Galling guilds associated with Acacia dealbata and factors guiding selection of potential biological control agents R.J. Adair1 Summary The Australian tree Acacia dealbata Link (Mimosaceae) invades natural ecosystems in both the Northern and Southern Hemispheres, including areas beyond its natural range in Australia. Biological control is under development in South Africa using the seed-feeding curculionid Melanterius macu- latus Lea. A diverse range of galling insects occur on A. dealbata in Australia, most exhibiting high levels of host specificity and niche partitioning within their host. Galling insects have successfully contributed to biocontrol of other Acacia species in South Africa. Australian galling insects from A. dealbata have considerable potential for adoption as novel or complementary biocontrol agents. Factors governing the selection of potential agents are considered in the context of impact on the host, efficacy and compatibility with the utilization of the host for timber, pulp, floriculture and fire-wood harvesting, particularly in resource-poor regions of the world. The potential for biological control of A. dealbata in invaded habitats in Australia is also discussed. Keywords: wattle, conflict of interest, agent selection. Introduction of A. dealbata occurs in South Africa where large-scale invasions make other forms of suppression difficult to Silver wattle, Acacia dealbata Link (Mimosaceae: implement. This paper examines the role of classical Botrycephalae), is a widespread and conspicuous tree biological control of A. dealbata using gall-forming indigenous to forests and woodlands of southeast- agents and how such agents may affect commercial and ern Australia (Costermans, 1983). The species has a utilitarian values of the host tree. broad habitat range with two sub-specific taxa (subsp. dealbata, subsp. subalpina) that are delineated by al- A. dealbata—the invader titude (Kodela and Tindale, 2001). A. dealbata is of- ten abundant in early post-fire vegetation succession, While A. dealbata is native to eastern Australia, ex- where mass germination of soil-stored seed is triggered tensive and expanding naturalized populations occur in by burning. In its native habitat, A. dealbata provides south-west Western Australia, where the species was ecosystem functions such as food and habitat for fauna introduced for horticultural purposes. Although south- (Broadhurst and Young, 2006) and fixing atmospheric ern Western Australia has an astoundingly rich native nitrogen. The species’ silvery bipinnate foliage and Acacia flora (Hnatiuk and Maslin, 1988), there are no abundant production of bright yellow flowers in winter native Botrycephalae, and very few Western Australian to early spring contributes to the popularity of A. de- acacias are large woody trees. Consequently, invasion albata in horticulture. Large, naturalized populations of A. dealbata into the native vegetation in Western of A. dealbata now occur in many countries and can Australia may have undesirable ecological impacts, al- require management to protect natural and social assets though quantitative impact data both in Australia and (Sheppard et al., 2006; Adair, 2008). Biological control elsewhere are lacking. In South Africa, A. dealbata has been problematic as early as 1915 (Henkel, 1915) and is now a weed of national importance due to negative impacts on water management and biodiversity conser- 1 Department of Primary Industries, PO Box 48, Frankston, Victoria, Australia 3199 <[email protected]>. vation (Le Maitre et al., 2002; Nel et al., 2004). More © CAB International 2008 recently, in Europe, A. dealbata was listed as one of the 122 Galling guilds associated with Acacia dealbata and factors guiding selection of potential biological control agents top 20 invasive plants suggested as targets for biologi- case, biological suppression programs were strongly cal control (Sheppard et al., 2006). Invasions in south- beneficial to the national interest. ern France post-1910 have progressively replaced local Where A. dealbata threatens important assets, bona vegetation including cork oaks, l`arbousier (Arbutus fide utilitarian values need to be taken into account unedo L.) and heather (http://www.worldwidewattle. when designing biological control strategies to reduce com/). A. dealbata is also naturalized in New Zealand, levels of conflict of interest. Historically, potential con- western North America, Madagascar, Japan and Chile flicts of interests are avoided by: (1) not initiating bio- (Randall, 2002). logical control programs, (2) undertaking a cost–benefit analysis and proceeding with biological control where Utilitarian values of A. dealbata it is in the public interest, or (3) by targeting specific organs on the host and