link 1 below), where the Valdivian rainforest is well developed and close to suburban and tourist housing. The superabundance of the bamboo seed, which started falling in January (2011), prolonged the breeding season of the "Colilargo" mouse (Oligoryzomys longicaudatus) (see the calphotos links in 2 below) into late winter. The number of these mice is now very high, and is a major public health concern. This mouse is the natural host of the Andes hantavirus, and causes a serious disease (HPS - hantavirus pulmonary syndrome) in humans that become infected (see links and publications in 1 below).

To the south of Bariloche, in the Río Manso area, mice left the forest in large numbers (the’exodus’ phase) and invaded barns and houses (see Picasa link in 3 below) during June and July, and there are cases of HPS in the infested areas at Aysen, Chile. Although their numbers ‘crashed’ at the beginning of August, the newspapers and television stations reported on the exodus event (see links in 4 below) when they were still so abundant. To the west of Bariloche the exodus phase began at the beginning of August in the area from Lago Frías (Argentina) to Lago Todos Los Santos (Chile). At Puerto Blest, only 5 km from Lago Frías the mice were still in the forest at the beginning of August, but at the end of the second week of August they began to leave the forest in high numbers. The differences in the beginnings of the exodus phase over such a short distance (i.e., between Lago Frías and Puerto Blest) is surprising.

At the beginning of the second week of September, in the Bariloche area the mice are still in the forests and the public is still focused (for now) on the catastrophe that the eruption of the Puyehue Volcano caused to the tourist business. It erupted on the 4-5th of June, 2011, and spewed out large, sand-grain sized ash during the first 24 hours. Since then the volcano continues to fume and send out much smaller, and more irritating dust particles. In the initial fall, about 2 cm of ash covered the forest floor at Llao Llao (see pictures in link 1 below). This small amount of ash did not have a negative effect on the mouse populations, as there were pregnant mice as late as the third week in August, and these females must have ovulated and implanted well after the ash fall. The delay in the ‘exodus’ phase near Bariloche may be because this area is at a higher elevation, and with colder temperatures than the localities further to the south and west, and so it is a bit ‘behind’ in the sequence. When the mice ‘swarm’ out of the forests and into the adjacent suburban areas and lakes (see pictures in link 2 below) then the photogenic aspect of “hordes of mice“ in the news will cause further difficulties for the tourist industry that is at the heart of the Bariloche economy.

  1. For continuing information on the bamboo flowering, the associated rodent outbreak near Bariloche, pictures and videos of the animals, and literature visit: https://sites.google.com/site/floracioncania/
  2. Photos of the “Colilargo” mouse, and their high numbers during a previous flowering to the north of Bariloche at: 
  3. Videos and photos of “hordes of Colilargo mice” at: https://picasaweb.google.com/113518369341044306238/RatadaRioMansoInferior?authuser=0&authkey=Gv1sRgCO2Z276iyLyWcg&feat=directlink
  4. Newspaper and television accounts of the ‘ratada’ and HPS at: