Acacia saligna (Port Jackson willow)-Management and Control September 2010 Contents 1.0 Introduction.................................................................................................Page 1 2.0 Physical Control..........................................................................................Page 1 3.0 Chemical Control........................................................................................Page 2 4.0 Biological Control........................................................................................Page 2 5.0 Other.............................................................................................................Page 3 6.0 References.....................................................................................................Page 3 1.0 Introduction Acacia saligna, or the Port Jackson willow, is a very adaptable and fast growing tree native to Western Australia (Midgely & Turnbull, 2003). These attributes have led to its widespread distribution as an important species used extensively for soil stabilisation, animal fodder, and a source of fuel wood in many countries around the world (Midgely & Turnbull, 2003). Acacia saligna has been identified as one of three priority multipurpose species for arid and semi-arid zones by FAO’s Silvae Mediterranea Network, with an estimated 300 000 ha planted globally (Midgely & Turnbull, 2003). In some areas, A. saligna has gone on to become an invasive species with a wide range of impacts. This is especially apparent in the unique South African fynbos systems, where A. saligna has displaced native species mainly through altering the fire regime (Musil, 1993; Holmes, 2002). Acacia saligna is a difficult species to manage, with control methods having to deal with its ability to resprout from its roots and the large, persistent seed banks it creates (Hadjikyriakou & Hadjisterkotis, 2002). Furthermore, there are often conflicts of interest over the desire to control acacias whilst continuing to commercially exploit them (Impson et al., 2009). 2.0 Physical Control MacDonald & Wissel (1992) analysed a number of physical clearing methods used for A. saligna in South Africa to determine cost effectiveness. These included cutting off the plants at ground level or below ground level, cutting off at 20 cm above ground level and then debarking to ground level, ringbarking all plants above 5 cm diameter and digging out those below 5 cm, and cutting off at ground level and then painting with either 5% Glyphosate in water solution or 1% Triclopyr in water with 0.5% Agripon (MacDonald & Wissel, 1992). Various combinations of these techniques over the 5-year experiment were found to be effective for different densities, the presence of coppicing plants and in the presence or absence of burning after the first year. Burning of the treated area after initial treatment raised costs of management markedly, but is an integral component of control to reduce the soil seed bank (MacDonald & Wissel, 1992) (see Other for more information on seed- bank reduction). The optimal strategy for A. saligna was determined to be a foliar arborcide application to post-fire regeneration followed in subsequent years by repeated mattocking operations. While the model developed by MacDonald & Wissel (1992) allowed for the maximisation of management efficiency, the recognised high costs and labour required illustrated the need for the development of biological control measures at the time of writing (MacDonald & Wissel, 1992). IUCN SSC Invasive Species Specialist Group Page 1 3.0 Chemical Control As mentioned earlier, both 5% Glyphosate in water solution and 1% Triclopyr in water with 0.5% Agripon were used with some effectiveness when painted onto freshly cut stems (MacDonald & Wissel, 1992). Subsequent studies on Glyphosate usage in South Africa determined that application in the late summer required a dose rate of 2880 g/ha using a high surfactant concentration to obtain satisfactory control whereas for the other seasons doses of 720 and 1440 g/ha using a low surfactant concentration controlled plants satisfactorily (Pieterse & McDermott, 1994). Pieterse & McDermott (1994) therefore do not advise applying Glyphosate during hot dry summers. 