A New Biological Control Agent for Acacia Mearnsii in South Africa
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Research in Action South African Journal of Science 104, July/August 2008 247 closely synchronized with the distinct Dasineura rubiformis (Diptera: flowering pulse exhibited by Acacia mearnsii during spring (September/Octo- Cecidomyiidae), a new biological ber). Adult D. rubiformis live only a few days and females require open flowers for control agent for Acacia mearnsii oviposition, so this synchrony is essential. The eggs are laid within the perianth tube in South Africa of the flower and, on hatching, the larvae start feeding on the surface of the ovary, F.A.C. Impsona,b*, C.A. Kleinjanb, J.H. Hoffmannb and J.A. Posta at the same time inducing gall-formation and preventing pod set by affected flow- ers. The flowers of A. mearnsii occur in cecidomyiid midge, Dasineura rubifor- programme against invasive Australian globular flower-heads, each with about 45 mis, is the most recent addition to the Acacia species in South Africa has been flowers. Afflicted flower-heads generally Asuite of biological control agents that fraught with conflict, particularly due to produce a small, tightly packed cluster of have been deployed in South Africa against the economic importance (for tannin up to 36 galls [10.5 ± 1.0 s.e., n =83at invasive Australian Acacia species. This insect 14–18 is associated with Acacia mearnsii (black and paper pulp) of black wattle. As a Stellenbosch in July 2007 (unpublished wattle), which is extremely invasive, but also compromise, the choice of biological data)] instead of pods. Each gall within an important agro-forestry species, in South control agents has been restricted to those the cluster contains 1–5 chambers, and Africa. It induces development of galls in the that reduce the reproductive capacity but generally a single larva develops within flowers of A. mearnsii, thereby preventing not the useful attributes of the plants.6,18,19 each chamber. Third-instar larvae emerge pod development and reducing the reproduc- The concerns about biological control from the galls during winter (June/July) tive capacity of the plants. The useful attributes potentially disrupting commercial supplies and drop to the soil where they pupate in of this economically important plant species of seed were resolved by demonstrating should not be affected by the introduction of silken cocoons. D. rubiformis. The midge is established in the that the prospective agents can be effec- vicinity of Stellenbosch, where it is increas- tively controlled with conventional insec- Host range ing in abundance. Studies have been initiated ticides,3,20 some of which are routinely Surveys in Australia of 147 native Acacia to (i) evaluate the performance of the midge; applied in wattle plantations.21–23 species, including 27 within the section (ii) confirm that galling does not cause a reduc- The first agent to be released against Botrycephalae, along with three intro- tion in vegetative growth of A. mearnsii; and A. mearnsii was a seed-feeding weevil, duced African species and one intro- (iii) determine the potential effectiveness of Melanterius maculatus, which is now estab- duced American species,3 showed that, D. rubiformis as a biological control agent lished at a number of sites across the besides A. mearnsii, D. rubiformis may be of A. mearnsii. All indications are that the 6 insect has the potential to become an excellent country. Although M. maculatus can associated with the following Botry- seed-reducing biological control agent of cause substantial levels of seed reduction cephalae species in eastern Australia: Aca- A. mearnsii. of A. mearnsii, considerable quantities cia parramattensis, A. irrorata, A. deanei, of seed are still produced annually (F. A. leucoclada and A. constablei. The struc- Background Impson, unpublished data). The addition ture of galls on these five species resem- A diverse array of gall midges (Cecido- of complementary biological control bles that of D. rubiformis on A. mearnsii myiidae) is associated with the Australian agents to further reduce reproductive and DNA sequences of larvae from the acacias in their native habitat.1 In the late output of A. mearnsii was therefore desir- first four species matched those of D. rubi- 1990s, attention was focused on them in a able. formis. Larvae from the fifth species were drive to find additional biological control Eight gall midge species are associated not sequenced. To confirm these putative agents for use against invasive Australian with the reproductive structures of black identifications, adults need to be reared Acacia species in South Africa with empha- wattle in Australia. Of these, D. rubiformis from the galls and examined.1 In Western sis on black wattle (Acacia mearnsii).2,3 was considered to be the most suitable Australia, where both insect and host Acacia mearnsii is one of South Africa’s candidate for use in the biological control plant are naturalized, D. rubiformis was most widespread and