Summary 1.Additional methods are needed in Australia to control the European rabbit Oryctolagus cuniculus , which continues to destroy valued native flora. A control option under development, immunocontraception, is intended to suppress the rabbit's high fertility. It would spread contagiously via genetically modified myxoma virus and European rabbit fleas Spilopsyllus cuniculi . An experiment with field populations of rabbits assessed whether suppressing fertility reduces their abundance. 2. In south-eastern Australia, four treatments in three replicates were applied to 12 subpopulations of rabbits. The treatments were surgical sterilization of 0%, 40%, 60% and 80% of the adult and juvenile females trapped before the annual breeding seasons of 1993-96. 3. The sterilized populations produced fewer young but the average adult population size remained unchanged in all treatments. Immigration was minimal in all treatments. 4. Sterilized adult female rabbits survived much better than fertile females, indicating a high cost of reproduction. Immature rabbits and unsterilized adults of both sexes also survived better in the sterilization treatments. The improved survival in all rabbit classes compensated for reduced reproductive output. 5. Fleas were fewer on both adult females and males in the sterilized populations but this did not impede transmission of myxomatosis. 6. Synthesis and applications . Imposing sterility on rabbit populations reduces breedingseason peaks of abundance. Improved survival compensates for the sterility of up to 80% of females and sustains populations, even in the presence of drought and myxomatosis. Immunocontraception alone has poor prospects for controlling rabbits. Cost-effective rabbit control requires multiple, integrated forms of attrition.

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An experiment compared effectiveness, cost and cost-efficiency of factorial combinations of the four commonly used methods of rabbit control on grazing properties in the Southern Tablelands of eastern Australia. Sixteen different treatment combinations were applied to 32 sites. The treatments comprised initial control, applied over four months, followed by repeated maintenance control on half the replicates, applied after intervals of 2,6 and 12 months. Initial control comprised no treatment, or poisoning (P) with sodium monofluoroacetate (1080), or warren-ripping (R), or chloropicrin pressure fumigation (F), or combinations of these (P+R, P+F, R+F, P+R+F). Maintenance control consisted of phosphine-diffusion fumigation (M). Indices of rabbit abundance were compared one month before treatments were implemented. Treatment effects were assessed one month after completion of the initial control, and one and 5-6 months after the three maintenance controls, and additionally nine months after the second maintenance control. Control combinations that were highly effective and cost-efficient included both warren-ripping and maintenance treatment. Poisoning prior to warren-ripping, or fumigating subsequently, or both, improved effectiveness and cost-efficiency. Warren-ripping interacted positively with one or more subsequent fumigations, improving effectiveness and cost-efficiency non-additively. Control combinations that excluded warren-ripping were ineffective and cost-inefficient, and one combination interacted negatively. Single treatments of poisoning or fumigation were cost-inefficient, allowing rabbits to recolonise rapidly to densities higher than original. Only multiple combination treatments or repeated applications were highly effective and cost-efficient; single applications of any method were inefficient and costly. The most effective and cost-efficient combination comprised the maximum six applications including ripping and maintenance treatment, namely P+R+F+M. The high effort and expenditure on the initial control resulted in high effectiveness and cost-efficiency, which maintenance control sustained at low cost. Maintenance treatments sustained or achieved effective control of rabbits; the cost of maintenance treatments halved on each repetition.

Demographic changes were monitored in free‐ranging European rabbit (Oryctolagus cuniculus) populations in Australia following the surgical imposition of four levels of female sterility (0%, 40%, 60%, 80%). Rabbit productivity decreased with increasing sterility level, but a greater proportion of offspring were recruited into populations with high levels of sterility. Adult rabbits, particularly sterile females, also survived better in the high sterility populations. Thus we were able to experimentally demonstrate that two density‐dependent processes were operating on our rabbit populations: one was acting on juvenile survival, the other on the survival of infertile adult rabbits. However, these compensatory mechanisms were insufficient to overcome the effects of sterility in the high sterility populations and the seasonal abundance of rabbits decreased. Female sterility levels of 60%–80% would benefit rabbit control programs by reducing the peaks of rabbit abundance. This effect is likely to be enhanced if fertility control could be integrated with other pest control strategies.

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