Laurie E. Twigg

Feral pigs in north-western Australia: basic biology, bait consumption, and the efficacy of 1080 baits

Bait consumption, and the efficacy of 1080-treated grain, were determined for feral pigs (Sus scrofa) during the dry season in the Fitzroy River region of north-western Australia. There were an estimated 250 pigs on the study site (15 000-ha paddock with beef cattle) before poison-baiting, and group size and the basic biology of these pigs were similar to that found elsewhere in Australia. All animals at the study site were naive to the test baits. Fermented wheat with added blood and bone was an attractant for feral pigs but added fish oil was not. Wheat and malted barley were the ‘preferred’ baits. Lupins and pig pellets were consumed in lesser amounts, suggesting that they are less/not acceptable to some feral pigs. Consequently, the efficacy of 1080-treated wheat and malted barley was determined (n = 3 sites per treatment). Three independent measures of pig activity/abundance were used. The daily sighting index before and after poison-baiting suggested that pig numbers were decreased by at least 81–100% (mean 89%) regardless of which bait was used. The take of both 1080-bait and non-toxic fermented wheat added to each station generally ceased within 1–3 days, and little take occurred during the post-poisoning follow-up. Pig tracks decreased to zero within 1–3 days of poisoning on the two sites where track plots were established. However, due to the arrival of ‘immigrant’ pigs ~6 days after poisoning on two sites, and the need to close down a third site before poison-baiting could be completed, we believe the absolute efficacy was greater than the 89% overall reduction. Even though they had access to bait, there was no bait-take by non-target species, either native (toxic and non-toxic bait) or domestic (non-toxic bait). The 61 pig carcases found after poisoning were within 20–610 m of active bait stations (mean 232 m), and most were found in clustered groups. These findings are discussed with respect to the development of management strategies for reducing the impacts of feral pigs, and in terms of their potential implications for developing wildlife disease (exotic and endemic) contingency plans.

The Effects of Microhabitat and Weather on House Mouse (Mus domesticus) Numbers and the Implications for Management

1. Linear multiple regression analyses revealed that the abundance of house mice in and around irrigated summer crops in the Macquarie Valley, NSW was associated with mild temperatures, adequate rainfall and an abundant ground cover and seed bank. 2. Using short-term data, these analyses aided the identification of which habitat characteristics were particularly important to mice. 3. While not replacing the need for long-term data collection, this approach can utilize short-term data to assist in the development of management or control strategies for vertebrate populations.

Seasonal changes in the diet of the European rabbit (Oryctolagus cuniculus) from three different Mediterranean habitats in south-western Australia

Seasonal changes in the diet of rabbits from three temperate (Mediterranean) areas in south-western Australia were identified using microscopic determination of the percentage occurrence of various food groups in sampled stom- achs. The sites differed in soil type and in the availability of summer perennials, native vegetation bush remnants (size of, and number of plant species), improved pastures, and summer rainfall, and hence, enabled a comparison of the diet of rabbits from the different vegetation communities. Although the diet of these rabbits was quite flexible, with some switch- ing in food items occurring between seasons, there were marked differences in the proportion of monocotyledonous and dicotyledonous species eaten in each habitat. There was a strong reliance on seeds (1-5 species) during late spring and summer in all three habitats. Guildford grass (Romulea rosea) leaf and corms were a major component of the diet in the two habitats where this species was common. Further, as a result of the summer die-off of pasture species, there was a shift in where rabbits sourced food items during winter and summer. Pasture species were eaten during winter, but rabbits fed mainly on those dicotyledons found only in the surrounding scrub during summer. This suggests that rabbits may impact negatively upon such remnant vegetation at this time. Rabbits in all three habitats consumed several plant species with high water content (>54%) during summer, presumably to help maintain their water balance. Rabbits also consumed the seeds and foliage of several weed/nuisance species in each habitat, but any role of rabbits in weed dispersal was not determined.

Sensitivity of some Australian animals to sodium fluoroacetate (1080): additional species and populations, and some ecological considerations

