Rodent outbreaks in the uplands of Laos: analysis of historical patterns and the identity of nuu khii.

Rodent outbreaks in the uplands of Lao PDR (Laos) are understood by traditional farmers to be triggered by the episodic and synchronised flowering and seeding of certain bamboo species. Historical data from 24 districts spread across four provinces indicate that these outbreaks have been a feature of the upland agricultural environment for at least 50 years. Although many outbreaks appear to be fairly local in scale, records from Luang Prabang and Oudomxay provinces appear to document at least one widespread and prolonged outbreak, over the period 1988–1993. Somewhat surprisingly, there is no suggestion that rodent ‘outbreaks’ have become more frequent in recent times, contrary to widespread reports that the level of chronic rodent damage to crops has increased over the last decade. This apparent ‘uncoupling’ of trends in rodent outbreaks and agricultural crop losses adds weight to the traditional perception that the outbreak events owe their origin to factors outside of the agricultural systems. A variety of rodent species are probably involved in the outbreak events. The identity of the ethnotaxon nuu khii, literally the ‘rat of bamboo flowers’, remains somewhat enigmatic. In some areas nuu khii may refer to one or more species of primarily forest-dwelling rat. However, in other areas, this term appears to describe an ecological phenomenon, namely the eruptive increase of forest rodent populations, with subsequent outpouring into adjacent agricultural landscapes. The historical records do not help identify the cause of the rodent outbreaks. The pattern of outbreaks shows no clear association with generalised El Nino Southern Oscillation cycles, and with the exception of one geographically widespread outbreak in 1988–1993, there is little to suggest a regional climatic influence of any kind. The traditional belief that rodent outbreaks occur in response to bamboo flowering events is plausible in terms of the general biology of Southeast Asian bamboos, but the historical data do not allow for any direct test of this proposition. Much more information is required on the identity, distribution and phenology of Lao bamboo species, and on the impact of mast-seeding events on small mammal communities in the Lao uplands, before this interesting and economically important ecological phenomenon can be properly assessed. Rats, Mice and People: Rodent Biology and Management 104 total area under rice cultivation (Lao PDR Ministry of Agriculture and Forestry records). Most upland rice cultivation is still based on the use of ‘slash and burn’, shifting cultivation systems. The productivity of the upland systems of production is generally in decline and problems associated with upland cultivation are on the increase (Schiller et al.1999; Roder 2001). Rodent damage and associated grain loss are cited by upland farmers as being second only to weeds as the most significant production constraints of the uplands. The damage attributed to rodents is a chronic annual problem in most of the Lao uplands. However, the severity of the problem varies from year to year and between localities. The occurrence of explosive rodent outbreaks in the uplands of Laos was reported by Singleton and Petch (1994) and Schiller et al. (1999), based on information obtained during interviews with farmers and agricultural officers, and on the returns from rat bounty systems. Schiller et al. (1999) noted that rodent outbreaks occur at “irregular intervals” but they did not speculate on the frequency of such events. Nor did they identify the outbreaking rodent species, known locally as nuu khii (literally ‘rat of bamboo flower’). Singleton and Petch (1994) suggested that the outbreaks might involve both Rattus argentiventer and a species of Mus, however this opinion was based on farmers’ descriptions rather than direct observation of specimens. In this paper, we present the results of historical investigations into rodent outbreaks in four upland provinces of Laos. The study area is characterised by shifting cultivation systems that produce a mosaic landscape of gardens, remnant forest and regrowth habitats (Roder 2001). We also shed new light on the identity of nuu khii, based on voucher collections made between 1998–2001. Although rodent outbreaks have been noted for at least 50 years, the reasons for these outbreaks are not adequately known. To gain a better understanding of the physical and biotic factors that regulate rodent population cycles and their impact on crop production systems, the analysis of historical records of rodent population fluctuations is essential. Materials and methods Our main body of historical information comes from five provinces: four of these provinces (Luang Prabang, Oudomxay, Houaphanh and Sayabouly) are in the northern agricultural region, while one (Sekong) is in the south-east of the country (southern agricultural region). Each of the provinces is unique in respect of climatic and/ or agronomic factors. Annual rainfall is highest in Sekong (>2000 mm) and lowest in parts of Luang Prabang and Sayabouly (<1440 mm) (Sisophanthong and Taillard 2000). Houaphanh is generally at a higher elevation than the other provinces and is considerably colder in the winter months than the other provinces. The ‘summer’ temperatures in Sekong are several degrees higher than in the more northerly provinces. Among the five provinces, the intensity of upland shifting cultivation is highest in Luang Prabang and Oudomxay (Sisophanthong and Taillard 2000). In Luang Prabang, Oudomxay, Houaphanh and Sekong, information was obtained from four to eight districts. The sources of information accessed were: (1) documentary records held by the provincial offices of the Ministry of Agriculture and Forestry; (2) interviews with current and former staff of these offices; and (3) interviews with farmers. In Sayabouly, information was gathered exclusively from farmer interviews. Information was sought on: the year and season of outbreaks; the rodent species involved and their approximate density; the geographical extent of outbreaks within the district; the crops affected and estimates of crop losses; and annual rainfall. Not all classes of information were obtained in all provinces or districts and the time period covered by the records also varies from 15 years (Sekong province) to 50 years (Luang Prabang and Houaphanh provinces). Estimates of crop losses are based on reported cropping areas and yields from particular districts. Not all of the yield loss is necessarily due to rodent damage, which may be compounded by other factors. On the other hand, yield loss estimates typically will not register ‘foregone’ loss where farmers fail to plant crops in anticipation of their complete destruction by rodent pests. In analysing the historical data, outbreaks that occurred in consecutive years were treated as a single, extended event. Estimates of crop losses during these extended outbreaks support this interpretation (see Results and Discussion). Rodent specimens were collected between 1998–2002 during the course of regular trapping programs in various agricultural and natural habitats in each of Luang Prabang, Oudomxay, Houaphanh and Sekong provinces. A smaller collection of voucher specimens was made in Sayabouly province in 2002. The trapping results and associated voucher specimens provide a detailed picture of the rodent communities in each province and some information on the pattern of habitat use during ‘nonoutbreak’ years (Khamouane et al., this volume). The occurrence of nuu khii outbreaks in Viengthong district of Houaphan province during 2001 provided an opportunity to collect voucher specimens directly referrable to this ethnotaxon. Results and discussion What constitutes an ‘outbreak’? The historical data provide a combined total of 155 ‘outbreak’ years across all four provinces. Estimates of crop losses during outbreaks show an exceptionally wide range, from as low as 2% to a maximum of 90% (mean + sd = 55% ± 23.5%). The low crop losses associated with some ‘outbreaks’ are intriguing, but make sense if rodent outbreak events are distinguished from chronic crop losses. Rats, Mice and People: Rodent Biology and Management