Hakea sericea
(silky hakea)

Hakea sericea, native to eastern Australia, is a major invasive woody species in South Africa, New Zealand and more recently in mainland Portugal and France. Characteristics promoting invasion include its prolific production of canopy borne seeds in the absence of natural enemies, the high seed longevity in the canopy and efficient seed dispersal. In dense stands, H. sericea seed densities of up to 7500 seeds per m² have been reported and the winged seeds can facilitate dispersal over several kilometres. An additional factor that contributes to invasiveness is the regularity of wildfires that occur in areas invaded by H. sericea. While H. sericea is a weed of pasture, it is principally an environmental weed and poses a serious threat to the biodiversity of the 'fynbos' Cape Floral Kingdom in the Western Cape and Eastern Cape provinces of South Africa.

Hakea sericea is native to Queensland (Mt. Barney, Mt. Maroon and Mt. Mee) and New South Wales in eastern Australia, with naturalized occurrences overseas in South Africa, New Zealand and southwest Europe (Barker, 1996). In South Africa, dense stands of H. sericea occur in the Western and Eastern Cape Provinces and isolated plants have been recorded in Kwa-Zulu Natal. In New Zealand, H. sericea occurs north of Auckland where it is considered an important invader because of its ability to invade Leptospermum and gumland communities (Beever, 1988) and is also seen in the north of South Island. H. sericea is reported to have been planted for reclamation of arid lands in Spain and Portugal and has become locally naturalized (Royal Botanic Garden Edinburgh, 2003) and is recorded as invasive in the south of France (EPPO, 2019). It is also reported from Angola (EPPO, 2019).

Further spread is possible if accidentally introduced as an ornamental. The following is taken from recent pest risk analysis for Europe (EPPO, 2019).

In Europe, H. sericea was added to the EPPO Alert List in 2007 and transferred to the EPPO List of invasive alien plants in 2012. In 2016, H. sericea was identified as a priority for risk assessment within the requirements of Regulation 1143/2014 (Branquart et al., 2016; Tanner et al., 2017). A subsequent PRA concluded that H. sericea had a high phytosanitary risk to the endangered area (EPPO, 2018) and was added to the EPPO A2 List of pests recommended for regulation (EPPO, 2019). In Spain, H. sericea is included in the Annex II of the Real Decreto (Royal Decree) 1168/2011, a list of potentially invasive species, inclusion meaning that the introduction of the species listed is prohibited and that necessary measures should be taken for management, control and eradication. In France, although there is no national regulation covering H. sericea specifically, at the department level, individual applications have been made for control orders against H. sericea that is also included on a regional ‘black list’. In Portugal, legislation was passed in 1999 (Decreto-Lei 565/99) to address the issue of invasive alien species, including H. sericea, meaning that cultivation, use as an ornamental plant, release, sale, exchange and transport are all prohibited.

In Israel, the species is considered to be a potential future risk and is included in a recent list of ‘Israel’s Least Wanted Alien Ornamental Plant Species’ which, although it does not currently have any legislative basis, is being used by the Israel Ministry of Environmental Protection to advise on non-native species to avoid in planting schemes (Dufour-Dror, 2013).

Hakea sericea is included on many weed lists in New Zealand (Howell, 2008) including the ‘consolidated list’ of Howell (2008), but that does not have regulatory status. In South Africa, the control of the species was enabled by the Conservation of Agricultural Resources (CARA) Act 43 of 1983, as amended, in conjunction with the National Environmental Management: Biodiversity (NEMBA) Act 10 of 2004. H. sericea is classified as a Category 1b invader species on the NEMBA mandated list (Government of the Republic of South Africa, 2014), meaning that it must be controlled and wherever possible, removed and destroyed and that any form of trade or planting is strictly prohibited.

Hakea sericea in its native range grows naturally in heaths and in the understorey of dry sclerophyllous forests in coastal regions from southeastern Queensland to southeastern New South Wales, Australia (Brown and Whelan, 1999). Where naturalized in South Africa, H. sericea occurs in sclerophyllous vegetation known as mountain fynbos (or 'macchia') which is virtually devoid of other tree species. In New Zealand, H. sericea invades native Leptospermum and gumland plant communities (Beever, 1988).

