|
The heaviest infestations covering ca. 30,000 ha. occur in the central and northern midlands on pastures grazed mainly by sheep. In these areas alone, losses in animal production are currently estimated at ca. $1 million per annum.
Biological control offers a long term solution to the gorse problem and the chance of returning land to sustainable and productive use through an environment friendly control system. The possibility of a biological control for gorse is not new and began with the introduction of the gorse seed weevil from England. The weevil was released in New Zealand in 1931 and in Tasmania and Victoria in about 1939. Although now widespread it has had no appreciable effect on gorse as it only attacks the spring crop of seeds, with the autumn crop being completely unaffected.
Several other biological control agents that attack gorse in their native environment of Europe have since been introduced into New Zealand and the United States. These include the gorse spider mite, Tetranychus lintearius Dufour which is the most damaging agent on gorse in Europe.
The gorse spider mite, which was found to attack only gorse, was originally imported into New Zealand from England in 1988. It was released in New Zealand in 1989 and is now widespread. Gorse spider mite was reassessed for its host specificity to make certain that it will only attack gorse if released in Tasmania or elsewhere in Australia. As a result of these tests, the mite was imported into Tasmania and Victoria in September, 1998 to enable mass rearing programs to commence in these States. The first field releases in Tasmania commenced in December 1998, with the Meander Valley Region receiving their first release near Meander on 11th December. Further field releases are planned throughout Tasmania over the next 2-3 years in an attempt to establish the mite at selected sites in all regions where gorse is a problem. Redistribution programs involving Landcare groups will then be carried out to accelerate its rate of spread.
Description
Gorse spider mites range in size from about 0.4-0.6 mm in body length (smaller than a pin head). Immature mites have green abdomens with mouthparts and legs ranging from orange to yellow. On maturing the body reddens often with a dark spot on either side of the abdomen.
 Biology
Gorse spider mite forms discrete, long lived colonies which live in a tent-like white web and move around the host plant en masse feeding and web spinning as they go. These colonies feed on mature gorse foliage.
The life cycle is temperature dependent and takes ca. 46 days at 15deg.C, ca. 32 days at 20deg.C and 20 days at 23deg.C. Because it is able to pass through several generations in a relatively short time at warmer temperatures it can quickly build up on large numbers. The mites pass through 6 immature stages before the adult stage is reached. Adults may feed from 1-23 weeks before the female starts egg laying. Eggs, which are brownish when mature, are scattered through the silk webbing of the colony and hatch in 1-23 weeks under warm conditions. In summer, each female lays ca. 40-50 eggs at the rate of 1 per day to a maximum of four per day, depending on temperature. The sex ratio is 1 male for every 20 females, although this can vary considerably. Climatic conditions, particularly rainfall, have a direct impact on the establishment and population growth of the mites. On warm days they can be seen moving actively through their webbing, however, on cold or wet days they protect themselves by clustering at the centre of the web, often on the leeward side of the gorse stems.
Prospects for control
It is expected that the gorse spider mite will be one of several agents that will be required to collectively reduce the vigour of gorse. Plans are under-way to introduce other agents into Australia once tests are completed to ensure that they attack only gorse. These agents are the gorse thrips, Sericothrips staphylinus (Haliday) and the gorse soft shoot moth, Agonopterix ulicitella (Stainton) which are foliage feeders, and the gorse pod moth Cydia succedana (Denis and Schiffemuller) which destroys the seed. If these agents can also be established, their effects will compliment those of the gorse spider mite and the already established gorse weevil.
It is important to remember that biological control will not eradicate gorse but it is hoped that the combined impact of complementary agents could significantly reduce plant vigour, making gorse more susceptible to grazing, weather stresses and herbicides.
Vigour reduction of gorse would enable beneficial plant species to compete with it more readily resulting in a substantial increase in pasture and animal productivity in agricultural areas. The control of existing gorse stand would become economic and gorse regeneration in existing or renovated pasture or conservation areas would be retarded or cease. The spread of gorse would also be restricted due to a greatly reduced seed output. There will also be a reduction in the use of gorse as a habitat for vertebrate pests, and it will be reduced as a fire hazard.
Traditional control methods, at much lower levels and therefore lower costs, will be integrated with biological control methods.
Funding support for this project is being provided by the Natural Heritage Trust
The text and images in this page were supplied by Dr. John Ireson, Senior Entomologist, Tasmanian Institute of Agricultural Research.
 
|
