Peter R. Brown

Mice, rats, and people: the bio‐economics of agricultural rodent pests

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Reproductive changes in fluctuating house mouse populations in southeastern Australia

House mice (Mus domesticus) in the Victorian mallee region of southeastern Australia show irregular outbreaks. Changes in reproductive output that could potentially drive changes in mouse numbers were assessed from 1982 to 2000. Litter size in females is positively correlated with body size. When standardized to an average size female, litter size changes seasonally from highest in spring to lowest in autumn and winter. Litter size is depressed throughout breeding seasons that begin when the abundance of mice is high, but is similar in breeding seasons over which the abundance of mice increases rapidly or remains low. Breeding begins early and is extended on average by about five weeks during seasons when mouse abundance increases rapidly. The size at which females begin to reproduce is larger during breeding seasons that begin when mouse abundance is high. An extended breeding season that begins early in spring is necessary for the generation of a house mouse plague, but it is not in itself sufficient. Reproductive changes in outbreaks of house mice in Australia are similar but not identical to reproductive changes that accompany rodent population increases in the Northern Hemisphere. We conclude that food quality, particularly protein, is a probable mechanism driving these reproductive changes, but experimental evidence for field populations is conflicting.

Ecologically based management of rodents in the real world: applied to a mixed agroecosystem in Vietnam.

Rodents cause significant damage to lowland irrigated rice crops in the Red River Delta of Vietnam. A four-year study was conducted in 1999-2002 to examine the effectiveness of applying rodent control practices using the principles of ecologically based pest management. Four 100-150 ha study sites adjacent to villages were selected and farmers on two treated sites were asked to follow a set of rodent management practices, while farmers on the untreated sites were asked not to change their typical practices. Farmers on the treated sites were encouraged to use trap-barrier systems (TBSs; 0.065-ha early planted crop surrounded by a plastic fence with multiple capture traps; one TBS for every 10-15 ha), to work together over large areas by destroying burrows in refuge habitats soon after planting (before the rats reestablish in the fields and before the onset of breeding), synchronizing planting and harvesting of the their rice crops, cleaning up weeds and piles of straw, and keeping bund (embankment) size small (<30 cm) to prevent burrowing. A 75% reduction in the use of rodenticides and plastic barrier fences (without traps or an early crop) was achieved on treated sites. The abundance of rodents was low after implementation of the management practices across all sites. There was no evidence for an effect of treatment on the abundance of rodents captured each month using live-capture traps, and no difference in damage between treatments or in yields obtained from the rice crops. Therefore, ecologically based rodent management was equally effective as typical practices for rodent management. Farmers on the treated sites spent considerably less money applying rodent control practices, which was reflected in the comparative increase in the partial benefit:cost of applying ecologically based rodent management from 3:1 on treated sites and untreated sites prior to the implementation of treatments to 17:1 on treated sites in the final year of the project.

Impacts of rodent outbreaks on food security in Asia

Since 2007, a spate of rodent outbreaks has led to severe food shortages in Asia, affecting highly vulnerable and food-insecure families. Little has been documented about wildlife-management issues associated with these outbreaks. The aims of the present study were to synthesise what we know about rodent outbreaks in Asia, and identify important gaps in our knowledge. We compiled information from agencies of the United Nations, non-government organisations and the authors. The authors conducted site visits to areas affected by outbreaks of rodent populations, and convened an international conference in October 2009 to share knowledge. Bamboo masting is clearly implicated as the primary cause of the rodent-population outbreaks that led to severe food shortages in Mizoram (India), Chin State (Myanmar), Chittagong Hill Tracts (Bangladesh) and upland provinces of Lao PDR. In Laos, emergency food assistance was required for 85 000–145 000 people. In 2009, high rodent losses occurred also in lowland irrigated rice-based systems in the Philippines, Myanmar and Indonesia, not related to bamboo masting. Asynchronous or aseasonal growing of rice crops was a common element in these outbreaks. In the Ayeyarwaddy delta, Myanmar, 2.6 million rats were collected in 3 months through community activities; this outbreak appeared to be related to an extreme climatic event, Cyclone Nargis. There are two key features of rodent outbreaks that make the future uncertain. First, climate change and extreme climatic events will increase impacts of rodents on agricultural production. Second, there is food-security pressure in some countries to grow three crops per year. Increased cropping intensity will reduce fallow periods and create ideal conditions for rodents to breed nearly continuously. Implications of the research are as follows: (i) rodent outbreaks are a consequence of enhanced reproduction and natural mortality is of minor importance, particularly with rapidly increasing populations; therefore, we need to focus more on methods for disrupting reproduction; (ii) a stronger understanding of the ecology of pest species and community dynamics will enable ecologically sustainable management; (iii) we need landscape approaches that focus on crop synchrony, and timely coordinated community action to manage pest species and conserve desirable species; and (iv) a simple monitoring system can help implement ecologically based rodent management.

