Rabbits on the increase
Dryland habitat in Central Otago with (right) and without (left) rabbits. Image - Don Robson
It is somewhat intriguing that in the Chinese Year of the Rabbit, there is growing concern about the numbers of rabbits infesting our drylands. In 1997, rabbit numbers over large areas of New Zealand were decimated by rabbit haemorrhagic disease (RHD). However, in the intervening years their numbers have recovered, due mainly to rabbits developing immunity to the RHD virus.
Such increases in rabbit numbers have led to a resurgence in the use of conventional control tools, including baiting with either 1080 or pindone. Landowners are required to control rabbits when numbers exceed specified population levels (see Parkes et al.) and some landowners are again incurring high rabbit control costs. It is therefore perhaps not coincidental that for the first time in more than 10 years, Landcare Research scientists, led by Bruce Warburton, have received funding from the Crown to extend recent research on possum control to the control of rabbits.
The research has three main strands: (1) reducing the cost and amount of toxic bait used in aerial baiting, (2) identifying cost-effective strategies currently used by farmers for secondary control, and (3) understanding the toxicology of pindone poisoning in order to reduce the cost of application and impacts on non-target species.
As a first step, published papers and reports from previous relevant research were catalogued within a searchable web-accessible bibliography. The catalogue helped identify key research that could be used to update current best practice for controlling rabbits with aerially sown 1080-carrot bait. Such control typically involves sequential sowings of 20–40 kg each of prefeed (non-toxic) and toxic bait per hectare, depending on rabbit density, and costs up to $100 per hectare. Based on research into baiting for possums, Bruce believes that current bait sowing rates and control costs for rabbits are too high and can be significantly reduced.
To test this hypothesis, Bruce and his team are working with staff from Regional Services (Otago Regional Council) to develop and run a series of trials this winter to test a range of different bait sowing rates and applications (Table). Even if the lowest toxic sowing rates (i.e. treatments 5, 6, 7 in the Table) are not effective, the team is confident treatments 3 and 4 will be. If so, the cost of control should be reduced by as much as 50% and the sowing rate of bait by 66%.
Table. Sowing rates of prefeed and toxic bait and flight-path spacing for each treatment (T1–T7). SR = sowing rate. Note T1 is current best practice as applied by Regional Services.
| Prefeed 1 SR (kg/ha) | Prefeed 2 SR (kg/ha) | Toxic bait SR (kg/ha) | Flight-path spacing (m) | Approximate cost/ha | Quantity of bait relative to T1 | |
| T1 | 30 | 30 | 30 | 25 | $75 | 100 |
| T2 | 30 | 30 | 10 | 75 | $56 | 33 |
| T3 | 30 | 10 | 10 | 75 | $40 | 33 |
| T4 | 30 | 10 | 101 | 75 | $40 | 33 |
| T5 | 30 | 10 | 5 | 75 | $39 | 10 |
| T6 | 30 | 10 | 5 | 75 | $39 | 10 |
| T7 | 10 | 10 | 5 | 75 | $23 | 10 |
1 The difference between T3 and T4 is that the swath width of toxic bait in T4 is reduced from 25 to 10.
Farmers and rabbiters engaged in control use a wide range of tools to limit rabbit populations. Shooting is the most common control method used, though its application varies. For example, in areas where access by 4WD or motorbike is possible, night shooting is effective when rabbit densities are low. On steeper, less accessible country, rabbits are shot from helicopters. Other complementary methods include fumigation of burrows and patch-poisoning with pindone.
Pindone (a first-generation anticoagulant toxin) is used by some farmers and contractors to control rabbits because of the regulatory and stock-withholding-period requirements when using 1080. However, baiting with pindone raises concerns over toxin residues and risk to non-target species. Penny Fisher has been assessing the residual concentrations of pindone in tissues from caged rabbits after they have eaten a lethal amount of pindone and has found relatively high levels of the toxin in rabbit liver and fat. Tissue testing is ongoing, and the final results will be used to review assessments of risk to non-target species that prey on or scavenge rabbits, such as Australasian harriers and dogs.
This work is funded by the Ministry of Science and Innovation (Programme C09X1007).