Abstract

This paper reports on some results of a field study of Rabbit Haemorrhagic Disease (RHD) since its illegal release in New Zealand in August 1997. The presence of fresh carcasses and seropositive rabbits of different ages were used to indicate the presence of RHD in rabbit populations, and changes in rabbit densities were used to infer its efficacy.Serological evidence from two Otago study sites where rabbits were sampled before and five times after the initial epidemic shows that (a) biociding left more seropositive, and so immunised, adult survivors than were left after a natural epidemic; thus natural epidemics are best if they persist, (b) the proportion of seropositive rabbits in cross-sectional samples declined over the 6 months after the initial epidemic, and (c) a second epidemic of RHD broke out naturally at both sites in September 1998. At 36 study sites around New Zealand, the initial 1997 epidemics caused declines in rabbit densities averaging 32±23 % (95% S.E.) in the North Island and 56±23% in the South Island. However, success was not universal and rabbits increased at a few sites.Causes of failure at some sites have been attributed to poor-quality biociding where rabbits were vaccinated rather than infected.Variable kills were also seen in natural epidemics at sites where the initial rabbit densities were less than c. 25 rabbits seen /spotlight km, while consistently high kills were evident at intial densities above this threshold. Overall, rabbit numbers increased at most sites measured between winter 1998 and early 1999, despite second epidemics being confirmed at some of these sites. The optimal strategy to manage rabbits as pests of pastoral production in New Zealand cannot be determined because we cannot yet predict the behaviour of RHD, and we have insufficient information on the relationships between rabbit densities and their impacts on production, although a single unreplicated trial we did in Otago showed that the reduction in rabbit numbers due to RHD reduced the grazing impacts of rabbits by 77%. Therefore, in the absence of any robust measure of benefits we cannot judge how much it is worth spending on conventional control. These information gaps are hindered by the old paradigm of rabbit control – “knock them down and hold them down”. If this strategy was ever the best for a boom-bust pest, it is certainly not so now that a biological control agent is at least partially successful and persistent.

J, Parkes, G Norbury, and R Heyward

Proceedings of the New Zealand Society of Animal Production, Volume 59, , 245-249, 1999