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Possums are furry animals of medium to stout build with thick, bushy tails. Their bodies are 38-45cm long (65-95cm long including the tail) and their weight varies greatly but averages 2-3kg. There are two main colour forms, grey and black. Possums have large eyes and catlike whiskers, which are characteristic of nocturnal animals.
Possums can live anywhere that has shelter and a varied food supply. Forests are their favoured habitat but forest/pasture margins can also support very dense populations of possums. Possums feed mainly on leaves but also take buds and flowers, fruits, ferns, bark, fungi, invertebrates (including land snails and weta), native birds and their eggs, and carrion. They are nocturnal, but in winter starving or sick animals may emerge to feed in the afternoon.
Possums are able to occupy a range of habitats and can survive on poor or irregular food supplies. By eating plant foliage they damage and destroy forests, and affect pasture, vegetable and horticultural crops. Possums can be a vector in the spread of diseases that affect domestic animals and humans, such as tuberculosis. In Northland, possums are currently free of tuberculosis; however, there is a high possum population in some areas and long bush boundaries on many farms, so stock are in close contact with possums. These factors may facilitate the spread of the disease.
Possum populations expand their range by the gradual spread of female offspring on the edge of occupied home ranges. Their ability to produce more than one offspring per year and the enhanced survival of juvenile females when conditions are good, allows possum populations to increase rapidly in newly invaded areas or after populations have been reduced by control pressure. Northland provides a graphic example of how fast possums can spread because they were virtually unheard of here in the 1960s.
The average life span of a possum is 7 to10 years. Most female possums breed from one year of age and can produce one or, less commonly, two young in a season if food supplies are adequate. Young possums spend the first part of their life in their mother’s pouch, feeding on rich milk. After weaning at 5-8 months, young females tend to remain close to their mother’s home range. Young males disperse randomly in search of receptive females and have been recorded migrating between 0.5 to 20 kilometres.
There are two main types of possum traps – live capture or kill traps. As with poisons, each has advantages and disadvantages depending on the situation.
There are many different designs but the principle is to quickly and humanely kill the possum. Some traps are better than others and often the way the trap is set can make a difference to its effectiveness. Timms traps – which are effective and easy to set – are one of the most common kill traps available.
The main advantage of kill traps is that they don't need to be checked daily. Traps may be left in place for long periods and they are cost-effective as a long-term control technique. Kill traps are seen as a danger to domestic cats however any risk can be reduced by using baits which cats won't eat or by keeping them indoors while traps are set. Traps can also become weak or worn over time and not kill humanely. Possums can also become trap shy if kill traps are not set correctly and they can be dangerous to other non-target species for the same reason.
Live capture traps
There are two main types of live capture traps. The first is usually a box or cage which is baited to lure an animal in, then the door is activated to close and lock behind it. The second are leg-hold traps which are small steel spring-loaded jaws that clamp and hold the possum's leg just above the paw when it stands on the trap.
Box or cage traps usually work well around built areas where non-target species may be at risk. Trap-shy animals can also be lured in by pre-baiting around the outside of the trap with favoured bait. Animals caught are unharmed but must be humanely disposed of. (It should be noted that the once common method of drowning has been found to be inhumane and is now illegal).
Leg-hold traps have a long history of being very effective at catching possums for both control and the fur trade. Leg hold traps with serrated jaws and large traps with unpadded jaws are now banned. Also as protection for cats, no leg-hold trap may be set within 150 metres of a dwelling without permission from the occupier.
For up to date changes to laws concerned with the use of leg-hold traps please check the animal welfare section on the Ministry for Primary Industries website: www.mpi.govt.nz.
With any live capture trap there is a legal obligation to check it within 12 hours of daylight each day. New possum traps are in constant development including self-setting and multiple-kill traps. For any news on new traps contact the Northland Regional Council.
Night shooting possums using a spotlight is a popular method of control in semi-open areas where access at night is easy. The red eye reflections of a possum can be easily seen and by knowing the seasonal feeding habits and targeting these areas, possums can be controlled. However, possums can become light shy and avoid hunters. Shooting can also become less effective if over-used particularly when possums are regularly missed. A .22 calibre firearm is usually best for shooting possums when limited to areas where firearms can safely be used by a licensed person.
Poisoning is the most cost effective way of controlling possum populations. The use of specially designed bait stations also ensures that control is carried out in a safe, effective way that does not put other non-target animals (i.e pets or birds) at risk.
Bait stations come in a variety of shapes, sizes and prices. Key features to look for are how well it will weather, how easily filled and how easy possums can feed from it. A bait station protects the bait from the weather which increases the effective life of the poison.
The table (below) lists four registered poisons that private landowners can use to control possums. Two of these require a controlled substance licence (CSL) while the other two are available for use without a licence however they are subject to some restrictions.
No licence required
Cyanide paste Encapsulated pellets (Feratox®)
Cholecalciferol paste (Feracol®)
Phosphorus double-strength paste
Each of the poisons has advantages and disadvantages depending on where they will be used and the degree of control you want. Council staff – or other pest control experts – can advise you about the best poison control for your situation.
Other toxins are currently in development and the council can also provide an up-to-date status on these. You can get a CSL licence through an Environmental Protection Authority (EPA) approved assessor. For more information contact the Biosecurity Team at the Northland Regional Council.