In its European invaded range, habitats include disturbed areas, particularly road margins, forest margins, coastal grasslands and pine forest are all highlighted as additional habitats (Fried, 2010; Marchante et al., 2014). In South Africa, H. sericea is reported as primarily a problem in the sclerophyll vegetation type known as mountain fynbos (Kluge and Neser, 1991). Characteristics of local habitats that enhance invasiveness of H. sericea (Kluge and Neser, 1991) include the virtual absence of competition from native tree species (Macdonald and Richardson, 1986), frequent fires (Kruger and Bigalke, 1984), human disturbance (e.g. altered fire regimes; Macdonald, 1984) and the lack of specialized natural enemies (Neser, 1968).

Hakea sericea is not a weed of crops but has been known to invade valuable pasture land in South Africa (Phillips, 1938). It is today, however, principally regarded as an environmental weed.

Genetics

Genetic variation in South African populations of H. sericea was examined by Dyer and Richardson (1992), using 11 populations representative of the range of invaded habitats where the species has spread from initial limited plantings in the mid-1800s, to occupy 4800 km² or 14% of the area of mountain fynbos in the Western, Southern and Eastern Cape, South Africa. Total genetic diversity (HT= 0.1157) was low compared with the average for plants (HT= 0.310), but similar to that of other serotinous species, with genetic diversity among populations (GST= 0.242) close to the average for plants (GST= 0.224). Inter-population diversity, due largely to differences between populations in the Western Cape and those in South and Eastern Cape, was discussed and the excess of heterozygotes in all populations implies an outbreeding mode of reproduction, suggesting significant heterozygote advantage.

 Physiology and Phenology

In its native range in eastern Australia, flowering occurs from winter to early spring (June to September) and produces woody fruits that can persist for several years (Brown and Whelan, 1999). Fruit development begins in October soon after flowering and fruits have been found to rapidly contribute to the availability of viable seeds in the canopy seed bank (Brown and Whelan, 1999) from which seeds are typically released from woody follicles (fruits) following the death of the plant, frequently caused by fire (Bradstock, 1991). The newly set fruit is green, soft and juicy but as fruits mature, they become woodier. The heat-resistant fruits, produced annually, are stored in the canopy and the seeds are only released once the plant has died. There is no seed bank in the soil. Seeds are released following the death of a branch; however, seeds can also be released from a small percentage of fruits that are on living branches (E. Marchante, pers. comm. 2017). The nutrient-rich, thin-coated seeds can germinate throughout the year provided the soil is moist from an initial good precipitation. After germination, a taproot develops even before the first true leaves are formed which may contribute to the seedling's resistance to drought (Fugler, 1979). The juvenile stage is 2 years and mature H. sericea trees can live for up to 30 years. The flowering period in part of its European invaded range (France and Portugal) is given as December to April, i.e. winter to early spring as in the native range (Paiva, 1997). H. sericea produces a tap root during the seedling stage and thick sub-surface lateral roots up to 4 m long as the plant matures. H. sericea also produces proteoid roots along the lateral roots.

Reproductive Biology

Propagation is by seeds, which accumulate on the plant throughout its lifetime. Large numbers of seeds are released following the death of the plant, usually by fire. The prolific seed production of H. sericea can result in estimated seed densities of up to 7500 seeds per m² in the ash bed following fires (Neser and Kluge, 1986). Beadle (1940) found that the seed of H. sericea was capable of withstanding temperatures of 110°C for 4 hours without a significant reduction in the germination percentage. No cases of vegetative reproduction have been found in South Africa although the plant can regrow vegetatively if damaged. The seed production of H. sericea is far more prolific in South Africa than in its native range. This has been attributed to the absence of natural predators in South Africa (Neser, 1968).
 

