S​ilky hakea (Hakea sericea) (Proteaceae)

The Weed

Hakea sericea, commonly known as silky hakea, is a tree species capable of reaching heights of 5 m. It is native to several parts of Australia, including Tasmania, New South Wales and Victoria, but has become invasive in large parts of South Africa, New Zealand and even some areas in Portugal. The species was introduced to South Africa as a hedging plant around 1858 and quickly became invasive. Within the space of 50 years, the area infested with H. sericea increased more than 30 fold, from an initial recorded estimate of 9000 ha in 1938, to over 350 000 ha in 1983. It has been declared as a Category 1b invasive alien species in terms of the 2014 Regulations for the National Environmental Management: Biodiversity Act (NEMBA) of 2004, and preceding this, in the Conservation of Agricultural Resources Act (CARA) of 1983. 

Hakea sericea is a member of the Proteaceae family and has long, needle-shaped leaves, which gives rise to the common name for most hakeas, needle bush. Like many Proteaceae, H. sericea is a serotinous species, which releases its seeds post-fire. This adaptation leads to some trees expelling in excess of 10 000 seeds when exposed to fire events. Germination post-fire is quite rapid and the species is a good competitor, adding to its invasive potential in Fynbos regions where it is a major threat to biodiversity. 

The mature fruits of H. sericea consist of two woody dehiscent halves which open only once the plant dies or in response to fire, releasing two black, winged seeds. The plants only develop fruits in their second or third year of life. Silky hakea flowers during the late autumn and early spring months, and produces silky wisp-like flowers which occur in curls, close to the tips of the spiky leaves. 

Control Options

Historically, H. sericea has been controlled by both slash-and-burn techniques, and by extensive clearing operations through the Working for Water Programme within the South African Department of Environmental Affairs. These techniques had varying levels of success, but ultimately proved too costly, given the extent of the hakea infestation. Biological control of H. sericea was initiated in the 1960s, following the first introductions of seed-feeding insects, the weevil, Erytenna consputa and the seed-feeding moth, Carposina autologa. An additional weevil species, Cydmaea binotata, was unsuccessful as a biocontrol agent and efforts at establishing the species were abandoned shortly after its original introduction.  Several other species are used in combination, in an effort to control the spread of hakea. The indigenous fungus, Colletotrichum acutatum, was noted as having an impact on hakea mortality at several sites, in 1969, and has since been developed further into a mycoherbicide. In the later years of biocontrol on H. sericea, two new agents were introduced from Australia. The stem-boring beetle Aphanasium australe was introduced in 2001 and the flower-bud feeding weevil, Dicomada rufa, was introduced in 2005. Investigations into the efficacy of each of these established agents are ongoing, but it is hoped that an integrated approach, adopting agents that impact on varying stages of the weed, will have the greatest impact in reducing the overall spread of H. sericea

Silky hakea​​Silky hakea​​Silky hakea

More information

  • ​​Copies of the following scientific reviews on silky hakea and its control can be downloaded: 
    • 1991 (biocontrol using insects) pdf​
    • ​​1991 (biocontrol using fungi) pdf
    • 1999 (biocontrol using insects) pdf
    • 1999 (biocontrol using fungi) pdf​
    • 2011 (biocontrol using insects and fungi)pdf​
  • Copies of the following leaflets can be downloaded:
    • Hakea sericea (1-page fact sheet) pdf
    • Hakea sericea ​(4-page leaflet) pdf​
    • Aphanasium australe pdf
    • Carposina autologa pdf​
    • Colletotrichum acutatum sheet 1 pdf​
    • Collectotrichum acutatum ​sheet 2 pdf
    • Erytenna consputa pdf

​Contact: Dr Candice-Lee Lyons at LyonsC@arc.agric.za​.