Gorse, Ulex europaeus, is a native plant to Western Europe and was introduced to Australia in the early 1800’s. Since its introduction gorse has become a significant agricultural and environmental pest and weed, it is now listed as a weed of national significance. The areas that are most heavily affected by gorse are Victoria and Tasmania, although it is also present through out Australia.
The annual cost of gorse management to the Australian agricultural and forest industries has been estimated at $7 million. The difficulty and expense of controlling gorse by traditional methods such as pesticides, clearing and cultivation and biological control are continually being revaluated as a possible cheaper and long-term control option for this species. Under biological control it has been found that the gorse seed weevil, Exapoin ulicis, that was introduced into Tasmaina from New Zealand in 1939 was the first biological control agent to be used in Australia.
The weevil is now widespread throughout Tasmania, Victoria South Australia and is heavily relied on as a major control to prevent the expansion of gorse. An adult weevil can live for up to 12 months and can be found of gorse bushes all year round, however there is only one generation born each year. The development times of the weevil’s offspring can vary depending on location and the prevailing weather conditions. Adults are known to over-winter then mate and commence egg-laying in the spring months. The females will bore a hole in the developing seed pods of the gorse through which they will insert on average nine eggs per pod (although this number can vary considerably).
Eggs of the gorse weevil are laid from early spring to the end of summer, peck egg production happens in mid to late spring, with eggs taking about 4 weeks to hatch. The hatched larvae will feed on the seed of the plant, it will take 6 weeks for the larvae to pupate, and this will occur within the seed pods and can take another 4 weeks to complete the metamorphosis to an adult. Pupation can occur throughout spring and summer depending on the site conditions. Adult weevils will emerge when the seed pods are mature and open to dispense the seeds, allowing the weevils to fly away. Adults will then travel to gorse plants to forage on the flowers and foliage of the plants, however the damage to the plant in this stage of life is negligible compared to the damage caused during the larval stages.
With the weevil consistently causing damage to the plant they are considered a good pest control option for the Australian government. However with the majority of the damage being done is targeted at the seeds once a year, it can fail to target the seeds that can develop in the Australian climate throughout winter, leading a low effect on plant densities. Flowering and pod production of gorse varies considerably not only between sites but on individual bushes within sites. At some sites, particularly those in cool, high altitude localities most gorse bushes flower in late winter/spring. At other sites, such as those in warmer, coastal localities, flowering occurs in autumn and winter as well as in spring. With the weevils only feeding on a proportion of the seeds produced in the spring and summer, as well as being not present during the autumn and winter seeds, a large amount of the annual seeds can escape weevil infestation due to timing. A study in Tasmania showed that the percentage of mature seed destroyed annually ranged from 12-55%. This is much lower than the estimated levels of seed produced in Tasmania.
Models in New Zealand have shown that the level of seeds that need to be destroyed is around75 to 85% to sustain a natural manageable decline in gorse densities. By introducing and additional gorse seed-feeding agent into the environment in combination with the weevils could further the seed production levels.
The other animals introduced to manage the gorse in Australia have been the gorse spider mite, Tetranychus lintearuis that was released in 1998 and is not widespread in Tasmania, South Australia and Victoria. The Mints feed on mature gorse foliage and further studies have shown that they could reduce the growth on a gorse bush be 36% due to foraging. Unfortunately the mites have predators such as the Chilean predatory mite, and species of mite eating ladybirds, Stethorus spp., have reduced its effectiveness in the field. Gorse thrips, Sericothirps staphylinus, was realised in 2001 and prefer to feed on the younger shoots and seedlings of the plants, in glasshouse studies in Tasmania showed that a combination of the gorse thrips, competition of ryegrasses and grazing of the areas caused a 93% mortality of the introduced gorse seedlings. In field tests they have struggled to increase the gorse thrip population density high enough to have observable damage to plants. The Agnonpterix umbellana, a gorse soft shoot moth attacks the developing gorse growth in spring; realised in 2007 was another foliage feeding agent to be used to control the spread and density of gorse growth.
With further released being planed it is still too early to tell whether the already released species are having a continuous effect on gorse in Australia. This impact that they may be having is something that will need to be investigated in oncoming years. Research is also being done in possible host specific fungal pathogens and seed feeding agents that can be realised in conjunction with the other animals. What does need to be monitored over the next few years is how these biological control agents will react to other native animals and plants after, hopefully, their completion of reducing gorse seed output and rate of spread.