Proceedings of the First Symposium on Marsupials in New Zealand
Damage by Possums Thichosurus Vulpecula to Erosion-Control Plantings
Poplar and willow cuttings, in the form of long poles, are planted extensively on farm land in order to control soil erosion without retiring the land from grazing. Reports on possum damage to plantings have shown that considerable damage can occur over a wide area. In some areas planting programmes have been abandoned because of this damage. Possums eat leaves in spring and late summer and break leaders. Bark biting and bud removal occur in winter.
The worst damage often occurs where poles are planted near scrub-filled gullies or bush patches but can also be severe in areas without bush or scrub where plantings are small and scattered. Both these situations present possum control problems either because of the large areas involved or because low density populations can cause severe damage.
Improved control would result from better co-operation between Catchment Boards, who are the planters, and possum control operators. Research into improving methods of protecting the poles follows along three lines. The selection of less palatable pole types is continuing. Experiments will be attempted with physical barriers in the form of sleeves around the poles and with lures in the form of highly palatable bait trees or crops dispersed amongst the plantings.
Poplar (Populus spp.) and willow (Salix spp.) trees are preferred for soil erosion control on hill country farmland primarily because of their extensive root systems and rapid growth rates, their ability to grow in poor soils and in harsh environments and their ability to strike from long poles. The last characteristic allows the young pole to grow in the presence of stock with minimal restrictions and avoids the need to retire large areas of land from grazing.
Catchment Boards have planted between 20000 and half a million poles annually over the last 20 years. It has been estimated (van Kraayenoord 1968) that a further 20 million poles will be needed in the next twenty years at a current cost of about $1.20 per pole.
This paper describes the damage to poles by common brushtail possums Trichosurus vulpecula and outlines research into methods of alleviating it.
The Nature and Degree of Possum Damage
There are four types of possum damage to the poles. The leaves are eaten and at the same time leaders broken, primarily shortly after leaf emergence in spring and before leaf fall in late summer. In winter possums bite off patches of bark and remove buds. As a result, some poles are killed and many others damaged to an extent that their growth is slowed or they become stunted and their effectiveness for erosion control is reduced or lost altogether.
Surveys of pole losses have illustrated the degree and extent of damage. An interdepartmental committee reported (Anon 1966) that, in the East Coast (North Island) district 6.7% of poles and trees inspected had been killed by possums and a further 21.2% damaged. A Ministry of Works survey (Anon 1970) of the Poverty Bay, Hawke's Bay, Wairarapa, Manawatu and Rangitikei catchment board districts found that 11.4% of the poles were definitely damaged by possums two years after planting. This figure is likely to be conservative because the cause of death or damage to many poles inspected some time after the damage occurred could not be determined. More recently (1974), the Rangitikei-Wanganui and Wairarapa Catchment Boards and the Waikato Valley Authority reported similar or greater possum damage (15–20% of poles lost - E.H.H. Kelman pers. comm.). They also reported that damage was so severe in some areas that planting was discontinued.
The value of poles lost to possums was estimated as $7,000 per annum in the East Coast District report (Anon 1966) and between $6,000 and $9,000 per annum for some boards in 1974. The resulting reduced effectiveness of erosion control measures makes the total cost of possum damage considerably greater.
The degree of damage by possums must be considered in relation to damage by other agents. The most important of these are believed to be desiccation and cattle rubbing and barking. No figures are available for losses due to desiccation but both surveys reported considerable cattle damage (East Coast survey: 29.5% killed by cattle; 6.7% killed by possums. Ministry of Works survey: 9.7% damaged by cattle; 11.4% damaged by possums). Although still a problem, this damage has now apparently been greatly reduced by encouraging farmers to keep cattle out of planted paddocks for the first one to two years. In addition, bark biting by stock is now largely prevented by attaching plastic netting sleeves to the poles.
Possum Control Problems
Damage is often worse where poles are planted near scrub-filled gullies or bush patches but can also be severe in areas without bush or scrub where plantings are small and scattered. Both these situations present possum control problems, firstly because of the large areas involved and secondly because low density populations can cause severe damage.
Some improvement in tree protection would result from better co-operation between the Catchment Boards and Pest Destruction Boards. Pest Boards should be furnished with the size and siting of all erosion control measures well before planting so that they can assess the control situation. Pest Boards have sometimes been reluctant to protect poles in possum prone areas through lack of appreciation that erosion control measures can be essential in such areas. Generally co-operation is better now than in the past but could still be improved.
A second limitation to the effectiveness of current control is that the funds of Pest Boards, derived from rates on farms and a 1:1 government subsidy, are very limited. Catchment Boards cannot vote money for possum control and grants from the Agricultural Pests Destruction Council are only available for catchment control schemes, which include only a small proportion of the planting. The cost of control is high because of the large area of poles planted, the need for extensive controlled buffer zones around the plantings, and the need to reduce possum populations to very low levels. There is clearly a need for cost-benefit analyses especially in areas where low density populations cause damage to small groups of plantings.
There are three approaches by which research can improve protection of the poles: the development of less palatable tree types, protection of the poles by physical barriers and the improvement of possum control techniques.
Research so far has been concentrated on breeding less palatable clones of poplars and willows. The Plant Materials Centre of the Ministry of Works and Development has had some success with willow breeding (Hathaway 1974). However, a vigorous, unpalatable, tree willow has yet to be produced (van Kraayenoord 1975). The arrival of the Melampsora spp. poplar leaf rusts have largely nullified the efforts with poplar breeding (van Kraayenoord 1975). All of the unpalatable clones in common use were susceptible to one or both rust species.page 208
Development of less palatable, rust resistant clones is being attempted by the Plant Materials Centre.
