Jens Jacob

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.

Is reproduction of the Australian house mouse (Mus domesticus) constrained by food? A large-scale field experiment

Food quantity and especially food quality are thought to be key factors driving reproductive changes in the house mouse, Mus domesticus, leading to outbreaks of house mouse populations in the Australian grain-growing region. Characteristic changes during an incipient mouse plague are an early start of breeding, a high proportion of females breeding at a young age and a prolonged breeding season. We conducted a large-scale food manipulation during an incipient mouse plague, which started with early breeding and relatively high spring numbers of mice. We measured background food availability in four farms throughout the study and conducted a food manipulation experiment from November to March in two of them. After harvest in December 100–200xa0kg/ha spilled grain remained in the stubble. This was depleted by March. In two treatment farms we added high-protein food pellets on a weekly basis between November and March and two farms served as controls. We measured changes in mouse numbers by capture-mark-recapture trappings and changes in reproduction by scoring embryos and recent placental scars at necropsy. Mouse numbers did not differ between treatments and controls. There were no differences in the litter size or the proportion of females breeding between treatments and controls. We observed the normal pattern of high litter size in spring and decreasing litter size towards the end of summer in treatments and controls. In all farms reproduction stopped in March. Mouse numbers were high but not at plague densities. Contrary to our prediction we did not observe food constraint affecting the reproduction of female mice. Our field experiment seems to rule out food quality as the driving factor for improved reproduction and formation of an outbreak of mice. We suggest that physiological mechanisms in mice might not enable them to take advantage of food with a high protein content in arid summers in southeastern Australian grain fields because of the lack of free-standing water.

What Affects Bait Uptake by House Mice in Australian Grain Fields

The main method of pest-rodent control is the distribution of rodenticide baits. Emerging management techniques such as fertility control also may rely on the distribution of bait, but we do not know what affects bait uptake by rodents in crop fields. From October 2000 to February 2001, we measured the importance of individual characteristics (sex, weight, length, reproductive status), mouse abundance, and environmental conditions (vegetation height and cover and food quantity and quality) for free-fed pellet bait uptake by house mice (Mus domesticus). Our study was conducted in a grain-growing region of southeastern Australia using pellet bait that contained Rhodamine B (RB) as a bait marker. High bait uptake was noted when fenceline vegetation was high and pellets were spread at high densities. Lower uptake was noted when the grain crop had high protein content. Vegetation cover, mouse abundance, the amount of spilled grain after harvest, and crop height had no impact on bait uptake. The proportion of RB-positive mice decreased during removal trappings, indicating effective removal of residents. Lactating females were more likely to be RB positive than nonlactating females, possibly because of higher food intake during lactation. Because many females (79.9%) ate bait pellets, bait formulations may be used to effectively deliver fertility-control agents targeted at female house mice.

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.

Rats on the run: removal of alien terrestrial predators affects bush rat behaviour

Predators can strongly influence the microhabitat use and foraging behaviour of prey. In a large-scale replicated field experiment in East Gippsland, Australia, we tested the effects of reduced alien red fox (Vulpes vulpes) and alien wild dog (Canis lupus familiaris) abundance (treatment) on native bush rat (Rattus fuscipes) behaviour. Bush rats are exposed to two main guilds of predators, namely mammalian carnivores and birds of prey. Tracking rat movements using the spool-and-line technique revealed that, in treatment sites, rats used ground cover, which provides shelter from predators, less often than at unmanipulated fox and wild dog abundance (non-treatment sites). In treatment sites, rats more frequently moved on logs where they would have been exposed to hunting foxes and dogs than in non-treatment sites. Furthermore, in treatments, rats showed a preference for understorey but not in non-treatments. Hence, bush rats adapted their behaviour to removal of alien terrestrial predators. Giving-up densities (GUDs) indicated no treatment effects on the marginal feeding rate of bush rats. Interestingly, GUDs were higher in open patches than in sheltered patches, suggesting higher perceived predation risk of bush rats during foraging at low versus high cover. The lack of treatment effects on GUDs but the clear response of bush rats to cover may be explained by the impact of predators other than foxes and wild dogs.

Ecologically-based rodent management: its effectiveness in cropping systems in South-East Asia

Ecologically-based rodent management (EBRM) has re-emerged as a paradigm for large scale rodent management campaigns. This concept has been tested in replicated, village-scale experiments over 4 years on rodent pests in lowland irrigated rice crops. In Indonesia, villages that practised EBRM had a mean increase in rice yield of 6%, whereas production levels were maintained in Vietnam but control costs were reduced. In both countries there was a substantial reduction in rodenticide use in villages practising EBRM. These studies provide strong evidence of the effectiveness of EBRM. The findings also highlighted the need to include end-users early in the development of management strategies. Lessons learned from these studies were extrapolated to the development of strategies for rodent management in intensive organic piggeries and poultry holdings in Europe. The challenge for these producers is not only the identification and then integration of different management actions based on our understanding of the ecology of specific rodent species, but also the integration of ecology, sociology and economics.