problematic invasive programme against black wattle in South found only on A. mearnsii.1 Of the species plants.4–7 Its presence in natural forests, Africa. This decision was based partly on listed above, only A. mearnsii currently grasslands and water courses continues observations that D. rubiformis substantially occurs in South Africa. to threaten ecosystems in terms of loss of reduces pod production of A. mearnsii in Host-specificity tests, conducted at the biodiversity, reduced water supplies, in- Western Australia, where both the midge Plant Protection Research Institute in creased soil erosion and by exacerbating and host plant have become naturalized Stellenbosch between 1999 and 2001, the intensity of fires.8–12 The success of after being introduced from eastern confirmed the restricted host range of the plant as an invasive weed and its Australia.3 In this article we summarize D. rubiformis. During these studies flow- persistence in the landscape is largely progress with D. rubiformis in South Africa ering branches of 10 Australian Acacia attributable to the annual production of and discuss the issues that need to be species [including four species from the enormous seed crops which accumulate addressed before the insect is exploited section Botrycephalae, namely, A. mearn- and persist in the soil for many years.6,13 fully as a biological control agent of sii, A. dealbata, A. baileyana and A. decur- Historically, the biological control A. mearnsii. rens, 13 African Acacia species, an Ameri- can acacia, and an additional four species aPlant Protection Research Institute, Private Bag X5017, Stellenbosch 7600, South Africa. Life cycle of D. rubiformis of test plant (Paraserianthes lophantha, bZoology Department, University of Cape Town, Private The life cycle of D. rubiformis has been Mimosaceae; Cydonia oblonga and Prunus Bag, Rondebosch 7701, South Africa. described by Adair.3 It is a univoltine armeniaca, both Rosaceae; and Vitis vini- *Author for correspondence. E-mail: [email protected] species in which adult emergence is fera, Vitaceae)] were exposed to cohorts 248 South African Journal of Science 104, July/August 2008 Research in Action of D. rubiformis adults and monitored for Africa. Qualitative assessments have Weeds, ed. N.R. Spencer, pp. 605–614. Bozeman, Montana. subsequent development of galls. Under shown that levels of galling are increasing 3. Adair R.J. (2004). Seed-reducing Cecidomyiidae as the test conditions, gall induction occurred (unpublished data). In 2006, galls were potential biological control agents for invasive Austra- only on A. mearnsii.3 detectable over a range that extended lian wattles in South Africa, particularly Acacia for about 800 m. In 2007 this range had mearnsii and A. cyclops. Ph.D. thesis, University Host plant growth of Cape Town, South Africa. increased sixfold, up to 5000 m. In Western 4. Stirton C.H. (ed.) (1978). Plant Invaders, Beautiful One of the concerns when using biolog- Australia D. rubiformis has demonstrated but Dangerous. Department of Nature and Envi- ical control agents that reduce seed pro- an ability to locate disparate populations ronmental Conservation of the Cape Provincial duction of invasive acacias by forming Administration, Cape Town. of A. mearnsii, including trees that occur in 5. Versfeld D.B., le Maitre D.C. and Chapman R.A. 3 galls is that the galling may also suppress isolation. The midge will probably show (1998). Alien invading plants and water resources in the growth rate of the host plants. A case the same dispersal ability and eventually South Africa: a preliminary assessment. WRC Report in point is that of the gall wasp, Trichilogaster become established widely in South No. TT99/98, CSIR No. ENV/S-C 97154. Pretoria. acaciaelongifoliae, which is an extremely 6. Dennill G.B., Donnelly D., Stewart K. and Impson Africa. At present no efforts are being F.A.C. (1999). Insect agents used for the biological effective biological control agent of A. made to distribute the insects manually control of Australian Acacia species and Paraserian- 18,24 longifolia in South Africa. Although until the completion of studies, to confirm thes lophantha (Willd.) Nielsen (Fabaceae) in South most T. acaciaelongifoliae galls are induced Africa. African Entomology, Memoir No. 1, 45–54. that D. rubiformis is no threat to the integ- 7. Henderson L. (2001). Alien Weeds and Invasive in flower buds, the wasps also stunt vege- rity of the wattle industry. Plants. A complete guide to declared weeds and tative growth of A. longifolia because: (i) These studies have begun and include invaders in South Africa. Agricultural Research Council, Pretoria. galls induced in vegetative meristematic monitoring of the extent and intensity of tissues destroy the growth points which 8. Macdonald I.A.W. and Jarman M.L. (eds) (1984). galling on A. mearnsii along with rates of Invasive alien organisms in the terrestrial ecosystems of would give rise to new stems; and (ii) galls spread and impact on pod production. the fynbos biome, South Africa.