Managing Murray-Darling Basin livestock systems in a variable and changing climate: challenges and opportunities.

The key biophysical impacts associated with projected climate change in the Murray–Darling Basin (MDB) include: declines in pasture productivity, reduced forage quality, livestock heat stress, greater problems with some pests and weeds, more frequent droughts, more intense rainfall events, and greater risks of soil degradation. The most arid and least productive rangelands in theMDBregion may be the most severely impacted by climate change, while the more productive eastern and northern grazing lands in theMDBmay provide some opportunities for slight increases in production. In order to continue to thrive in the future, livestock industries need to anticipate these changes, prepare for uncertainty, and develop adaptation strategies now. While climate change will have direct effects on livestock, the dominant influences on grazing enterprises in the MDB will be through changes in plant growth and the timing, quantity and quality of forage availability. Climate change will involve a complex mix of responses to rising atmospheric carbon dioxide levels, rising temperatures, changes in rainfall and other weather factors, and broader issues related to how people collectively and individually respond to these changes. Enhancing the ability of individuals to respond to a changing climate will occur through building adaptive capacity. We have, via secondary data, selected from the Australian Agricultural and Grazing Industries Survey, built a national composite index of generic adaptive capacity of rural households. This approach expresses adaptive capacity as an emergent property of the diverse forms of human, social, natural, physical and financial capital from which livelihoods are derived. Human capital was rated as ‘high’ across the majority of theMDBcompared with the rest of Australia, while social, physical and financial capital were rated as ‘moderate’ to ‘low’. The resultant measure of adaptive capacity, made up of the five capitals, was ‘low’ in the northern and central-west regions of the MDB and higher in the central and eastern parts possibly indicating a greater propensity to adapt to climate change in these regions.

Can farm-management practices reduce the impact of house mouse populations on crops in an irrigated farming system?

The impacts of a range of farm-management practices on house mouse (Mus domesticus) populations were tested in a large replicated field study in a complex irrigated farming system in southern New South Wales, Australia. An advisory panel, made up of farmers, extension officers, industry representatives and scientists developed a series of best-practice farm-management actions to minimise the impact of mice. Twelve experimental sites were split into six treated sites, where farmers were encouraged to conduct the recommended practices, and six untreated sites, where farmers conducted their normal farming practices. Mouse abundance was generally low to moderate for the 4-year project (5–60% adjusted trap success). We found significant reductions in population abundance of mice on treated sites when densities were moderate, but no differences when densities were low. Biomass of weeds and grasses around the perimeter of crops were significantly lower on treated sites because of applications of herbicide sprays and grazing by sheep. We could not detect any significant difference in mouse damage to crops between treated and untreated sites; however, levels of damage were low (<5%). Yields of winter cereals and rice crops were significantly higher on treated sites by up to 40%. An analysis of benefits and costs of conducting farming practices on treated sites compared with untreated sites showed a 2 : 1 benefit to cost ratio for winter cereals, 9 : 1 for rice and 4 : 1 for soybeans.

Pre-sowing control of house mice (Mus domesticus) using zinc phosphide: efficacy and potential non-target effects

Zinc phosphide was tested on populations of house mice in cereal stubble and pasture paddocks in the Central Mallee region of Victoria, in Autumn 1997. There were three replicates of two application methods: aerial and ground (perimeter) baiting. The response of mouse populations to baiting was monitored by live-trapping; estimates of population size and survivorship were compared between baited and unbaited sites (n = 3) taking prebaiting population sizes into account. Zinc phosphide was effective in decreasing the abundance (adjusted trap success) of mice on aerially baited sites (by 51%), but the reduction observed on ground-baited sites (24%) was not significant. There was no change in abundance on the untreated sites. There was a significant reduction in the survivorship of mice on both aerially and ground-baited sites compared with unbaited sites. Non-target species were monitored before and after baiting. Only four bird deaths were recorded as a result of the baiting program. Given that birds are highly mobile, with deaths possible many kilometres from the bait sitesit was difficult to fully assess the impact of poisoning on bird species in the area without more rigorous searching of vegetated areas further from baited paddocks.