avoiding negative impacts on In Australia, A. dealbata is utilized in habitat resto- utilitarian interests. ration programs and urban landscaping projects. The species is not utilized commercially, although pollen used by honey bees contributes to the apiary industry. Biological control of Australian acacias In New Zealand and North America, A. dealbata is uti- Classical biological control of Australian acacias lized in horticulture but with limited economic value. was pioneered in South Africa, where eight species In contrast, the exploitation of A. dealbata is well de- are currently subject to active research, development veloped in southern Europe and South Africa where or agent redistribution programs. All of these pro- the species services quite different industries in each grams have succeeded in the establishment of one of these regions. or more agents, and several targets are now subject In Europe, A. dealbata was introduced around 1816 to satisfactory levels of suppression (Dennill et al., (Cavanagh, 2006) where acacias (‘mimosa’) are grown 1999; Hoffmann et al., 2002). Two general approach- for horticultural and floricultural purposes. The ‘mi- es to biological control of acacias have been adopted mosa’ cut flower industry in France occupies around in South Africa, each largely governed by the level 200 ha with an estimated value of €3–4 million/year of conflict of interest with commercial or utilitar- (Roland, 2006). Hybrids of A. dealbata and selected ian interests. Economically important species (A. cultivars form the basis of the industry and produce mearnsii, Acacia melanoxylon R. Br., A. dealbata, flower crops between December and March. Whether A. decurrens Willd.) are targeted solely for biologi- these selections and hybrids have naturalized in Eu- cal control of reproductive organs with seed-feeding rope is uncertain, but the creation and invasion of de curculionids (Melanterius spp.) that have no nega- novo genotypes by hybridization can complicate clas- tive impacts on vegetative growth of the host plant. sical biological control programs. Essential oils from In contrast, acacia species of little or no economic the flowers of A. dealbata are used as a fixative and value (Acacia cyclops A. Cunn. ex G. Don., Acacia blending agent in the manufacture of high-grade per- longifolia (Andrews) Willd., Acacia saligna (Labill.) fumes and soaps, and the industry consumes around €1 H.L.Wendl., Acacia pycnantha Benth.) are subject to million of refined ‘mimosa’ absolute per year (Roland, biological control of a range of plant organs where 2006). More recently, the French tourism industry has Melanterius spp. or flower-galling Cecidomyiidae promoted the virtues of the ‘Route de Mimosa’ during are used to target reproductive organs along with the main flowering season with numerous festive acti- Trichilogaster (Hymenoptera: Pteromlaidae) or Uro- vities linked to this period, undoubtedly contributing to mycladium (Fungi: Uredinales), which gall vegeta- local economies in the Bormes-les-Mimosas to Grasse tive organs. region. In Australia, biological control of invasive acacia In South Africa, silvicultural operations use Acacia species that have transgressed substantial geographical mearnsii De Wild. And, to a limited extent, Acacia de- barriers (trans-continental invaders) is advocated. A. currens Willd. A. dealbata is not commercially culti- dealbata invasions in Western Australia are suggested vated, but extensive areas of naturalized and invasive as targets for biological control (Adair, 2008). Biologi- populations of A. dealbata in eastern South Africa are cal control of A. longifolia in Portugal has commenced the legacy of early experimental and development pro- following successful control in South Africa (Sheppard grams. Resource-poor communities utilize A. dealbata et al., 2006). for fuel wood, charcoal and construction timber where harvesting is carried out ad hoc and driven by local- ized domestic needs (de Neergaard et al., 2005). The Galling agents and biological control contribution of A. dealbata to the regional and national strategies for Australian acacias economies of South Africa has not been calculated, al- though limited and careful extrapolation from the cost– Galling organisms vary in their impact on the host benefit analysis undertaken for A. mearnsii (de Wit et plant depending largely on the mode of physiological al., 2001) could possibly be made. In the A. mearnsii interaction with the host (innate impact), gall densi- 123 XII International Symposium on Biological Control of Weeds ties, phenological synchronization, location on the host The average dry weight of galled tissue compared and capacity to divert and accumulate resource alloca- to the average weight of the same un-galled