The sensitivity to fluoroacetate (1080) of a number of species of rodents and dasyurids with and without evolutionary exposure to fluoroacetate-bearing vegetation was determined. Rattus fuscipes, and species of Pseudomys from populations with exposure to this vegetation, were particularly tolerant to fluoroacetate. However, the level of tolerance varied among the different populations of each species, depending on the degree to which the toxic plants were present in their microhabitat. The tolerance of the F1 offspring of sensitive R. fuscipes (South Australia) crossed with tolerant conspecifics from Western Australia was mid-range between those of the parental populations. The sensitivity of introduced R. rattus and Mus domesticus from areas with fluoroacetate-producing plants in Western Australia was similar to that reported elsewhere for these rodents. This suggests that their relatively short coexistence with the toxic plants has had little obvious impact on their level of sensitivity to fluoroacetate. The dibbler, Parantechinus apicalis, which coexists with the toxic vegetation, was exceptionally tolerant for a native carnivore/insectivore (LD50 ~35 mg 1080 kg–1). In contrast, however, Phascogale tapoatafa from southern Western Australia was more sensitive to 1080 than was expected, with an estimated LD50 of 7 mg 1080 kg–1. Although the level of tolerance to fluoroacetate was seen to vary depending on the level of exposure of each species/population to fluoroacetate-bearing vegetation, our findings provide further evidence of the evolutionary impact that fluoroacetate-producing plants appear to have had on the genetic composition of indigenous Australian fauna.

The epidemiology of rabbit haemorrhagic disease, and its impact on rabbit populations, in south-western Australia

The impact of rabbit haemorrhagic disease (RHD) on free-ranging rabbit populations, their immunological response, and the abundance of insect vectors, were monitored in detail in the southern agricultural region of Western Australia. Further, a broad-scale rabbit monitoring program was also established at nine locations across a rainfall gradient in the southern half of Western Australia to monitor the natural arrival, or controlled release, of rabbit haemorrhagic disease virus (RHDV). Changes in rabbit populations and the immune status of RHDV antibodies were monitored in these areas to enable further understanding of the epidemiology of RHD, and its impact on rabbit numbers. RHDV had the greatest impact on rabbit populations in the arid and semi-arid areas (<360 mm per annum), where rabbit numbers were reduced to, and maintained at, 10% of pre-RHD levels. Conversely, the effects of RHD on rabbit numbers in higher-rainfall areas (360–700 mm per annum) were highly variable and patchy, and in some instances RHD had little apparent impact. In higher-rainfall areas where RHD was effective, rabbit numbers were reduced by 50–78%. RHDV was first confirmed in the southern agricultural region of Western Australia in early September 1996, ~1 year after its escape from Wardang Island, South Australia. At the detailed monitoring site (485 mm rainfall per annum), rabbit numbers declined by 65% within 2 weeks of RHDV being detected. However, ~70% of the remaining rabbits had antibodies against RHDV, indicating that they had survived the disease. There was also a demographic shift towards young rabbits (<1 year old) at this time. Further, even though there was no clinical evidence of RHD recurring in this population during the 3-year study, the presence of IgM antibodies in some rabbits well after the initial epizootic suggests that low-level transient outbreaks of RHDV had occurred at this site. Although the impact of these low-level outbreaks on population dynamics were uncertain, rabbit numbers at this site had recovered to pre-RHD levels within two breeding seasons. The abundance of insect vectors on the detailed monitoring site was similar between years with and without RHD outbreaks. Thus, the failure of RHDV to develop clinical disease on this site after the initial epizootic was unlikely to have been caused by the lack of suitable transmission vectors. The apparent lack of disease recurrence at the detailed monitoring site may have been caused by the presence of a non-pathogenic form of RHDV, which seemed to impart at least some cross-immunity to RHDV in these rabbits. The presence of RHDV also caused a shift in the timing of natural epizootics of myxomatosis at this site.

Sensitivity to sodium fluoroacetate (1080) of native animals from north-western Australia

The sensitivity to sodium monofluoroacetate (1080) of 9 species of native animals from north-western Australia was assessed using the increasing dose procedure to determine the Approximate Lethal Dose for each species. Granivorous birds from this region (e.g. ducks, corellas) were generally more sensitive to 1080 than their counterparts from southern Australia, and would be theoretically at risk from primary poisoning during 1080 grainbased baiting programs. However, the tolerance to 1080 of birds of prey from these areas is sufficient that these species face little risk of secondary poisoning during pest-control programs aimed at rodents or rabbits. The risk of primary poisoning to raptors from meat baits containing 6 mg 1080 per bait or less also appears to be low. The coexistence of brown falcons and barn owls with fluoroacetate-bearing vegetation over parts of their range has probably contributed to their development of tolerance to fluoroacetate.

Evaluation of the association of parasitism with mortality of wild European rabbits Oryctolagus cuniculus (L.) in southwestern Australia.