Farmers' knowledge, attitudes, and practices for rodent management in Myanmar

Abstract A survey of 350 farmers was conducted in the lowland rainfed agricultural system of central Myanmar (Burma) to examine the importance of rodents, farmers’ perception of the causes of yield loss, and their beliefs as to why they undertake rodent management. Farmers grew monsoon rice, summer rice and mungbeans with the major constraints upon production identified as pests (29.4% of respondents), followed by insufficient water (19.4%). The main pests were insects (48.6%), followed by rats (40.9%); however, farmers thought that rats caused most damage to their crops (47.7% of respondents; insects 30.3%), and were the most important pest to control. Farmers estimated that rodents caused 13% yield loss. Most farmers believed they could increase yields if they were to work together to control rats, but 61.1% applied rodent control individually. Most farmers (81%) thought that rodenticides were not safe despite 33% using them. Rodents were clearly identified as a significant problem by farmers. With an understanding of the main rodent pest species, ecology and crop damage, ecologically based management strategies for the lowland rainfed rice system in Myanmar can be developed.

Can outbreaks of house mice in south-eastern Australia be predicted by weather models?

Outbreaks of house mice (Mus domesticus) occur irregularly in the wheat-growing areas of south-eastern Australia, and are thought to be driven by weather variability, particularly rainfall. If rainfall drives grass and seed production, and vegetation production drives mouse dynamics, we should achieve better predictability of mouse outbreaks by the use of plant-production data. On a broader scale, if climatic variability is affected by El Nino-Southern Oscillation (ENSO) events, large-scale weather variables might be associated with mouse outbreaks. We could not find any association of mouse outbreaks over the last century with any ENSO measurements or other large-scale weather variables, indicating that the causal change linking mouse numbers with weather variation is more complex than is commonly assumed. For the 1960-2002 period we were only partly successful in using variation in cereal production to predict outbreaks of mice in nine areas of Victoria and South Australia, and we got better predictability of outbreaks from rainfall data alone. We achieved 70% correct predictions for a qualitative model using rainfall and 58% for a quantitative model using rainfall and spring mouse numbers. Without the detailed specific mechanisms underlying mouse population dynamics, we may not be able to improve on these simple models that link rainfall to mouse outbreaks.

Ecologically based management of rodents in lowland irrigated rice fields in Indonesia.

Context.Overabundantrodentscauseconsiderablecropdamageand,indevelopingcountriesofSouth-eastAsia,rodents can be an obstacle to attempts at alleviating poverty. Management is often based on the use of chemicals that can harm non-target species. Therefore, an effective and environmentally benign management approach such as ecologically based rodent management (EBRM) is desirable. Aims. We compared the effectiveness of EBRM to that of conventional management on populations of rice-field rats (Rattus argentiventer). Methods. The study was conducted as a large-scale replicated field trial in lowland irrigated rice fields in West Java, Indonesia. EBRM actions included habitat manipulations, removal of rats with trap barrier systems, coordinated rat-control campaigns and synchrony of cropping on the village level. We measured abundance, population structure, and breeding of rice-field rats as well as rice production and crop damage caused by rats. Key results. Although there was no overall effect of the EBRM treatment on rat abundance, we found decreasing rat abundanceinrice-fieldhabitatsatthelatecroppingstageintreatedvillagesandadecreaseinbodysizeofrats.Inaddition,we foundfewerreproducingfemaleswhenEBRMwasappliedthanwiththeapplicationofconventionalmethods,whereasmale reproductive condition did not decrease. Overall, there was a reduction in mean crop damage when EBRM was applied (4.4 � 0.4% in treatments v. 2.5 � 0.4% in experimental controls), which translated into 6% higher rice production. Conclusions. The results demonstrated that EBRM is an appropriate approach to manage overabundant rodents in irrigated lowland rice-based agro-ecosystems and possibly in other agro-ecosystems. This will provide substantial benefits for smallholder farming communities in developing countries and most likely benefits for ecosystem health. Implications.The EBRM approach should be used routinely in irrigated lowland rice crops that are at risk of damage by rice-field rats.