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++ I ¥ +I -...-Con-Se-Rv-~-Tionn-Ew-S ++ I ¥ +I -......-C-O-N-SE-R-V-~-T-I-O-N-N-EW-S----.- ~ _~ ¥ .:-==~~=1 ~-,--~ ----- - -------~ GO RE Biological control of Australian acacias in South Africa by John Hoffmann Zoology Department, University of Cape Town Golden wattle (Acacia pycnantha) that threaten our environment. takes pride of place as the national The impact of the long-leafed wattle­ floral emblem of Australia where it gall wasp has been gradual because in occurs naturally in the south-eastern most infestations, vast quantities of parts of the country. Indeed, in his seeds had accumulated in the soil over book A field guide to Australian trees, extensive areas. However, because of Ivan Holliday gives top billing to the insect, very few seeds have been golden wattle as 'the Acacia that most produced in the last twenty years and enlivens the Australian bushland and seed banks have become depleted to roadsides after a drab winter'. The the extent that decreasing numbers of bright yellow flowers occur in clusters new plants are recruited following on stalks and are prominently fires. exhibited in masses above the green In the case of golden wattle, the foliage. This impressive display of effect of the wasps is expected to be flowers has become a part of the spring more immediate because the weed is at vista in some areas of the Western Cape a much earlier stage of invasion and is and Eastern Cape following deliberate still confined to a few isolated areas in plantings of golden wattle in South the Western and Eastern Cape. Golden Africa during the 1800s. wattle occupies much the same habitat Ironically, golden wattle is not easy Golden wattle flowers. Photo: Tony Gordon. as another invasive species, Port to cultivate in Australia because Jackson willow (Acacia saligna) which attacks by specialist, gall-forming process the vigour of the plants is is currently being brought under insects stunt the growth and flower drastically curtailed. control by an introduced fungus. production of the plants. One of these The first releases of golden wattle­ Without the wasps to control it, golden gall-forming insects has now been gall wasps were made in 1987 by wattle was expected to spread rapidly introduced into South Africa in a entomologists from the Plant into areas vacated by Port Jackson deliberate attempt to check the natural Protection Research Institute (PPRI). willow so that one invasive species dispersal of golden wattle here. The Initially the insects were slow to would have replaced another with no galls are induced by the grubs of a increase in numbers but since 1995 apparent gain. The wasps which are species of wasp (Trichilogaster, massive populations have developed effectively sterilizing their host plants Pteromalidae) which can only develop around the original release sites and will largely prevent any such take-over on golden wattle. The adult wasps are the wasps have been manually redis­ by golden wattle. active throughout November but tributed throughout the range of the There is no doubt that golden wattle individuals only live for a few days. weed. Galls are now extremely will virtually stop spreading naturally. During this period, the females insert common and flowers are becoming In time the densities of existing infesta­ eggs into the small immature inflores­ increasingly scarce so that the once­ tions of plants will decline, with cences which are destined to become prominent displays of golden flowers substantial benefits for the flowers and seeds during the following are no longer apparent. Although this environment and the sustainable flow spring. On hatching, the grubs feed in loss may be felt by some, it is a small of water in our alien-infested rivers. the inflorescence and secrete chemicals price to pay' for the benefits that will All this will have been achieved at that mimic the plant's hormones and accrue from- not allowing golden wattle very little expense and with no detri­ induce abnormal growth. to spread further. mental ecological side effects. In the Inflorescences with grubs become In the case of the golden wattle-gall mean time, other insects, especially distended and develop into character­ wasp, history is repeating itself in seed-feeding weevils, are being used to istic galls instead of normal flowers many ways. This project follows the enhance the biological control of these and seed pods. The grubs live in successful introduction of a closely and other invasive alien acacia species. hollow chambers within the galls and related species of gall wasp The aim is to have a complex of two to are thus protected from environmental (Trichilogaster acaciaelongifoliae) to three biological control agents on each extremes and predation while being control long-leafed wattle (Acacia weed species so that the damage ensured an abundance of food. Besides fongifolia) in 1982. Before this, long­ caused by the plants will be minimized destroying the reproductive tissues, leafed wattle was one of the major and the integrity of our natural galling stunts the growth of plants invasive alien plant species in the ecosystems can be restored. because more resources are required to country. Nowadays the weed is not produce galls than would normally be even mentioned when rankings are Acknowledgements allocated to flowers and seeds. In the assigned to the many invasive species Studies to monitor the effectiveness of the wasps are being undertaken in the Zoology Department, University of Cape Town in conjunction with the Plant Protection Research Institute of the Agricultural Research Council. Generous financial support for the project has been provided by the Department of Water Affairs through the Working for Water Programme. Far left A female adult of the golden wattle gall-wasp. Photo: Tony Gordon. Left Galls induced by the larvae of the golden wattle gall-wasp. Photo: John Hoffmann. Veld 8-' Flora June 2001.
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