Pest Control and Management in Tasmania - A Review

Transcription

1 81 9. CONTROL OF PEST ANIMALS This issue was ranked ninth by the community group and fifth by stakeholders (DCAG). 9.1 Feral vertebrate pests In the Derwent catchment rabbits, deer and hares are prolific exotic herbivorous pests which can cause losses to the productivity of agriculture and forestry. Shooting and poisoning are used to avoid damage to crops, pasture and trees. Fencing is effective but very expensive and time-consuming to establish and maintain. The shooting of deer is under strict legislative control and a permit is required which limits the numbers and gender to be taken. These restrictions are unpopular with landholders in the upper Clyde and Ouse catchments as deer do a lot of damage on some properties in these areas. The rabbit calicivirus and myxomatosis virus have generally depleted rabbit populations although there can be large seasonal variations in rabbit population numbers. The rabbit population on the Central Plateau is cause for concern as it seems to be less affected by the biological controls and impacts on the fragile soils, causing soil erosion. Hares can cause localised problems and are controlled in the same way as rabbits. Poisoning is discussed below. Feral cats are a menace to Tasmania s small native animals, which have not evolved the means to protect themselves from fast predators. Their prey includes fauna such as bettongs, potoroos, bandicoots, marsupial rats and mice and some ground-feeding birds, many of which have become extinct or endangered on mainland Australia due to the spread of the European fox. The threat to these animals posed by cats would multiply enormously if foxes were to become established in Tasmania. Protection of Tasmanian fauna from foxes and wild cats should be considered an urgent priority. 9.2 Native vertebrate pests No environmental issue is as difficult to resolve as the question of how to deal with native animals which pose a threat to forestry and farming enterprises. There are strong arguments for both an ethical and an economic stand on the issue and few compromises that are not already in place. Through the increase in food supply after land clearing and the removal of their main predator (the thylacine), population sizes of the native herbivores have greatly increased. Since 1985 numbers of Bennett s wallaby have increased by 2.5 times, the Tasmanian red-bellied pademelon by 3.5 times and the brush-tailed possum by 3 times (G. Hocking, DPIWE). Browsing

2 82 of young trees, crops and pasture by these animals can have a large impact on the profitability of agricultural and forestry enterprises. Methods of dealing with the problem are fencing, shooting and poisoning with sodium monoflouroacetate (1080). Fencing can be effective in small scale situations, but for forestry and other large areas the intensive upkeep needed makes it impractical. Shooting is useful in some situations, particularly in agricultural areas, but many forestry areas are not suited to shooting. It is doubtful whether shooting is a humane control method in wooded terrain where an animal is more likely to be injured than killed. Other options for forestry companies are the use of deterrents and the integrated planting of more palatable plants, and research into these alternatives continues to be undertaken. The amount of 1080 used in Tasmania has declined from a peak in of 18 kg. A minor rise to kg occurred in , before dropping to 12kg in Proportionate use by different industries has changed from approximately 60% agricultural use and 40% forestry use to 45% agricultural use and 55% forestry use in (G. Hocking, DPIWE, pers. comm.). The Statewide rise in use in forestry applications appears to be related to the increase in plantation establishment in the north of the State. Permits to poison are issued by the Parks and Wildlife Service (DPIWE) only where farmers or foresters can demonstrate significant browsing damage is occurring. A Code of Practice for the Use of 1080 outlines strict procedures and controls required for its application. The use of 1080 causes four main concerns : Non-target wildlife; Secondary poisoning of carnivores; Environmental effects Animal welfare The direct poisoning of non-target wildlife is currently being researched to minimise the risk to other native animals. Bait type, bait size and uniformity, positioning of bait stations, use of blue dye and low poison concentration have been found to diminish the poisoning of other species. The risk of secondary poisoning of birds of prey or native carnivores is very low (M. Statham, DPIWE, pers. comm.). Dogs are highly susceptible to 1080 poisoning, however, and it is recommended that owners within 500m of the poison line pen or muzzle dogs for a three month period, as it cannot be guaranteed that all carcases will be located and removed. There is a concern that the distance requirements for notification of neighbours are inadequate (see Table 9.1) as there is a lag period between the animal taking the bait and the poison taking effect, during

3 83 which time the animal may travel closer to houses. Most forestry operators will not use 1080 close to occupied land unless there is no other feasible alternative. No direct environmental effects have been found from the use of 1080, which breaks down readily in soil. Poisoned animals tend to look for shelter and may be attracted to riparian buffer strips, so baits should be placed well away from streams with bush around them. They are not attracted to water (M. Statham, DPIWE, pers. comm.). Animal welfare and other moral issues related to 1080 use should be addressed by the wider community. The forestry industry relies on the use of 1080 and its importance as an industry in this state is a political and economic matter as well as an ethical one. The Browsing Damage Management Group is an Agricultural Research Advisory Committee of the Tasmanian Institute of Agricultural Research, and defines priorities for research as well as providing a forum for the exchange of ideas between representatives of the farming, forestry, conservation, science, animal welfare and government sectors. It focuses on research into improved practices, animal behaviour and trials of alternatives to poisons. Table 9.1 Distance and timing requirements for 1080 use Action Requirement Notify neighbours in writing All adjoining neighbours and other neighbours if property boundary within 500 m of proposed bait Notify neighbours in writing At least 4 but no more than 7 working days before laying of bait Laying of bait near permanent stream More than 20 m from edge of stream Laying of bait near neighbour s boundary More than 5 m from boundary Laying of bait near public road More than 5 m from edge of road Laying of bait near occupied house More than 200 m, unless occupant gives written permission Disposal of uneaten baits (by prescribed means) Within 7 days Recovery of carcasses and disposal by burial or Within 24 hours (all reasonable effort must be made) incineration 9.3 Invertebrate pests Many growers of agricultural and forest crops minimise the use of chemical insecticides through integrated pest management programs. Integrated pest management relies on knowledge of agricultural or forest ecosystems (which include pest species) to target the pest with a number of simultaneous different pest control strategies. These may include crop rotation, the use of sex

