Proceedings of the First Symposium on Marsupials in New Zealand
Tuberculosis and the Control of Possums Trichosurus Vulpecula - An Expensive Business
The recent identification of bovine tuberculosis in widely scattered populations of possums throughout New Zealand has led to a re-thinking of possum control policy and control technology, and to dramatic increases in the areas under control and monies expended.
Two organisations are involved in possum control. The Agricultural Pests Destruction Council is responsible for possums on rateable lands, while the N.Z. Forest Service is responsible for those on lands of other tenure. Together, the organisations accounted for $1.38 million in direct Tb/possum control between 1972 and March 1976, and anticipated spending a further $1.3 million in the 1977-78 financial year. Even with this effort, the elimination of local foci of bovine tuberculosis has been achieved in few instances. It is apparent that a greater understanding of the epidemiology of the disease in possums is required to reduce future cattle losses and enable the best use to be made of manpower and resources.
The common brushtail possum Trichosurus vulpecula has long been identified with the destruction of New Zealand's indigenous forests. More recently the species has also been blamed for economic losses in exotic plantings, in catchment plantings, and in improved pastures and fodder crops. Perhaps most devastatingly, possums have also been identified as a reservoir of leptospirosis and as both a vector and reservoir of bovine tuberculosis (Tb) Mycobacterium tuberculosis var. bovis; the latter from such scattered areas as the central North Island, Wairarapa, Westland, Methven, Banks Peninsula, and Southland (Fig. 1).
The progressive reduction of bovine Tb amongst New Zealand cattle has led inevitably to the need to "control" farm-dwelling, tuberculous possum populations. However, traditional methods of possum control have often proven unsatisfactory in the elimination of the disease from possum populations although dramatic changes in control planning have taken place.
The present paper outlines the history, function and funding of possum control agencies in New Zealand and examines the cost and success of individual control operations.page 212
The Organisation of Possum Control
The basis for successful possum control is adequate legislation - the Wildlife Act of 1953 contains in its sixth schedule a list of animals "declared" to be noxious (and hence to be destroyed) under the Noxious Animals Act of 1956. This list includes nine species of deer (Cervidae), thar Hemitragus jemlahicus, chamois Rupicapra rupicapra, goat Capra hircus, pig Sus scrofa, wallabies (Macropodidae) and the possum. Local efforts at the control of any of the so declared species vary markedly throughout New Zealand, but in terms both of the finance and control effort expended, the possum is now pre-eminent.
Two organisations are involved in possum control. The Agricultural Pests Destruction Council (APDC) with its associated pest boards is responsible for the control of possums on all rateable lands, while the New Zealand Forest Service (NZFS) is similarly charged on lands of other tenure, e.g. state forest, unoccupied crown lands, national parks and flora and fauna reserves.
The involvement of the APDC and of its predecessor, the Rabbit Destruction Council (RDC) in possum control began in 1958. Initial action was taken by the Banks Peninsula and Northern Hawkes Bay Pest Boards, who became alarmed at possum densities on local farmland and mounted pilot control programmes funded by direct government grants. The success of these operations, together with the concern growing for the numbers of possums present over much of New Zealand's rateable land, led in 1960 to an amendment of the Rabbit Act of 1955. Boards then became able through revised legislation, to legitimately seek revenue both from central government and from rate payers for the control of possums wheresoever they are declared "a pest of local importance". Many boards made use of this amendment - 61 had done so by 1962, and 87 out of a total of approximately 100 by August 1976 (Fig. 2). Surprisingly, possums have been declared "pests" in areas where the nature of the farmland and land management is such that the species is unlikely ever to cause serious economic losses, viz. boards in inland South Canterbury and Central Otago. Conversely, other boards exist which have yet to declare the possum a pest, even though they are contiguous with, or are themselves, thought to be harbouring Tb infected populations, viz. South Otago, Waitomo and Hutt.
exotic plantings within state forests, with the exception that since the early seventies and the isolation of bovine Tb from possums, some control of diseased populations on rateable lands has taken place. Involvement of the NZFS in control on rateable lands has resulted from (1) the absence of active pest boards in key Tb areas, particularly the Buller, Inangahua and Grey counties, (2) the need to control possums on non-rateable forested lands contiguous with lands of other tenure and, (3) the control experience gained in past years by its management staff.
