Page 8 – Biological control
The call for myxomatosis
Myxomatosis (a viral disease of rabbits, caused by the myxoma virus) had been introduced to New Zealand in 1952, but failed to become established as there was no vector to spread it through the rabbit population.
In the 1980s there were changes in the funding and management of rabbit control, along with an outbreak of rabbits. Farmers in badly affected areas demanded new tools to control the pests. They urged the government to release myxomatosis again, but in a more controlled way so it would become established effectively.
The Parliamentary Commissioner for the Environment heard submissions in 1987, but found the risks of introducing the virus outweighed any advantages.
The Rabbit and Land Management Programme
Instead the government introduced the Rabbit and Land Management Programme. This assisted farmers in the most rabbit-prone parts of the South Island to make their properties rabbit-proof and their farms sustainable. The programme included rabbit control on a grand scale. Netting was supplied to secure farm boundaries, stopping rabbits invading from elsewhere, and containing them so they could be destroyed.
Farmers were also funded to improve their pastures, with the aim of strengthening farm finances so they would no longer need help to control rabbits after the programme finished. It was also a pest-control measure, as it was thought that rabbits did not thrive in areas with heavy vegetation cover.
The rabbit revival
The Rabbit and Land Management Programme ran from 1989 to 1995. Despite the outlay of $28 million by the government, regional councils and landowners, the gains were short-lived. The initial poisoning was highly successful, as were some of the land improvement efforts. But after a series of dry seasons rabbit numbers began to grow, and, in the user-pays environment, farmers faced expensive poisoning programmes to try to reduce numbers. At the same time another biological control appeared that landowners thought could offer a cheap, long-term solution.
Rabbit haemorrhagic disease (RHD)
Rabbit haemorrhagic disease (RHD), formerly known as rabbit calicivirus disease (RCD), was first reported in China in 1984. It soon spread through Asia, Europe and North Africa, killing millions of rabbits. In 1991 a laboratory trial of the disease began in Australia, and field trials began on Wardang Island off the Australian coast in March 1995. The virus soon spread beyond the perimeter of the testing site, and in October 1995 RHD was found on the Australian mainland.
RHD on the menu
The remarkable speed at which RHD spread through Central Otago and the Mackenzie Country was largely due to the actions of some farmers. They collected the internal organs – in particular the liver, heart and lungs – from rabbits that had died from RHD, minced them in kitchen blenders, covered bait with the diluted slurry and spread it over their farms.
Illegal release of RHD
In mid-1996 a group that included 10 regional councils lodged an application to the government for the release of RHD in New Zealand. In July 1997 the deputy director-general of agriculture declined the application. Some farmers responded by illegally releasing the disease. In late August RHD was found to be killing rabbits in the Cromwell area of Central Otago, and the disease quickly spread through the district and into the Mackenzie Country. It has since spread throughout New Zealand.
At first the disease was very effective in reducing rabbit numbers in most parts of the country. Vegetation recovered well, especially in the semi-arid areas of the Mackenzie basin and Central Otago. However, in 2007, rabbits were showing signs of becoming immune to RHD, and some farmers had to resort to poisoning. It is clear that in drier areas where rabbits thrive, the battle is far from over.