Physical barriers in the form of sleeves can be attached around the trunks of the poles. The sleeve design must allow for expansion of the trunk over the critical first 4–5 years of the pole's life. They must also be designed to be recovered and re-used or to break away when their maximum diameter is reached. It is believed that metal sleeves are unsuitable because of the excessive heat transferred to the trunk of the young tree but formal trials are lacking. Several designs of both plastic and metal sleeves will be tested as barriers to possums and for any deleterious effects on the poles.
Research is also required to try to produce possum control techniques that avoid the expensive practice of blanket poisoning large areas. Highly palatable bait trees or crops dispersed amongst plantings will be assessed as lures where the possums can be subsequently poisoned in confined areas. A variety of methods of presenting the poison, such as bait stations or poison in gel form, will also be tested. There is also a need for research on poisons other than 1080 which are safer in farm land conditions, particularly in the presence of farm dogs. A lure in the form of highly palatable poles is suitable for systemic poisoning.
It is likely that only a combination of all three approaches will result in greatly reduced damage.
We are grateful to Mr E.H.H. Kelman of the Soil and Water Division, Ministry of Works and Development for providing us with unpublished reports. We would also like to thank Mr C.L. Batcheler and Dr J.D. Coleman of the Forest Research Institute for reviewing this manuscript.
Anon. 1966. Departmental report on opossum damage in the East Coast Rabbit District 1965–1966 . Inter-Departmental Report (unpublished).
Anon. 1970. Survey of damage to poplar and willow poles planted in 1967 for erosion control. Report of Soil and Water Division, Ministry of Works, Wellington (unpublished).
Hathaway, R.L. 1974. Palatability of Salix spp. to opossum. National Water and Soil Conservation Organisation Report, Ministry of Works and Development, Wellington.page 209
Van Kraayenoord, C.W.S. 1968. Poplars and willows in New Zealand with particular reference to their use in erosion control. International Poplar Commission 13th Session, Montreal, September 1968.
Van Kraayenoord, C.W.S. 1975. National report on activities related to poplar and willow cultivation. Period: 1971–1974. XV session of the International Poplar Commission, Madrid, 1975.
ANONYMOUS. Could you tell us more about the heat effect of the protective bands on the trunks of the plantings?
JOLLY. I believe in trials carried out by the Plant Materials Centre, tin sleeves were found to "cook" the live pole plantings and they simply would not grow.
WODZICKI. Are you looking for a species or variety of tree that would be very unpalatable to possums, that would show less damage from them?
JOLLY. I think the Plant Materials Centre has explored the ground very thoroughly. You need a pole 3 m high that will sprout from the top so that stock cannot reach the foliage. Also you need a plant that will strike easily in tough country from such a long pole. Your choice of suitable species is limited and you come back to poplars and willows every time. The alternative is to retire land and there is probably more retirement going on in North Island hill country now than there was in the past.
FITZGERALD. The Plant Physiology Division of DSIR have been working on meristematic tissue and have developed a polar strain highly resistent to Melampsora. Unfortunately it is highly palatable to possums. I gather they are going on to try and find a strain resistent to both possums and Melampsora.
WODZICKI. How does the frequency of damage by possums to plantings vary - have you considered the matter quantitatively?
JOLLY. Most of the poles were planted in mudstone country across the central and eastern part of the North Island - from Wanganui to the Rangitikei and Manawatu catchments, then north up the east coast from Wairarapa to East Coast-Poverty Bay. The damage has been pretty bad in all the catchment districts involved. It does vary locally, in that you tend to get less damage further away from normal possum habitats; conversely in Wairarapa tussock country, lacking in scrub or forest, we found significant damage despite low numbers of possums. There is also a seasonal component - spring is probably the main period of attack, after the leaf emergence.
WARD. You have mainly referred to poles. Does this mean the damage occurs only over the first 2 or 3 years, or is it only critical then? If you can nurse your pole through this early period of establishment can it then hold its own?
JOLLY. The damage is critical over the first three to four, possibly five, years but it continues in older trees too.page 210
BROCKIE. Dr Gibb has suggested it might be possible to incorporate a poison into a sticky glue and simply paint this in a band around a pole. As the animals climb up and down, the poison is transferred to the fur and when the possum later grooms itself the poison takes effect.
JOLLY. Yes, Dr Spurr also had the same idea. There is the prospect of applying 1080 as a gel onto the top of the pole above the height the stock can reach. I would like to do trials on this aspect.
COLEMAN. A suggestion that the N.Z. Forest Service use 1080 gel on the bark of some forest trees was turned down for on smooth-barked trees, at least, the life of the 1080 gel would be very short indeed.
ANONYMOUS. Would a band of sticky paper wrapped round the trunk be quite an effective deterrent?
JOLLY. The problem is it would have to cover every pole, which themselves cost $1.20 each now, so the catchment boards are not prepared to add to their labour costs unless satisfied it would be worthwhile. I think you need to draw the possums to a particular site and try and kill them there.
SPURR. The Plant Materials Centre has worked on repellents, ranging from rotting seaweed to mechanical devices, but so far they have turned out nothing really effective.
* Present address: Wildlife Service, Department of Internal Affairs, Private Bag, Wellington.