4 84 pheromones to disrupt breeding cycles, imported biological control organisms, specially timed releases of large numbers of native predators, conservation of predator habitat, the use of crops bred for pest resistance and chemical controls. Specific techniques in use in the Derwent catchment include crop and pasture rotation and the planting of shelter belts to attract insectivorous birds and animals. Forestry Tasmania is also testing biological control methods to protect young eucalypts from the eucalypt leaf-eating beetle. (Derwent District Forest Management Plan, 1999). Many insecticides, including the commonly used pyrethroids, are not target-specific, and can impact on all levels of an ecosystem, leading to boom-bust cycles in some animal populations and in extreme cases to contamination of drinking water. Insecticide use can be self-defeating if natural predator populations are reduced through either direct contact or secondary poisoning. The Tasmanian Agriculture and Veterinary Chemicals Act (1995) and the Pests and Diseases Management Plan for State Forests in Tasmania (Wardlaw, 1990, in the Derwent District Forest Management Plan, 1999) control the use of pesticides in the agriculture and forestry industries. Guidelines apply to the aerial spraying of insecticides, including buffer zones around waterways and houses, and notification distances. Work by Barton and Davies (1993) showed that where pyrethroid insecticides were sprayed aerially, a well-vegetated (i.e. multi-level) riparian buffer strip of at least 50 m width was necessary to avoid mortality of stream fauna. In other studies on aquatic fauna, short-term low concentrations of pyrethroids or organophosphates caused physiological stress (in some cases lethal) in brown trout, native and introduced fish and stream invertebrates (Davies and Cook, 1993; Davies, Cook and Goernaso, 1994). Guidelines for the use of aerial spraying do not specify the nature of the streamside vegetation when setting requirements for buffer zone width. 9.4 Aquatic fauna pests The introduced aquatic pest species carp (Cyprinus carpio), redfin perch (Perca fluviatilis), tench (Tinca tinca) and the mainland yabbie (Cherax destructor) have all been recorded in the Derwent System. Carp has been declared as a 'controlled fish' under the Inland Fisheries Act 1995 and have, so far, been found to be restricted to Lakes Sorell and Crescent. Considerable funding has been made available to manage the population, minimise the risk of their spread and research means of eliminating them. Carp compete with native fish and have the potential to contribute to significant environmental degradation if left unchecked.

5 85 The extent of the distribution of tench and redfin perch is unknown at present. In the areas where populations are known to be present, these species appear to be maintaining selfsustaining populations which coexist with native species. Redfin perch have the potential to have a high impact on native Galaxias populations with restricted distributions if they gain access into those areas through modification of waterways or translocation of the fish. The mainland yabbie has been recorded in the New Norfolk area and has also been declared a 'controlled fish'. Yabbies compete with native species, could carry disease and parasites harmful to Tasmania's native lobster and can contribute to degradation of native habitat. See Priority 3.2 for recommendations regarding limiting the spread of aquatic pests. Control of Pest Animals - Management Recommendations Goal To control pest animals in an acceptable and sustainable way. Priority ranking (community) : 9 Priority ranking (DCAG) : 5 Strategies Priority 9.1 To prevent damage to farming and forestry activities by excluding or deterring browsing native vertebrate pests. a. Agreements between neighbouring forest growers and farmers on pest exclusion to be negotiated for their mutual benefit. b. Trials of innovative means of deterring native pests to be set up. Priority 9.2 To minimise the impact on biodiversity of feral vertebrate pest species. a. Information to be made available on the impacts and control of feral pests. b. All reasonable measures to be taken to remove feral pests, within current legislative requirements. c. Commercial harvesting of feral pests to be promoted where appropriate. d. Councils to consider the need to licence and/or desex pet cats in rural areas. e. Feral pest populations on the Central Plateau to be reduced as a matter of urgency.

6 86 Control of Pest Animals - Management Recommendations (cont.) Priority 9.3 To ensure culling of all pests including invertebrate pests is done by speciesspecific methods. a. The use of shooting and commercial harvesting to cull pests to be encouraged and promoted where practical and in response to neighbours concerns. b. Research into alternatives to 1080 to be encouraged and promoted. c. Research on avoiding the accidental poisoning of non-target species to be encouraged and promoted. d. Integrated pest management of invertebrate pests to be encouraged. Sawfly larvae infesting a young eucalypt