Fire is a key part of the life cycle of H. sericea, with the heat-resistant fruits accumulating on a plant throughout its lifetime. The plant itself is ‘absolutely fire sensitive’ (Morrison and Renwick, 2000). However, after plant death, typically through fire, the fruits release their seeds (Kluge and Neser, 1991). The strategy of storing seeds in the canopy in fire-resistant woody fruits is not unusual in fire-prone ecosystems (Cowling et al., 1987) and has been referred to as ‘serotiny’ (Lamont et al., 1991) or ‘bradyspory’ (Whelan, 1995). The strategy has been viewed as an adaptation to fire by some authors (Bradstock et al., 1994), although it is found in many parts of the world and is not always associated with fire (Bond and van Wilgen, 1996). Fire frequency, seasonality and intensity are all important for the natural regeneration of H. sericea (e.g. Brown and Whelan, 1999), as frequent fires may kill seedlings after the initial stimulation of seed release and germination. Fire dynamics are, therefore, important determinants of community composition in any ecosystem which is burnt at a frequency that regularly influences the regeneration cycles of any of its constituent species (Bond and van Wilgen, 1996). Studying H. sericea in its native Australia in the context of fire seasonality and community diversity, Brown and Whelan (1999) for example, found that fire too early in the fruit ripening process could reduce the supply of viable seeds due to the unripe fruits still containing enough moisture to make heating lethal to young tissue. H. sericea has been identified as influencing fire regimes both positively and negatively (Mandle et al., 2011), increasing fuel loads and intensity, but decreasing spread and frequency (van Wilgen and Richardson , 1985; Holmes et al., 2000; van Wilgen et al., 2007).

Environmental Requirements

Hakea sericea is native to areas in Australia where the rainfall is relatively evenly distributed throughout the year. In South Africa, however, it occurs in rainfall regimes including a Mediterranean-type climate with a summer drought, a summer rainfall region and an area with rainfall throughout the year. H. sericea occurs mainly on well-drained soils derived from sandstone and quartzite with low nutrient levels. These soils are low in available nitrogen and phosphorus and often lack copper, zinc and molybdenum (Specht and Rayson, 1957). The spread of H. sericea in South Africa has been restricted to some extent by barriers of unsuitable nutrient-rich substrata (Fugler, 1979).

In its native range, H. sericea is a widespread in areas with a warm temperate climate (Specht, 1994), having good drought resistance, although water limitations or heavy soil may lead to stunting (ANBG, 2017) and it is resistant to frost to -7°C when established (Moore, 2004). The native range mapped by Barker (1996) corresponds to Koppen–Geiger climate zones Cfb (warm temperate, fully humid, warm summer), with a small overlap with Cfa i.e. the same but with a ‘hot’ rather than warm summer (Kottek et al., 2006).

Parts of the native range is characterized by nutrient-deficient sandstone soils, typical of those on which heathland plant communities are found (Specht, 1994) and H. sericea like other Proteaceae is well adapted to the acidic, highly weathered soils of such areas (Lambers et al., 2008). Richardson (1984) also found quartzite and sandstone substrates to be correlated with the occurrence of Hakea spp. in South Africa. In its European invaded range, Martins et al. (2016) showed that schist was an important predictor of the distribution of H. sericea at smaller scales.

Surveys by South African researchers have discovered more than 40 insects in over 20 families that attack H. sericea in Australia (Kluge and Neser, 1991), beginning with surveys in 1962 (Moore, 1964). Four host-specific herbivorous insects have been introduced from Australia for the biological control of H. sericea in South Africa. In addition, an indigenous fungus has also been utilized in the H. sericea biological control programme. Recent progress in determining the host range of a flowerbud-feeding weevil, Dicomada rufa, may soon allow the release of this agent. Thirteen indigenous herbivorous insects in nine families have been recorded on H. sericea in South Africa (Swain and Prinsloo, 1986), although none of these do any significant damage to the plant.

Environmental

• Amenity
• Erosion control or dune stabilization
• Landscape improvement
• Revegetation
• Shade and shelter
• Soil conservation
• Soil improvement
• Windbreak

Fuels

• Fuelwood

Human food and beverage

• Honey/honey flora

Ornamental

• Christmas tree
• garden plant

Hakea sericea is very similar to H. decurrens and has also been confused with H. gibbosa. H. sericea can be distinguished from the other main Hakea species naturalized outside of Australia (H. gibbosa, H. drupacea and H. salicifolia) according to a key adapted from Webb et al. (1988) on non-native plants of New Zealand. However, in some cases, the genus Hakea may be hard to distinguish from some morphologically similar Grevillea species (Barker, 2010). The most obvious characteristic distinguishing H. decurrens from H. sericea are its thin fruits. H. gibbosa has longer, distinctly hairy leaves and larger fruits than H. sericea and H. decurrens. For botanical descriptions of native Australian Hakea spp., H. propinqua, H. macraeana, H. teretifolia, H. gibbosa, H. sericea, H. constablei, H. microcarpa, H. dactyloides, H. eriantha, H. salicifolia and H. ulicina, see Stacy (1977).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Mechanical Control

