A Green Solution for a Gold Problem
Severe dieback in a 10 m tall Sydney golden wattle tree in South Africa resulting from heavy galling of flower buds by the gall wasp.
An Australian plant, Sydney golden wattle (Acacia longifolia), is the focus of a proposed application to the Environmental Protection Authority (EPA) for approval to release two new biocontrol agents into New Zealand. The application is expected to be submitted by the Northland Regional Council (NRC) later this year, following consultation with stakeholders. Richard Hill, a research associate with Manaaki Whenua – Landcare Research, is assisting NRC to prepare this application.
Sydney golden wattle (SGW) is a shrub that can reach 10 m tall. It grows particularly well in dune ecosystems, where it forms dense, shady stands that displace native species and increase fire risk. Like so many other weeds here, it was introduced as an ornamental plant in the late 1800s. Although not considered a major problem in New Zealand yet, SGW has the potential to occupy many coastal habitats and it is steadily getting worse. The Department of Conservation has invested considerable effort to reduce its prevalence in the nationally significant Kaimaumau swamp in Northland. SGW is also a problem in new pine plantations on Matakana Island (Bay of Plenty) and in the far north, at Muriwai (Auckland), and in dunes near Wanganui and Levin. SGW grows as far south as Christchurch, and is now considered too widespread to control manually. “Biocontrol seems to be the only way now to prevent the adverse effects of SGW spreading widely through coastal areas of the North Island,” said Richard.
As its name suggests, SGW is native to Australia but it is weedy in a number of countries, especially South Africa and Portugal. This shrub was introduced into South Africa around 1827 with the aim of stabilising drift sands on the Cape flats, but it became a major weed, invading the delicate and unique ‘fynbos’ vegetation as well as mountain and river ecosystems. There was some resistance to the release of biocontrol agents in South Africa, as wattles (Acacia species) are commonly used as a source of wood and tannins. “As a compromise two agents were introduced in the 1980s that just attack the reproductive structures,” explained Richard. The gall-forming wasp (Trichilogaster acaciaelongifoliae) and seed-feeding weevil (Melanterius ventralis) proved highly effective and hence are now being considered for release in New Zealand. The wasp has also more recently been released in Portugal.
The gall wasp creates golfball-sized galls in the flower buds, preventing flower production, while the weevil larvae finish the job by destroying the seeds in those few pods that escape gall wasp attack. Working together, these two agents limit seed production in South Africa by over 90%, and galling by the wasp is often so severe that it causes the whole SGW plant to die. The success of the agents appears to be closely associated with climate. The performance of the wasp can be negatively affected by long, dry periods with high temperatures, which result in desiccation of the eggs. The performance of the seed-feeding weevil in South Africa is dictated by the short period of time the seed pods exist on the host plant (allowing only one generation of weevils per year), as well as the success of the gall wasp at destroying the flowers. “Conditions in New Zealand appear to suitable for both insects,” confirmed Richard.
Comprehensive host-range testing prior to the introduction of these agents to South Africa and Portugal means that no further testing for New Zealand is required. Although they belong to the same family, New Zealand’s native legumes are only remotely related to the acacias. There are a number of exotic Acacia species grown as ornamentals. Acacia melanoxylon is the only species grown commercially here, but it makes up <2% of forest plantations. There is strong evidence to suggest that the risk these insects pose to non-target, economic or ornamental legume species here is negligible. If approval to release these agents in New Zealand is granted by the EPA, it is anticipated that both agents will be imported into containment late next year and released shortly thereafter.
Funding to support this project is being provided by the Northland Regional Council and a Medium Envirolink Advice Grant (NLRC213). For more information about this project and the EPA approval process see: https://www.landcareresearch.co.nz/science/plants-animals-fungi/plants/weeds/biocontrol/approvals/current-applications/sydney-golden-wattle
CONTACT
Richard Hill – hillr@landcareresearch.co.nz