Spatial distribution of feral house mice during a population eruption

Abstract: Seasonal movements of rodents in agro-ecosystems from refuge habitats to impact habitats could reflect tracking of resources by mice and may be linked to population eruptions. We monitored the abundance of house mice in refuge habitat (fencelines) and impact habitats (crops) at four farms in southeastern Australia using capture-mark-release trappings during a population eruption. For most of the year, mice did not prefer fencelines to the adjacent “sea of grain crops”, but 3 to 4 months post-harvest more mice populated refuge habitats than impact habitats. This preference for refuge habitat coincided with a considerable increase in mouse abundance and the depletion of food in the stubble of the harvested crops. Therefore, habitat choice by mice seems to predominantly track resources such as food and shelter. The percentage of recaptures within months was highest in the crop habitat, indicating higher site fidelity for mice caught there. Mice captured in crop habitats were generally representative of the demographic structure of those mice living along the crop margin. Six months post-harvest, mice living in crops were smaller, and possibly lower-quality emigrants from the fencelines. Foraging movements measured with the fluorescent biomarker Rhodamine B commonly extended about 20-30 m from the fenceline into the crop.

Shifting age structure of house mice during a population outbreak

A technique to age wild house mice, Mus domesticus, in Australia using the dry weight of the eye lens based on known-age mice from semi-natural enclosures is described and presented for 3-32-week-old mice. At four sampling periods from November 2000 to September 2001, the age frequency distributions of free-living house mice were determined using this relationship. The distributions of ages shifted between seasons from relatively young animals at the beginning of the breeding season (November 2001), coinciding with low mouse abundance, to progressively older distributions in each sample as breeding continued, ending with the cessation of breeding and a population crash before the last sample. No significant difference was detected between the sexes at any of the four periods. These results are consistent with the suggestion that the formation of mouse outbreaks requires a shift in age structure towards younger mice.

Who eats first? Uptake of pellet bait by target and non-target species

House mice (Mus domesticus) are an important vertebrate pest in Australian agriculture. We studied the uptake of non-toxic placebo bait pellets targeted on house mice from bait stations in the grain-growing region of southeastern Australia. Bait stations allowed access for either ants; ants and mice or ants, mice, and birds. Soy meal bait pellets offered in December were of low preference for both ants and mice, but were eaten by birds in one study plot. In January, there were no differences between bait stations in the amount of wheat bait pellets removed indicating that the pellet bait had been primarily removed by ants. Most pellet bait was removed during the first 12 h after distribution. The results indicate that non-target species consumed pellet bait quicker than house mice. This has to be taken into account if similar bait pellets are to be used as carriers of lethal or sterility agents to manage house mouse populations.

DOES CONTINUOUS REMOVAL OF INDIVIDUALS SEPARATE HIGH- AND LOW-QUALITY RICEFIELD RATS?

In many open populations of small rodents such as multimammate rats (Mastomys natalensis; Mwanjabe and Leirs 1997) and voles (Microtus spp.; Krebs et al. 1976, Sullivan 1979), conspecifics rapidly reinvade an area after the removal of residents. Rapid recolonization may act through 2 processes: (1) True dispersers immigrate into the area (Halle 1993). In some small mammal species predominantly juveniles and subadults disperse (Andreassen and Ims 2001, Sullivan et al. 2001), and dispersing red-backed voles (Clethrionomys gapperi) can be reproductively inferior to residents (Schieck and Millar 1987). (2) Conspecifics living adjacent to removal areas may occupy the area by expanding their territories (Boutin et al. 1985, Schieck and Millar 1987). Rapid recolonization can be a major problem for the management of overabundant populations if the removal techniques are short term (Stenseth 1977). Continuous removal of individuals may minimize rapid recolonization, but it could profoundly alter the population’s composition and may in fact separate individuals of high quality (breeding adults of good body condition) from individuals of low quality. If residents were removed and immigration occurred quickly, immigrants would be present in the population and create a bias towards individuals of low quality, assuming that immigrants are of low quality (e.g., reproductively inferior or of low body mass). In addition, one would expect a male bias because dispersal in mammals is often biased towards young males (e.g., Bollinger et al. 1993, Clout and Efford 1984, Bollinger et al. 1993). Trap barrier systems (TBS) were introduced to some Southeast Asian countries for rodent management (Lam 1988, Singleton et al. 1999a). A TBS consists of traps associated with a plastic fence, which encloses an early planted lure crop. TBS allow continuous removal of rodents from rice fields from the time the farmers’ crop is transplanted to the time it is ripening (Singleton et al. 1999b). As with many trapping systems, it is unclear whether residents or dispersers enter the TBS traps and whether the composition of the surrounding populations changes because of the continuous removal of individuals. If a TBS removed mainly dispersers of low quality and left the resident population intact, the quality of residents near a TBS would be similar to rats in areas without a TBS. If a TBS removed resident rats also, replacement by immigrants may occur and result in the presence of individuals of lower quality in areas near the TBS compared to areas with no TBS. We used TBS to remove individuals from populations of ricefield rats (Rattus argentiventer). Ricefield rats occur throughout Southeast Asia. They are the most common species of rodent in lowland irrigated rice agro-ecosystems in West Java, Indonesia. There, they cause 10–20% pre-harvest damage to rice crops (Geddes 1992). The breeding season of ricefield rats is closely linked to the crop stage (Lam 1983). It starts spatially synchronized about 1–2 weeks before the maximum tillering stage of the rice crop, lasts 7–9 weeks per cropping season, and finishes soon after harvest (Leung et al. 1999). Female ricefield rats are territorial with home ranges of 1–2 ha (Tristiani et al. 2000). We compared the demographic characteristics of ricefield rats removed with TBS and of residents sampled in areas with and without a TBS to determine whether TBS remove mainly residents or dispersers and whether continuous removal separates highand low-quality individuals.