The most successful method to control H. sericea is the fell and burn technique where adult plants are cut down and left for 12 to 18 months before they are burnt. Shortly after the plants are cut down, the fruits, which have accumulated over the plant's lifetime, split open and fall to the ground. The released seeds germinate the following winter. The area is then burnt before the seedlings become reproductively mature. One or two follow-up operations are necessary after the burn to eradicate any regenerating or coppicing plants. This is an extremely important facet of the operation as it ensures that no plants are left to produce viable seeds. Although this is a very effective control method, the increased fire intensities using this technique can have a negative effect on sensitive ecosystems (Breytenbach, 1989). It is not advisable just to cut and then leave the cut plants unburnt, as fire is needed to kill any seedlings that may germinate. Burning standing plants can be effective in some cases but may result in dense stands of seedlings and widespread dispersal. The manual eradication of seedlings is both time consuming and expensive.

A massive campaign was initiated in South Africa in 1996 called the 'Working for Water' programme. The objective of this programme is to increase and sustain water supplies by eradicating alien invasive plants from all rivers, wetlands, catchments and other water resources countrywide. Alien invasive plants, especially Hakea spp., Pinus spp. and Australian Acacia spp. are known to reduce water runoff and restrict stream flow and are being cleared as part of this campaign.

Chemical Control

Chemical control has not played a large role in the control of H. sericea as it can have a negative effect on the indigenous vegetation. The costs of chemical control are also high as H. sericea occurs in dense thickets and inaccessible areas. Tebuthiuron is recommended for the control of H. sericea in South Africa. Triclopyr, glyphosate and an 8:1 glyphosate:picloram mixture were tested for the control of H. sericea seedlings, with triclopyr at 960 g/ha giving effective control and was cheaper per unit area than the other products and was recommended for use following burning as an alternative control system to the cut and burn technique (Donald and Nel, 1989).

Biological Control

A biological control programme against H. sericea was initiated in South Africa in 1962. Priority was given to seed-attacking insects and the first insect releases were made in 1970 (Kluge and Neser, 1991). Priority was given to seed-attacking insects and the first insect releases were made in 1970, being the hakea seed weevil, Erytenna consputa (Curculionidae: Erirhininae), the larvae of which destroy the green developing fruits (Neser, 1974). The adults of E. consputa live for 2-3 years and feed on shoots, buds, flowers and young fruit as these become available. The eggs are laid in or near young developing fruits from late winter to mid-summer. The larvae develop singly in the young fruit and can consume more than one fruit before they complete their development. Following its establishment, E. consputa has spread throughout the South African range of the plant and has drastically reduced the annual seed production of H. sericea at some sites (Neser and Kluge, 1985; Kluge and Neser, 1991; Gordon, 1999).

The hakea seed moth, Carposina autologa (Lepidoptera: Carposinidae), the larvae of which destroy the seeds in mature fruits of H. sericea, was introduced in 1972 to augment E. consputa. The moth is univoltine with no diapause or quiescent phase. Eggs are laid singly on the surface of mature fruits in autumn. The larvae enter the fruits along the suture on the axial surface of the fruit. Only one larva develops per fruit. The mature larva emerges from the fruit in summer and pupates in the soil. Despite difficulties rearing and releasing this agent, it has now become established at a number of sites in South Africa (Dennill, 1987; Dennill et al., 1987; Gordon, 1993). Studies and observations at release sites in the field showed that the moth has reduced the mean number of accumulated seeds on H. sericea by up to 80% (Gordon, 1999). Despite these promising results, several factors are limiting its effectiveness. Firstly, an indigenous fungus, Colletotrichum gloeosporioides [Glomerella cingulata] causes death and die-back of H. sericea in some areas and the fruits on infected trees split open and seeds fall to the ground resulting in larval mortality as the larvae are unable to move to new fruits (Gordon, 1993; Fourie et al., 2012; Gordon and Lyons, 2017). Secondly, the moths are unable to distinguish between healthy and previously attacked fruits for oviposition resulting in larval mortality as few neonate larvae successfully find and enter pristine fruits. Thirdly, regular wild fires in the Western Cape cause local extinction of C. autologa and they take a long time to recolonize regenerating plants (Gordon and Lyons, 2017).

