Current research
This research project is being funded by the Working for Water (WfW) Programme and the KwaZulu-Natal Department of Agriculture and Environmental Affairs.
Research addresses all the usual aspects of a comprehensive biological control programme, notably (i) the identification, importation and host-specificity testing of additional potential biocontrol agents in quarantine, (ii) the release of suitable agents into the field and the monitoring of their establishment and initial effects, (iii) the evaluation of the long-term impacts of biological control on the population dynamics of the target weed species and (iv) the publication of the research results in the appropriate scientific literature.
Two universities, in Venezuela and Jamaica, are carrying out contract research on behalf of this project. In Venezuela, research is being conducted on the biology and host-specificity of the stem-tip galling weevil Conotrachelus reticulatus and the stem tip-wilting moth Carmenta sp. In Jamaica, studies are looking at host-specificity of several insects in the field, andC. odorata biotype preference, with particular emphasis on the stem-galling fly Polymorphomyia basilica and the stem tip-wilting fly Melanagromyza eupatoriella.
Recently, a gratifying find was made at the release site of a previously released insect. Pareuchaetes insulata, a butter-yellow arctiid moth with black antennae and legs, which lays batches of pale yellow eggs on the underside of chromolaena leaves, was collected as a potential biocontrol agent in Florida, USA. After extensive host specificity testing the moth was released in KZN in early 2001. In the initial stages 20 000 to 30 000 caterpillars were released at a number of sites for periods of 4 months or less with 380 000 released near Umkomaas on the KZN South Coast between November 2001 and March 2003. In December 2004 a large outbreak of the insect was found about 0.5km from the Umkomaas release site, with high levels of damage. In June 2005, numbers of the insect increased throughout most of the area lying within 0.5 to 1km of the release site, with patches of chromolaena largely stripped of leaves, and by September 2005 damage was found up to 2km away. In shadier areas, chromolaena plants have died en masse, while even in sunny areas, defoliation by the larvae has reduced the growth rate of chromolaena plants so much that they have decreased dramatically in height and have been overgrown by other plant species. In late summer the insect was also found at least 20km along the coast to both the north and south of the Umkomaas release site, and 10km inland.
Pareuchaetes insulata is the first biocontrol agent established on chromolaena in South Africa. A second biocontrol agent, a small fly (Calycomyza eupatorivora) whose larvae tunnel inside the leaves of chromolaena, was established at Amanzimtoti on the KZN South Coast in late 2003. However, it is a much less damaging species. Research into the development of a number of other promising agents including a root-feeding beetle (Longitarsus sp.), a stem tip-galling weevil (Conotrachelus reticulatus), a sessid moth (Carmenta n. sp.), a stem tip-galling pyralid moth (Mescinia n. sp.), a longhorn beetle (Recchia parvula), an agromyzid fly (Melanagromyza eupatoriella), and a small tortricid moth (?Dichrorampha sp.), continues.
Research is also underway on two promising species of pathogens that were collected from chromolaena in Jamaica, Pseudocercospora eupatorii-formosanii and Mycovellosiella perfoliata. (Ms Estianne Retief, e-mail: retiefe@arc.agric.za)
The Lead Agent for this project is ARC-PPRI; Team leader: Dr Costas Zachariades, e-mail: zachariadesc@arc.agric.za
Earlier research
Progress with the biological control of chromolaena has been hampered by the failure to match our southern African form of chromolaena with plants growing naturally anywhere else in the world. Natural enemies that control chromolaena successfully in other parts of the world have constantly failed to become established on the weed in this country. The results of recent surveys, followed by genetic and morphological studies, confirm that the southern African form of chromolaena originates from either Jamaica or one of the other islands of the Greater Antilles. Close ties have recently been developed with several countries that might prove valuable research partners: Venezuela, Jamaica and Cuba.
Two species of leaf-feeding moths, Pareuchaetes aurata aurata and P. pseudoinsulata, failed to become established after repeated releases some ten years ago. Despite considerable efforts to mass-rear, release and monitor the latter species once more, the agent did not establish. The first releases anywhere in the world of a related moth, P. insulata, were made at several sites in KwaZulu-Natal, but for a long time it appeared as if the Florida P. insulata biotype was not establishing, despite releases of more than 730 000 individuals at 17 sites in KZN province. (But see above.) Therefore it was replaced with Cuban and Jamaican biotypes which are likely to be better suited to the climate or local biotype of C. odorata. Results are being awaited.
Recently, the leaf-mining fly Calycomyza eupatorivora has been cleared for release by NDA and DEAT, and trial releases of both pupae and adults have been made around Durban. A starter culture was also supplied for mass-rearing at the Sappi KwaMbonambi nursery. A release application for the damaging stem-boring weevil Lixus aemulus has been compiled. Culturing techniques and biology studies continued in quarantine on the root-boring flea beetle Longitarsus horni. The stem tip-wilting fly Melanagromyza eupatoriella, collected in Florida, was cultured for a few generations for the first time at the Cedara quarantine laboratory; however, the culture died out and a new one will be imported from Jamaica. (Dr Costas Zachariades, E-mail: zachariadesc@arc.agric.za). A review of this biocontrol project was published in 1999.
Research into the role of chromolaena on coastal grasslands in KwaZulu-Natal confirms that, in the absence of fire, these species-rich communities are easily invaded and transformed by the weed, resulting in up to 100% loss in species diversity. Even with regular burning, which is effective in eradicating chromolaena, these grasslands take many years to fully recover their diversity and structure. (Mr Jeremy Goodall, E-mail: goodallj@arc.agric.za)
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