Grant R. Singleton

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

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MOVEMENTS AND SOCIAL ORGANIZATION OF WILD HOUSE MICE (MUS DOMESTICUS) IN THE WHEATLANDS OF NORTHWESTERN VICTORIA, AUSTRALIA

Abstract From September 1996 to May 1997, 187 wild house mice (Mus domesticus) were fitted with radiotransmitters at an agricultural site in the wheatlands of northwestern Victoria, Australia, to examine movements and social organization. Males had slightly larger home-range areas than females. Home-range size was highly variable (0.0002–8.024 ha) but could not be predicted from body size or body condition in males and females, or by whether females were breeding. Mice were site-attached during the breeding season, with extensive intersexual overlap of home ranges but variable intrasexual overlap. Home ranges were significantly larger during the nonbreeding season compared with the breeding season. Evidence existed for exclusive home-range use by females at all densities of mice, low to moderate home-range overlap for males when densities were low and increasing, and an apparent switch to a more gregarious phase in male mice when the breeding season ceased and densities were high. Nonbreeding mice seemed to be nomadic when densities were low, which is consistent with an earlier study of home ranges and social organization of mice on the Darling Downs, Queensland.

Movements of feral house mice in agricultural landscapes.

From September 1992 to May 1993 we radio-collared 155 house mice (Mus domesticus) on agricultural fields in southern Queensland to measure movements and to determine social organisation. During the breeding season most individuals were site-attached and home ranges of both sexes overlapped extensively. There was no sign of exclusive space use for breeding individuals. Breeding males had home ranges that were larger than those of breeding females (0.035 ha v. 0.015 ha), and moved about more. After breeding ended, home ranges increased over tenfold in area, and most mice became nomadic and not site-attached.

Fertility control of wild mouse populations: the effects of hormonal competence and an imposed level of sterility

We report on a study of confined populations of wild mice in which 67% of females were surgically sterilised to simulate the possible effects of fertility control on population dynamics. Social structure can influence the breeding performance of female mice and, as this may be hormonally controlled, we examined whether the maintenance of hormonal competence by sterilised female mice was necessary to achieve a significant decrease in population size. We compared two methods of surgical sterilisation – tubal ligation, which leaves the animal’s reproductive hormone regulation intact, and ovariectomy, which disrupts the normal regulation of the hormones of the pituitary–ovarian axis. There was no difference in the population sizes produced by the two methods of sterilisation and thus the maintenance of hormonal structure is unlikely to influence the population’s response to fertility control. If anything, the population response to the presence of hormonally competent but sterile females was different from that expected – populations with tubally ligated females had slightly higher growth rates, recruitment of young, and breeding performance, than populations with ovariectomised females. The 67% level of infertility amongst females in the population successfully reduced population size and growth rate when compared with unsterilised populations. This reduction in population size was not related to the level of sterility imposed. Compensation occurred through improved breeding performance of unsterilised females, particularly in the tubally ligated populations.

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.

ESTIMATING THE ABUNDANCE OF MOUSE POPULATIONS OF KNOWN SIZE: PROMISES AND PITFALLS OF NEW METHODS

Knowledge of animal abundance is fundamental to many ecological studies. Frequently, researchers cannot determine true abundance, and so must estimate it using a method such as mark-recapture or distance sampling. Recent advances in abundance estimation allow one to model heterogeneity with individual covariates or mixture distributions and to derive multimodel abundance estimators that explicitly address uncertainty about which model parameterization best represents truth. Further, it is possible to borrow information on detection probability across several populations when data are sparse. While promising, these methods have not been evaluated using mark-recapture data from populations of known abundance, and thus far have largely been overlooked by ecologists. In this paper, we explored the utility of newly developed mark-recapture methods for estimating the abundance of 12 captive populations of wild house mice (Mus musculus). We found that mark-recapture methods employing individual covariates yielded satisfactory abundance estimates for most populations. In contrast, model sets with heterogeneity formulations consisting solely of mixture distributions did not perform well for several of the populations. We show through simulation that a higher number of trapping occasions would have been necessary to achieve good estimator performance in this case. Finally, we show that simultaneous analysis of data from low abundance populations can yield viable abundance estimates.

Towards sustainable management of rodents in organic animal husbandry

From 26 to 28 May 2004 an international seminar was held in Wageningen, the Netherlands, about current knowledge and advice on rodent management on organic pig and poultry farms in Western Europe. This paper summarizes the discussions. Rodent management is necessary to protect the food production chain from health hazards to livestock and humans. Some organic farmers prefer biological rodent control, but since rodents can also transmit diseases this bears certain risks for the production of healthy livestock and safe food. Effective rodent management requires a thorough understanding of the biology of the pest species concerned. These can be divided into two groups: field rodents, such as voles, and commensal rodents like house mice and rats. The objective of managing field rodents is to mini- mize livestock exposure to these vectors, and to regulate their populations in case their density is expect-

An experimental field study to evaluate a trap-barrier system and fumigation for controlling the rice field rat, Rattus argentiventer, in rice crops in West Java

Abstract The effectiveness of a trap-barrier system (TBS), which enclosed a crop planted 2–3 weeks early (trap-crop), and fumigation (sulfur dioxide) was assessed for managing pre-harvest damage by the rice field rat, Rattus argentiventer , to rice crops in West Java, Indonesia. The TBS was a 50 × 50 m plastic fence with live-multiple-capture traps inserted intermittently at its base. Damage to tillers and yield loss were assessed within the trap-crop and at 5, 50, 100, 150 and 200 m from the TBS. Two crops were monitored: dry season crop when rat densities were high and 20–55% of rice tillers were cut by rats; wet season crop when rat densities were low and 0–4% of rice tillers were cut. Over the two crops, rats caused a 20% annual loss in potential rice production. The benefit—cost ratios for using a TBS were in the range of 20:1 to 7:1 for the dry season and 7:1 to 2:1 for the wet season. Fumigation was not effective in reducing rat losses. Damage assessment provided a phenology of rat damage for the two crops but, unlike the yield data, differences were not significant between treatments. The benefits of the TBS need to be weighed against high labour input, initial cost, logistics of growing a trap-crop, and whether the technology can be transferred to growers. Research on how rats respond to a TBS-plus-trap-crop is required before it can be recommended to manage rats.

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 2u2009:u20091 benefit to cost ratio for winter cereals, 9u2009:u20091 for rice and 4u2009:u20091 for soybeans.