A second weevil, Cydmaea binotata (Curculionidae: Erirhininae) was introduced into quarantine as a contaminant in one of the shipments of E. consputa and was successfully cultured in quarantine. They were found to be so damaging to H. sericea seedlings that it was felt that their feeding would enhance the infection rate of C. gloeosporioides. Their biology and host-specificity were studied and when the weevils were found to be host-specific permission was granted for their release in 1979. The larvae are most damaging as they tunnel down the leaves or distal sections of soft stems. Releases were made at 36 sites throughout the range of the weed but weevils have only since been recovered at four sites (Kluge and Neser, 1991). The impact of the weevil on H. sericea has not been investigated in South Africa because their effect on the density of seedlings has been negligible (Fourie et al., 2012).
 

The stem-boring beetle, Aphanasium australe (Coleoptera: Cerambycidae) was introduced not only to add impetus to the biocontrol of H. sericea project but also because it attacks the related weed Hakea gibbosa (Gordon, 2003). The larvae tunnel gregariously at the base of stems and in the sub-surface roots of the plant leading to stem bases developing a characteristic thickening due to the formation of scar tissue. Although A. australe does not kill mature plants growing under natural conditions, it is envisaged that trees subjected to additional stress i.e. from drought, may be killed by larval damage. The first releases of this agent were made during January 2000 (Fourie et al., 2012).

A flower bud-feeding weevil, Dicomada rufa, is a promising agent that is being considered for release to negate perceived weaknesses in the programme for biological control of H. sericea. The effectiveness of E. consputa and C. autologa is being hampered by periodic wildfires. Regenerating H. sericea plants only set seed 2-3 years after a burn, causing local extinction of E. consputa and C. autologa as both agents require fruits for development. As D. rufa feeds on buds, flowers and succulent growth, it is believed D. rufa could make a significant contribution by limiting fruit production at this critical stage (Gordon, 1999). The adults of D. rufa are 2-3 mm in length and dull grey-brown in colour. The larva is a cream-coloured, legless grub that feeds on the flowers and succulent growth. Due to problems culturing this insect in quarantine, data on its host-specificity was obtained by developing a type of open-field testing method, the fixed-plot survey method, to show that D. rufa is host specific to H. sericea (Kluge and Gordon, 2004).

An indigenous fungus, Glomerella cingulata f.sp. aeschynomene, has been exploited for the biological control of H. sericea in South Africa. The fungus causes stem and branch lesions exuding quantities of colourless gum. Plants may be killed if lesions occurring on the lower regions of the main stem girdle the stem (Morris, 1981; 1991). Individual branches that are girdled can also die. The growing points of young seedlings are most susceptible resulting in necrosis extending down the stem killing the plant (Morris, 1981). Various methods of culturing and inoculating plants have been successfully developed. These include wound inoculations, knapsack and aerial applications of spore suspension of the fungus and the application of a dried preparation of fungal-colonized wheat bran onto young seedlings (Morris, 1989; Morris et al., 1999).

Integrated Control

The 'Working for Water' programme in South Africa is primarily involved with the mechanical control of H. sericea but has identified biological control as the only long-term solution to prevent further spread of the weed and the re-invasion of cleared areas. Although mechanical control is an extremely efficient method to control H. sericea, biological control needs to be in place to prevent re-invasion of the weed and limit the need for follow-up operations. However, large-scale eradication of H. sericea can lead to the local extinction of established biocontrol agents. The seed-feeding agents are particularly at risk because seedlings recolonizing burnt areas take a number of years before they set fruit. It is, therefore, essential that insect refuges or reserves be established in areas to be cleared. These insect refuges can then act as foci from which recolonization of re-invading H. sericea populations can occur and collections of agents for redistribution can be made. These reserves should be 1-5 ha in size, 10 km apart and consist of reproductively mature plants (Gordon, 1999).

01/10/2019 Update by:

Nick Pasiecznik, Consultant, France