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Proceedings of the First Symposium on Marsupials in New Zealand

The Nature and Degree of Possum Damage

page 206

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.