4.0 Biological Control The difficulty and cost associated with physical and chemical control methods for A. saligna led to it being targeted for biological control (Wood & Morris, 2007). Following host specificity testing, the gall-forming rust fungus Uromycladium tepperianum was introduced into South Africa from Australia in 1987, establishing into 174 localities throughout the range of A. saligna from 1988 to 1996 (Morris, 1987; in Wood & Morris, 2007). Through wind dispersal, U. tepperianum is now found wherever A. saligna occurs, resulting in the lowering of population densities by at least 80 % in the absence of fire (Morris, 1997) and reducing longevity and reproductive output (Impson et al., 2009), with few trees living more than 10 years (Wood & Morris, 2007) when the lifespan has been reported to be between 30 – 40 years (Milton & Hall, 1981; in Wood & Morris, 2007). It is postulated that U. tepperianum does not kill the individual plant by itself, but rather the high number of galls formed on the plant as a result of infection stress the plant to such an extent that it cannot cope with additional environment stressors such as drought (Morris, 1999; in Wood & Morris, 2007). The follow up study performed at the same sites as Morris (1997) by Wood & Morris (2007) determined that U. tepperianum has continued to be an effective biological control agent against A. saligna in South Africa with predicted densities continuing to decrease over time (Wood & Morris, 2007). The large soil seed banks produced by A. saligna prior to the introduction of U. tepperianum however has led to the recognition of the need for a second biological control agent to target the remaining seeds (Impson et al., 2009). As such the seed-feeding weevil Melanterius compactus was released in South Africa in 2001, with preliminary monitoring indicating that they are playing an important supplementary role in curbing the production of viable A. saligna seeds (Impson et al., 2009). A number of other potential biological control agents have been discussed over the years. These include Eriophyid mites, with 17 species potentially being of use for the biological control of a number of invasive plants in South Africa including A. saligna (Craemer et al., 1997). Furthermore A. saligna is susceptible to white scale (Coccidae) which attacks the leaves and stems (Midgely & Turnbull, 2003) and termite attacks are known to cause serious problems in some tropical countries (Michaelides, 1979; in Midgely & Turnbull, 2003). 5.0 Other Reduction of the seed-bank is seen as an important factor of A. saligna control, with or without the application of biological control methods (Holmes, 1990; in Cohen et al., 2008). For Acacia species in fire-prone ecosystems, water-impermeable seed dormancy is broken by exposure to a heat- pulse (Richardson & Kluge, 2008). A slow, hot fire depletes more seeds than a rapid one, as in addition to killing seeds on the soil surface it also promotes germination of deeply buried seeds (Milton & Hall, 1981; in Richardson & Kluge, 2008). Emergent seedlings can then be treated with herbicide or selectively grazed (Campbell, 2000; in Richardson & Kluge, 2008). It is important to note that the use of intense fires can have adverse effects on the environment such as destroying IUCN SSC Invasive Species Specialist Group Page 2 the soil structure, and sterilising the soil for up to three years (Campbell et al., 1999; in Richardson & Kluge, 2008). Soil solarisation has been found to be effective in reducing A. saligna seed-banks (Cohen et al., 2008). It involves the solar heating of soil by covering it with a transparent polyethylene sheet during the hot and dry seasons. This raises the temperature of the upper layers of soil up to 40 – 55 °C and can lead to the elimination of pests, pathogens and weeds (Cohen et al., 2008). In experimental plots, Cohen et al. (2008) demonstrated that soil solarisation was capable of reducing A. saligna seed viability to 45 % in moist soil up to a depth of 12 cm. In contrast, heating of dry soil was determined to be ineffective in reducing seed viability, but was able to increase germination rates from 8 % to 67 % (Cohen et al., 2008). 6.0 References Cohen, O., Riov, J., Katan, J., Gamliel, A., & Bar (Kutiel), P. (2008). Reducing persistent seed banks of invasive plants by soil solarisation - the case of Acacia saligna. Weed Science, 56(6), 860-865. Craemer, C., Neser, S., & Smith Meyer, M.K.P. (1997). Eriophyid mites (Acari: Eriophyoidea: Eriophyidae) as possible control agents of undesirable introduced plants in South Africa. Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie, 1(3), 99-109. Hadjikyriakou, G., & Hadjisterkotis, E. (2002). The adventive plants of Cyprus with new records of invasive species. Zeitschrift fuer Jagdwissenschaft, 48(Supplement), 59-71. Holmes, P.M. (2002). Depth distribution and composition of seed-banks in alien-invaded and uninvaded fynbos vegetation. Austral Ecology, 27(1), 110-120. Impson, F.A.C., Moran, V.C., Kleinjan, C., Hoffmann, J.H., & Moore, J.A. (2009). Multiple-species introductions of biological control agents against weeds: look before you leap. In M.H. Julien, R. Sforza, C. Bon, H.C. Evans, P.E. Hatcher, H.L. Hinz, & B.G. Rector (Eds.) Proceedings from the XII International