Abundances of the parasitic nematodes Trichostrongylus retortaeformis and Passalurus ambiguus, and 8 Eimeria species were estimated by fecal egg and oocyst output in 12 discrete free-ranging populations of wild rabbits (Oryctolagus cuniculus) in southwestern Australia. Comparisons of parasite egg and oocyst counts were made between those rabbits known to have survived at least 2 mo after fecal samples were collected and those rabbits that did not survive. There were significant negative relationships between parasite egg and oocyst counts and survival when all age groups and collection periods were pooled for several species of coccidia and for T. retortaeformis. However, when the same comparisons were made within rabbit age groups and within collection periods, there were very few significant differences even where sample sizes were quite large. The differences indicated by the pooled analysis for coccidia were most likely due to an uneven host age distribution with respect to survival, combined with an uneven distribution of the oocyst counts with rabbit age. The result for T. retortaeformis was similarly affected but by a seasonal pattern. Parasitism by nematodes and coccidia did not appear to be an important mortality factor in these rabbit populations, at least at the range of host densities we examined. This suggests that other factors must have been responsible for the observed pattern of density-dependent regulation in these rabbits.

The effect of sodium monofluoroacetate on plasma testosterone concentration in Tiliqua rugosa (gray)

1. 1. Administration of multiple or single doses of sodium fluoroacetate (1080) to male Tiliqua rugosa caused a decrease in plasma testosterone concentration. 2. 2. A single dose of 100 or 250 mg 1080 kg-1 body weight decreased plasma testosterone by 52%. However, 25 mg kg-1 had little apparent effect on testosterone levels. When lizards were given the multiple dose equivalent of these doses over 12 days at 3 day intervals, the effect was much less dramatic with plasma testosterone concentration steadily declining over 15 days for the two higher doses. 3. 3. There was a suggestion of degeneration of seminiferous tubules in some individuals

Factors influencing the fecal egg and oocyst counts of parasites of wild European rabbits Oryctolagus cuniculus (L.) in southern western Australia

Abundance of intestinal parasites was monitored by fecal egg and oocyst counts for samples of wild rabbits Oryctolagus cuniculus with different levels of imposed female sterility from 12 populations in southwestern Australia. Differences in egg counts of Trichostrongylus retortaeformis between seasons and age groups were dependent on the sex of the host. Pregnancy may have been responsible for these differences because egg counts were consistently higher in intact females than in females surgically sterilized by tubal ligation. Egg counts for Passalurus ambiguus were influenced by season and host age but there were no differences between sexes or between intact and sterilized female rabbits. No differences were detected in the oocyst counts of the 8 species of Eimeria between male and female rabbits or between intact and sterilized females. Seasonal differences were detected in oocyst counts of Eimeria flavescens and Eimeria stiedai. The overwhelming determinant of coccidian oocyst counts was host age, with 6 species being much more abundant in rabbits up to 4 mo of age. There was a suggestion that egg counts of T. retortaeformis and oocyst counts of several species of Eimeria were reduced in populations where rabbit numbers had been depressed for at least 2 yr, but there was no evidence that short-term variations in rabbit numbers had a measurable effect on parasite abundance.

Rabbit Haemorrhagic Disease Virus: serological evidence of a non-virulent RHDV-like virus in south-western Australia

Although several different cELISAs have been used to assess the exposure of European rabbits to rabbit haemorrhagic disease (RHD), the interpretation of the results of such assays is not always straight-forward. Here we report on such difficulties, and on the likely presence of a non-virulent rabbit haemorrhagic disease virus–like virus (nvRHDV-LV) in south-western Australia. Analysis of sera collected from European rabbits at Kojaneerup (near Albany) in Western Australia provided the first serological evidence of the likely presence of a nvRHDV-LV in wild rabbit populations outside the east coast of Australia and New Zealand, before the deliberate introduction of RHDV as biological control agent in both countries. Six out of 30 rabbits (20%) sampled 1–2 months before the known arrival of RHDV at Kojaneerup were seropositive to RHD on the basis of their IgG isoELISAs. However, none of these positive samples were positive for the RHDV antibody cELISA (1 : 10), indicating likely exposure to nvRHDV-LV. Subsequent serological analysis of 986 rabbits sampled between September 1996 and August 1999 at Kojaneerup indicated that nvRHDV-LV persisted in these rabbits following the natural arrival of RHDV in September 1996. At least 10–34% of rabbits appeared to have been exposed to nvRHDV-LV during the 3-year study. The presence of nvRHDV-LV seemed to offer only limited protection to rabbits from RHDV during the initial epizootic; however, persistence of nvRHDV-LV may have mitigated further RHDV activity after this epizootic. Fewer than 1% of rabbits (9 of 986) showed evidence of RHDV-challenge during the 30 months following the initial RHDV epizootic. Furthermore, except for the epizootic in September 1996, no clinical signs of the disease were apparent in the population until RHDV was deliberately reintroduced in April 1999. Mortality of rabbits exposed to RHDV at this time appeared to be correlated with their IgG isoELISA titre.