Tailor-made solutions to tackle rodent pests of rice through community-based management approaches in Cambodia
Introduction
Rodents are a major pest of rice throughout Southeast Asia, causing both pre- and post-harvest losses that can cause devastating impacts to smallholder farmers’ livelihoods and to food security (John, 2014; Singleton et al., 2010). In Cambodia, where 90% of the cultivated land is used for rice production, a rice-crop health survey conducted in 2016 recorded a mean of 9% rodent damage across four surveyed provinces, with damage levels of 57% recorded in one of the surveyed fields (Castilla et al., in press). The infliction of rodent damage to rice from the reproductive stage onwards directly translates into rice yield loss due to the inability of the rice plant to compensate in time for harvest (My Phung et al., 2010; Singleton et al., 2005b). For example, during 1996, Jahn et al. (1999) reported the occurrence of a rodent outbreak in Cambodia which caused a yield loss of 12,600 t of rice; enough to feed more than 50,000 people for a year. The potential for such high crop losses due to rodents necessitates rodent management action. However, in Cambodia, farmers often apply indiscriminate methods such as the acute rodenticide zinc phosphide, abamectin-based insecticides mixed with motor oil, and electric fencing, despite their awareness of the hazardous risks to people and other animals. The efficacies of such methods are also questionable, including the common practice of applying zinc phosphide in tropical agroecosystems (Buckle, 1999; Hoque and Sanchez, 2008).
To develop rodent management strategies that are sustainable and have minimum environmental impact, ecologically-based rodent management (EBRM) strategies are recommended (Singleton et al., 1999a). Through a solid understanding of the species composition and the biology of the pest species, as well as the ecological characteristics of the agro-ecosystem and the local farming and cultural practices, the optimal times, locations and scale of actions can be identified (Brown et al., 1999; Fiedler and Fall 1994; Leung et al., 1999; Palis et al., 2008). For example, it is known that the breeding seasons for the most important rodent pest species of rice in Southeast Asia are closely linked to rice-cropping cycles due to the abundant availability of food provided by the growing rice crop (Brown et al., 2017). Thus, EBRM strategies for rice ecosystems in this region generally include synchronous planting, community action, and extended fallow periods to reduce pest population build-up. However, an increasing pressure to produce more food with less land and labour availability is leading to intensified cropping frequency and changes to cropping systems that can pose challenges for rodent management.
In Cambodia, the introduction of faster maturing varieties and improvements in irrigation infrastructure has enabled an increase in the frequency of rice crops per year, from one wet season crop to two or three crops per year in some areas (Jahn et al., 1999), and also an expansion of rice crop production to river and lake margins as the floodwaters recede in the dry season (Frost and King, 2003). For example, between 2002 and 2012, the total annual harvested area for rice paddy increased by 50%, from 2 million to 3 million hectares (FAO, 2017). Such conditions increase the availability of food for rodents during the year, which can thereby exacerbate rodent problems to rice crops. The problem is then compounded by the limited knowledge of rice farmers on how best to manage rodents.
One effective EBRM tool for rice-based agroecosystems is the trap barrier system (TBS; Singleton et al., 2003; Singleton et al., 1998). The TBS was originally developed by Lam (1988) as a method to protect an individual farmer's field. However, it has since been adapted to include a trap crop that is planted several weeks before the surrounding crops. This is often known as the Community-TBS (CTBS) due to its ability to protect 8–10 ha surrounding the trap crop (Singleton et al., 1998, 2003). Another variant of the TBS is to apply it as a linear barrier known as a Linear-TBS (LTBS) to intercept rodent movement into or within agricultural crops. There are currently no published studies that have examined the effectiveness of this method in rice. However, a recent study in maize fields in China found it to be as effective as a CTBS applied with no trap crop (Wang et al., 2017).
In smallholder farming systems, community participation is needed for successful implementation of EBRM (Palis et al., 2008). In Cambodia, King et al. (2003) conducted a study to determine how an adaptive research approach can be applied to promote effective adoption of EBRM at the community level. Involvement of farmers in the decision making process allows them to combine local knowledge and experience with information and technology options offered by researchers. The farmers test options and then decide on how to adapt and integrate them on their farms to suit local agroecological and socioeconomic conditions. Through this adaptive process, the value of both process and technical knowledge of farmers is highlighted, helping to ensure the sustainability of the learning process in communities (King et al., 2003). Such local experimentation has been found to have a learning effect not only for farmers and researchers, but also for other stakeholders such as policy makers and service providers (Flor et al., 2017; Krupnik et al., 2012). The co-production of knowledge is intended to align these stakeholders and enable innovations, such as EBRM (Leeuwis, 2004).
In this paper, we report on two different adaptive research experiments for EBRM in two villages in Takeo province, Cambodia, where high rodent losses were previously reported. The main aim of the study was to determine whether rodent damage could be decreased and rice yields increased following implementation of integrated EBRM approaches that were locally adapted to village-specific agroecological and social conditions. We also conduct an economic assessment to determine the economic viability of each approach for the farmer.
Section snippets
Study sites
As part of an adaptive participatory research platform, field trials were established in two villages in Takeo province, Cambodia, namely Ro Vieng village in Traeng district (10°87‘N 104°77E) and Kandaul village in Batie district (11°19′N 104° 55′E). Rice farming is the primary livelihood in both villages with an average farm size of 1.2 and 1.1 ha, respectively. In Ro Vieng village, year-round irrigation allows for three rice crops per year: a dry season (DS) crop from December to March, the
Rodents captured
The majority of species caught were Rattus argentiventer. However, Rattus rattus species complex, Bandicota sp. and Mus caroli were also present. Across all three treatment sites in Ro Vieng, a total of 1234 and 914 rodents were caught during the EWS and WS, respectively (Fig. 1). In both seasons, the majority of these were caught in the CTBS and by community hunting. The CTBS was most effective from the reproductive stage onwards, with more rodents caught in the CTBS as compared to the LTBS or
Reducing rodent damage and rice yield loss
The level of rodent damage recorded in our untreated sites confirms that rodent damage to rice poses a significant threat to food security and smallholder farmer livelihoods in Cambodia (Castilla et al., in press; Jahn et al., 1999; King et al., 2003). The implementation of a CTBS along with LTBS in intensive rice monoculture and a LTBS with bromadiolone rodenticide in recession rice growing areas reduced rodent damage by at least 84% for all cropping seasons evaluated. The level of success for
Conclusions
Our results show that rodent damage to rice in Cambodia and the associated yield loss can be significantly reduced following the implementation of cost-efficient EBRM approaches that are locally adapted to village-specific agro-ecological and social conditions. By working closely with farmers in a participatory adaptive research approach, we successfully demonstrated that different rodent management options are suitable for different conditions, even within the same geographic region. To enable
Acknowledgements
The authors wish to thank team members from the Plant Protection Division, Cambodian Agricultural Research and Development Institute (CARDI) and extension staff from the General Directorate of Agriculture and the Provincial Development of Agriculture Forestry and Fisheries of Takeo that helped to coordinate activities with the farmers and collect the data. We also sincerely thank all the farmers involved in the research and Prof. Grant Singleton for his advice during the planning stage and for
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