Secondary control:
RHD is still effective and making a difference but adequate secondary control measures will always be necessary to take advantage of RHD on rabbit prone land - killing the survivors before they breed and limiting the potential for viral attenuation and evolutionary adaptation in rabbits.
The RCD Applicant Group made it clear that ongoing rabbit control with conventional control tools would still be required on rabbit prone land if RHDV were to be introduced and regional councils have reinforced this message since its arrival.
When considering the implications of RHD immunity for rabbit management, the key factor is not the proportion of immune survivors, but the number of immune survivors, especially females (Lough, 1998). When epidemics
do not achieve good kills and a high proportion of the survivors are immune, it is particularly important to target residual populations with other control measures. The 1970′s and 80′s saw rabbits reaching problem levels over most of the semi-arid lands, not seen since before the introduction of 1080 in the early 1950,s.
Some of the highest numbers were seen in the Alexandra District where rabbits had transformed large areas of pastoral land into bare ground or degraded the vegetation to the extent that the only plant species remaining were those unpalatable to rabbits, such as scabweed and stonecrop.
The most publicised property in the Alexandra District was the 25,000-hectare Earnscleugh Station, which regularly featured in the media to show the ‘hillsides moving with rabbits’. Approximately 14,000 of the 18,000 hectares of rabbit prone land on this property are recognised as high (20%) or extreme (80%) in rabbit proneness. From 1956, up to 80 percent of the cost of rabbit control on this property was met by public funding (Campbell pers. comm.).
During the 1980s, large inputs continued to be directed to rabbit control together with the landholder’s own Pest Destruction Board rates of more than $1 per hectare for the worst affected land (1988 dollar value). However problems of poison and bait shyness resulting from a history of frequent poisoning, and the sheer scale of the problem, meant that no headway was being made.
The injection of substantial public funding through the RLMP enabled Earnscleugh to reduce rabbit populations to very low levels. Internal and boundary rabbit proof fencing was erected to create manageable blocks and boundaries for control.
The control effort focussed initially on the higher less rabbit prone country and then moved progressively downhill. However bait and poison shyness in the rabbit population meant that kills of only 70 percent were being achieved (c.f. up to 98 percent on other properties) leaving the seemingly insurmountable task of killing the survivors and their offspring by other means.
The runholder has no doubt that the turning point came when ‘we took ownership of the operation ourselves; in the days of the rabbit boards it had always been someone else’s problem. Two skilled staff were hired and rabbit control became (and continues to be) the first priority in all business decisions because we knew that rabbits were the only thing that could wipe us out (Campbell, per comm.).
After a concerted effort, much of it on very difficult terrain, rabbit populations suddenly declined over a period of a few months (possibly enhanced as the predators present had an increasing impact on the declining population). This was achieved prior to the arrival of RHD.
Good farm management practices were in operation, such as keeping stock off land recently cleared of rabbits, so that a seed bank could again build up over a number of years. Rabbit habitat was successfully modified by prolonged spelling from grazing of the worst areas; this increased the vegetation cover and enhanced predator success.
As the sward of vegetation increased so did the incidence of coccidiosis, a disease of the liver that can cause high mortality in young rabbits when they are exposed to a thick wet sward. Briar and scrub was removed to make areas less favourable to rabbits and the reduced cover made shooting more effective.
Commitment to secondary control measures after the RLMP maintained rabbit populations on Earnscleugh at very low levels and the property recovered, with dramatic improvements in vegetation and ground cover. Blocks that had been unable to carry stock for 10 years began to contribute to farm income.
The advent of RHD provided further assistance in maintaining the low rabbit levels that had already been achieved. Earnscleugh has continued to employ one fulltime skilled rabbiter. The predominant secondary techniques used are night-shooting (70%), day shooting with dogs, and some helicopter shooting.
No poison programmes have been necessary since 1995. Maintenance of rabbit proof boundary fences has been essential to prevent re-infestation from the many neighbouring smallholdings and lifestyle blocks. A recent incursion led to a concerted effort being required to bring the resulting local infestation under control (Campbell, pers. comm.).
Most of the approximately 18,000 hectares of rabbit prone land on Earnscleugh are of high and extreme proneness. The fact that rabbit populations have been maintained at the low levels that were achieved before the arrival of RHD in 1997, using existing secondary technologies, demonstrates that rabbit control can be viable in the post-RHD environment. Annual expenditure on rabbit control has been stable since 1997 at about $3 per stock unit.
Earnscleugh is not the only extremely rabbit prone property in the district demonstrating that rabbit control is achievable and financially viable - Galloway, Matangi, Little Valley and Kawerau are other examples of stations rated as extremely rabbit prone and all are currently maintaining rabbits at low levels. As stated earlier, most of the Otago RLMP properties of extreme rabbit proneness have kept rabbit populations well under control for at least 12 years. These Otago properties are considered to be among the most rabbit prone in NZ.
The Otago and Canterbury Regional Councils and the Marlborough District Council have all advised that they are not aware of any properties, under similar levels of control to Earnscleugh, on which rabbit populations have got out of hand.
On the 11,600-hectare Galloway station, approximately 10,000 hectares are classified as moderately to extremely rabbit prone. The property carries 10,000 stock units. Rabbit populations are being maintained at negligible levels and this can be seen on the Otago Regional Council RHD Upper Manorburn site depicted in Figure 9. The whole property requires rabbit control and this equates to annual expenditure of $70,000, including helicopter time.
Before the arrival of RHDV on a 10,000-hectare station of 5,000 stock units in the Mackenzie Basin, rabbit populations were rising (as on many other properties) despite secondary rabbit control expenditure of $10 to $12 per stock unit. The station is now (post RHD) maintaining rabbit populations at low levels with expenditure of between $2 and $4 per stock unit - predominantly by night-shooting with some patch poisoning (Simpson, pers. comm.).
For the last eight years, another rabbit prone Mackenzie station has employed a skilled part-time rabbiter. His monthly day and night-shooting has helped maintain rabbit populations at low levels (Fastier, pers. comm.). Day shooting is matched to the terrain – some hill slopes in the morning sun, others in late afternoon. All night-shooting takes place when there is no visible moon. Again, in the three years prior to the arrival of RHD, an immense commitment to secondary control was failing to hold a rising rabbit population on this farm.
Helicopter shooting has been in use for nearly 20 years and its use is increasing, especially in less accessible terrain. At about $11 per hectare, it is considered to be cost effective when preventing or delaying poisoning costs of up to $100 per hectare. Maniototo Pest Management Ltd has recently begun to use this technique on the more rugged hill country over which they carry out control. It is essential that the pilot and hunter are both skilled and experienced in working together as a team.
Secondary control methods are most effective when used under the correct conditions by experienced staff. The most common cause of problems is when a method is used where rabbit numbers are too high or where the terrain is not suitable. For example, a shooting programme should not occur when rabbits have been allowed to reach too high a level.
In the examples given above, an ongoing commitment to rabbit control has been essential to the successful suppression of rabbit populations in the post-RHD environment, managed within the farming business and without the need for public funds. Another important factor has been the employment of mature, motivated, skilled and methodical rabbit control staff; there is some concern in the farming community about the availability of such skilled people in the future.
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