Acacia Mearnsii (Black Wattle) New Bio-Control Agent

Weeds in our Area (Part One Hundred and Twenty Five) By Bob and Ena McIntyre – Garden Route Acacia Mearnsii (Black Wattle) New Bio-control Agent

In our series of articles during the first half of last year we examined various biological control agents. One that stood out in terms of success as control agent was the little gall midge Dasineura dielsi. Released in 2002 this species has had a major impact on the seed production of Acacia cyclops (Rooikrans). Among the control agents we covered, the efficacy of the seed eating weevil (Melanterius maculates) on Acacia mearnsii (black wattle) had not yet been assessed. Nevertheless science is ever evolving and a cousin of gall the midges Dasineura rubiformis has progressed to an advanced stage of testing as an agent for the control of black wattle, probably the worst of the invasive species in our region.

Assessing a new control agent is a rigorous process as the precautionary measures are exceptionally stringent. It is essential to ensure that the new agent is not likely to become an invasive pest in its own right. For this reason initial assessment of the likely candidate happens in the agent’s country of origin. The suitable candidate is then imported, again under strict control, and tested in quarantine to ensure that the agent is host specific (will feed only on its particular plant host) and is not likely to threaten indigenous and other desirable vegetation types. This is done by exposing the potential agent to a variety of plants with characteristics similar to the host during the quarantine period. Even the slightest indication of risk will lead to the immediate destruction of the insects and the scrapping of the plan. Successful quarantine studies are subsequently followed by field trials to monitor the breeding cycles and determine the likelihood of predation on the new agent by indigenous species. Happily Dasineura rubiformis (nicknamed “Ruby”) currently under test in the battle to control black wattle has completed the quarantine stage and is now undergoing field trials. A researcher from University of Cape Town working with the Plant Protection Research Institute of the Agricultural Research Council at Stellenbosch is conducting this work.

What happens? The midge lays its eggs in the black wattle flowers. The larvae feed on the flower ovary causing the forming of galls that prevent the flowers from fertilisation and producing seed. The entomologists are also studying the biology of the midge and the possibility of it working in concert with the seed-eating weevil mentioned above. Black wattle produces vast numbers of seeds that can remain dormant for up 50 years, therefore any clearing project must include a well- planned and thorough follow-up programme and should “Ruby’s” trials prove 100% successful, she’ll still have plenty to cope with for many, many years. We wish the scientists every success in their efforts to stall the spread.

Identification: These are the multitudes of grey-green trees along our water-courses, road verges and mountain passes that sport an abundance of pale yellow or cream globular flower heads in large fragrant sprays during August and September. The leaves of A. mearnsii are bipinnate, dark olive green, finely hairy with short leaflets (1.5 – 4.0 mm). A. mearnsii has raised glands on the central leafstalk and between the junctions of the pinnae pairs. Unlike all other invader acacias with bipinnate leaves that have glands on the central leafstalk the glands are at the junctions of the pinnae pairs and not between. The fruit pods are dark brown with marked constrictions between seeds.

Control: Black wattle is resilient and coppices readily. Felling a sapling or mature tree necessitates prompt stump treatment with a suitable herbicide, while frilling with herbicide application will kill the tree standing. Young plants are easily strip-barked – strip bark from the stem from about 50cm above ground down to the root ball. No herbicide is necessary provided no bark remains on the root ball (very eco-friendly method). Another option is a basal stem treatment (painting the stem with the herbicide mixture from about 50cm to ground level) with a suitable herbicide mixed with diesel.

References: With sincere thanks to: “Wild” (Your Wild Card Magazine) incl photographs by Leonie Joubert (Leonie is a science journalist who writes about environmental change. Her books include Scorched: South Africa’s changing climate and Boiling Point: people in a changing climate. “ALIEN WEEDS AND INVASIVE PLANTS”: Lesley Henderson (Copyright © 2001 Agricultural Research Council). Problem Plants of South Africa: Clive Bromilow.