The Cost of Tb/Possum Control
The cost to the nation of the control of Tb/possums has steadily increased since 1970, as new areas of infection have been discovered and as larger numbers of cattle become at risk. Thus, while the NZFS and APDC together spent approximately $1.38 million in direct possum control up to March 1976 (Salisbury, MAF, pers. comm.), a further $1.3 million was requested for control in the 1977-78 financial year (from "Surveillance" 1977, No. 2, p. 13). These estimates do not include any of the substantial costs incurred by the replacement of diseased stock, herd testing, the loss of production, or the wages of NZFS staff employed in possum control.
The control activities of both the APDC and NZFS have been made possible by increased funding from central government, and made workable by close inter-organisational liaison. Finance for APDC control programmes has been derived from additional rates struck for possums, from the related increase in government subsidies, and from direct government grants. Grant monies for the control of disease-free populations increased steadily during the sixties to a high in 1967 (Fig. 3). This trend reversed in 1968 concurrent with the general economic recession of the time, but dramatically increased once again in 1976 and 1977 as the need for Tb/possum control in the North Island became apparent. Rate and subsidy monies available for Tb control have also been substantial - a total of $88,000 was spent in the 1976/77 year (P.L. Burston, APDC, pers. comm.). Thus for the period April 1976 to April 1977, the APDC spent a total of $408,000 directly on Tb/possum control, with by far the greater amount being spent in the North Island.
Fig. 3 Grant monies requested by the Agricultural Pests Destruction Council from central Government for the control of possums. Data from APDC and RDC reports.
It is apparent that control costs vary locally, regardless of whether they are expressed in terms of cost per unit area under control, or cost per unit of stock. Aerial control for possums has been costed out at $6–8/ha in the South Island and is suggested to be a little higher in the North Island (about $8.00/ha, P. Nelson, APDC). Further, the "kill" obtained is normally in the range of 50–80% and alone is usually considered inadequate for disease control. Ground control is often equally as expensive as aerial control, as it is rarely a "one-hit" operation and may continue for several years. It is also apparent that the economics of farming beef or dairy cattle in areas under Tb control are likely to be less than profitable. Any apparent profitability is eroded further by basic farm costs such as labour expenses and mortgage repayments which are not included in the profit margins quoted above, as well as by stock replacement.
The Success of Tb/Possum Control
Success in Tb/possum control can ultimately only be measured in terms of a reduced incidence of tuberculosis amongst cattle herds. This has been most accurately documented for the "Block 1" test area near Westport (Fig. 4, from Stockdale, 1975, Report AH26-1175). In that area, tuberculin testing of herds at three monthly intervals revealed incidences amongst reactor and tuberculous cattle of 5–6% and 2–3% respectively. Reduced incidences of cattle of either status were obtained only with the attempted elimination of infective animals outside of individual herds. Control of possums, initially by aerially applied toxin and later by repeated ground operations has led to a steady reduction in the incidence in both reactor and tuberculous cattle. In at least two instances, viz. the Monument area on Banks Peninsula and the Matiri Valley near Murchison, repeated possum control has led indirectly to disease-free herds.
Long term changes in the disease status of possum populations have not been monitored. However, in the two instances quoted above - the Monument area and the Matiri valley - it is reasonable to infer that a dramatic reduction if not a total elimination of all foci of infection amongst possums occurred in each area.
Short term changes have been monitored at least once, viz. in the Hohonu region, central Westland, by the author. In that instance, the possum kill obtained by aerial poisoning was estimated to be 67± 14% (C.L.) while the percent incidence of tuberculous possums remained unchanged.
The eradication of the disease from dairy and beef herds remains an uphill fight, with successes such as those discussed above counter-balanced by the need for intense possum control throughout many beef and dairy farming areas of New Zealand. It is obvious that both a greater understanding of the epidemiology of the disease in possums and some revision of the traditional methods of possum control are required, to reduce further cattle losses and enable the best use to be made of the available manpower and resources.