Four Gorse biological control agents have established in Tasmania.
Gorse Spider Mite (Tetranychus lintearius )
The first releases in Tasmania of the Gorse Spider Mite, (which attacks the mature foliage) commenced in December 1998 and by late March 1999, releases had been carried out at 25 sites throughout the state. It is now widespread in the state.
It forms colonies on the plant which spin a tent-like white web and move around the host plant as a group, feeding and web spinning as they go. These colonies feed on the leaves of mature gorse plants. By spring 2001 the spider mite had become widely established throughout most of the major gorse infestations in Tasmania. Although the mite is having an impact, it is being reduced by other mites and insects which feed on it and can destroy entire colonies.
There are a number of established sites for the Gorse Spider Mite in the Tamar.
Gorse Soft Shoot Moth (Agonopterix umbellana)
The Gorse Soft Shoot Moth was first released in Tasmania in spring 2007 and was subsequently released at 81 different sites by Tasmanian Institute of Agricultural Research (TIAR) researchers.
Adult Gorse Soft Shoot Moths are approximately 1 cm long with a 2 cm wingspan. Their forewings are a uniform light brown colour and have dark brown veins with occasional black flecks. The early instar larvae will cause some damage but the later instar larvae have the greatest impact. If larval feeding fails to kill the shoot, the destruction of spines from larval feeding destroys the plant’s reproductive buds thereby reducing or preventing flowering and seed set the following spring.
Gorse Soft Shoot Moths have only one generation per year. Eggs are laid in October/November and larval stages are present from October to January. Larvae start to pupate during January. Adults emerge during February and eventually move into the gorse bushes to begin their winter diapause. Once temperatures start to increase in early spring the adults become active, mate and lay eggs near buds in the axils of gorse spines and stems. Egg hatching coincides with the availability of succulent new growth. Newly hatched larvae migrate to young buds, spinning a protective silken tube-like shelter whilst feeding on the developing spines of the apical tip. As the larvae develop they spin larger shelters and feed on the lateral spines of the growing shoot. (Tasmanian Institute of Agriculture Weed Biological Control Pamphlet July 2012)
There is one established nursery site for the Gorse Soft Shoot Moth in the Tamar. Tamar NRM and DPIPWE, with the assistance of Landcare groups and local government are currently attempting to establish more.
Gorse Seed Weevil (Exapion ulicis)
The Gorse Seed Weevil was introduced into Tasmania in 1939, and is now widespread. Adults appear greyish due to minute hairs, but the underlying colour is black. Adult weevils live for 12 months and can be found on Gorse bushes all year, however there is only one generation each year. Adults over-winter then mate and commence egg-laying in spring. The females bore a hole in the developing seed pods in early spring, through which they insert their eggs. Although adults feed on flowers and foliage it is the larvae feeding on the developing seeds that causes the main damage. However, the impact of the larvae on seed production is not high enough to significantly affect plant densities because the larvae only feed on pods in the spring and Gorse can produce seeds at different times of the year between sites and even within a site. (Tasmanian Institute of Agriculture Weed Biological Control Pamphlet No.5, April 2008)
Gorse Thrip (Sericothrips staphylinus)
The Gorse Thrips (Sericothrips staphylinus) was first released in Tasmania and Victoria in 2001. Post-release surveys in Tasmania show that it had successfully established but its spread was slow. Increasing its spread will depend on planned redistribution programs.
It is important to remember that biological control is a long-term process that will not eradicate Gorse. However, it is hoped that once all the biological control agents become widespread their combined impact will eventually result in a reduction in Gorse vigour, seed output and rate of spread, making it more susceptible to grazing, weather stresses and herbicides as part of an integrated management program.