Browsing by Author "Hoffmann, J H"

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    A proposed prioritization system for the management of invasive alien plants in South Africa
    (2003) Robertson, M P; Villet, M H; Fairbanks, D H K; Henderson, L; Higgins, S I; Hoffmann, J H; Le Maitre, D C; Palmer, A R; Riggs, I; Shackleton, C M; Zimmermann, H G
    Every country has weed species whose presence conflicts in some way with human management objectives and needs. Resources for research and control are limited, so priority should be given to species that are the biggest problem. The prioritization system described in this article was designed to assess objectively research and control priorities of invasive alien plants at a national scale in South Africa. The evaluation consists of seventeen criteria, grouped into five modules, that assess invasiveness, spatial characteristics, potential impact, potential for control, and conflicts of interest for each plant species under consideration. Total prioritization scores, calculated from criterion and module scores, were used to assess a species' priority. Prioritization scores were calculated by combining independent assessments provided by several experts, thus increasing the reliability of the rankings. The total confidence score, a separate index, indicates the reliability and availability of data used to make an assessment. Candidate species for evaluation were identified and assessed by several experts using the prioritization system. The final ranking was made by combining two separate indices, the total prioritization score and the total confidence score. This approach integrates the plant's perceived priority with an index of data reliability. Of the 61 species assessed, those with the highest ranks (Lantana camara, Chromolaena odorata and Opuntia ficus-indica) had high prioritization and high confidence scores, and are thus of most concern. Those species with the lowest ranks, for example, Harrisia martinii, Opuntia spinulifera and Opuntia exaltata, had low prioritization scores and high confidence scores, and thus are of least concern. Our approach to ranking weeds offers several advantages over existing systems because it is designed for multiple assessors based on the Delphi decision-making technique, the criteria contribute equally to the total score, and the system can accommodate incomplete data on a species. Although the choice of criteria may be criticized and the system has certain limitations, it appears to have delivered credible results.
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    Biological control in the management of invasive alien plants in South Africa, and the role of the Working for Water programme
    (2004) Zimmermann, H G; Moran, V C; Hoffmann, J H
    The first biological control project against invasive alien plants in South Africa was in 1913. Initially, invasive cactus species were the only plants in South Africa targeted for biological control. By the early 1960s, the emphasis expanded to include problem plants that threatened the ‘Fynbos’ biome, and latterly to invasive species in other types of natural habitats. Many of the South African projects have been innovative, for example: the use of gall-forming and seed-feeding insect species that have not been used elsewhere; the emphasis on weeds in conservation areas; and the predominance of woody invaders that have been targeted for biological control. Most of these woody plants originated as forestry or agro-forestry introductions, which has created a relatively high incidence of conflicts of interest between conservationists and growers. Recent benefit:cost analyses have demonstrated exceptionally high returns on investment for biological control, even for the least successful of the projects. The inception of the Working for Water programme in 1995 has significantly enhanced biological weed control in South Africa. The benefits include: sustainable funding; investment in research on emerging weeds; a well-organized implementation programme that has increased the impact of biological control in the field; improved international cooperation, particularly in Africa, in concert with the NEPAD initiative; and the recent re-organization of research personnel into cooperative teams that include entomologists, plant pathologists, resource economists and plant ecologists.
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    Dasineura rubiformis (Diptera: Cecidomyiidae), a new biological control agent for Acacia mearnsii in South Africa
    (Academy of Science of South Africa, 2008) Hoffmann, J H; Impson, F A C; Kleinjan, C A; Post, J A
    Acecidomyiid midge, Dasineura rubiformis, is the most recent addition to the suite of biological control agents that have been deployed in South Africa against invasive Australian Acacia species. This insect is associated with Acacia mearnsii (black wattle), which is extremely invasive, but also an important agro-forestry species, in South Africa. It induces development of galls in the flowers of A. mearnsii, thereby preventing pod development and reducing the reproductive capacity of the plants. The useful attributes of this economically important plant species should not be affected by the introduction of D. rubiformis. The midge is established in the vicinity of Stellenbosch, where it is increasing in abundance. Studies have been initiated to (i) evaluate the performance of the midge; (ii) confirm that galling does not cause a reduction in vegetative growth of A. mearnsii; and (iii) determine the potential effectiveness of D. rubiformis as a biological control agent of A. mearnsii. All indications are that the insect has the potential to become an excellent seed-reducing biological control agent of A. mearnsii.
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    Environmental factors influencing the distribution of Opuntia stricta, an invasive alien plant in the Kruger National Park, South Africa
    (2007) Hoffmann, J H; Foxcroft, L C; Viljoen, J J; Kotze, J J
    Opuntia stricta (Cactaceae), an alien weed, has invaded an area of more than 35 000 ha in the Skukuza region of the Kruger National Park [KNP]. The distribution of the plant and features of the environment were used to identify biotic and abiotic factors which may be affecting the distribution and density of the plant. A Canonical Community Analysis revealed that none of the environmental factors that were monitored influenced the distribution or abundance of O. stricta in KNP. There were no apparent natural barriers that might limit the occurrence of O. stricta within the KNP. The study provides insight into the relationship between the invader and host environment, showing that there is a high probability that, unless there is appropriate intervention, O. stricta will eventually colonise the entire KNP.
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    Factors influencing the distribution of Cactoblastis cactorum, a biological control agent of Opuntia stricta in Kruger National Park, South Africa
    (2007) Hoffmann, J H; Foxcroft, L C; Viljoen, J J; Kotze, J J
    Cactoblastis cactorum, a phycitid moth, has been used with considerable success as a biological control agent against several different cactus species in many parts of the world, including Opuntia stricta in Kruger National Park [KNP], South Africa. Although the moth has become widespread and abundant in KNP, its populations have not reached levels where it is able to provide the desired level of control on the alien species which has invaded more than 35 000 ha of the park and continues to spread. Although several factors, especially acquired predators and parasitoids, are known to curb the abundance of C. cactorum in South Africa, the extent to which this applies in KNP is unknown. Logistic regression and Canonical Community Analysis were used to determine the biotic and abiotic factors affecting the distribution and abundance of C. cactorum in KNP. To do this, maps of the ranges of the insect and host plant were constructed and integrated with other environmental features. The logistic regression showed that O. stricta density had no influence on whether or not C. cactorum was present in an area. The Canonical Community Analysis suggested that a high fire frequency, the herbicide control programme and the presence of small host plants may be important in limiting the distribution of C. cactorum, either directly or indirectly, through impacts on associated predators or parasitoids.
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    Oviposition behaviour of Neltumius arizonensis Schaeffer (Coleoptera: Bruchidae) : a biological control agent of Prosopis spp. in South Africa
    (1995) Strathie, Lorraine Wallace; Hoffmann, J H; Moran, V C
    Little has been documented on the biology of Neltumius arizonensis Schaeffer. This bruchid is native to the south-western United States of America and is a prospective biological control agent of Prosopis spp. in South Africa. The primary objective of this study was to examine and quantify the oviposition behaviour of N. arizonensis females under different conditions to determine whether eggs of conspecifics or of another bruchid species, Algarobius prosopis (LeConte), affected oviposition. Diet, the number of mates, the duration of access to mates, and the variety of Prosopis pod provided for oviposition all affected the fecundity of N. arizonensis. Optimal conditions for N. arizonensis oviposition included: a diet of pollen pellets in solution, constant access to a limited number of mates, and mature, undamaged Prosopis pods of the 'mottled-purple' variety. The physical structure of the surface of Prosopis pods, observed by scanning electron micrography, did not reveal trends in characteristics among pod varieties that could be linked to the oviposition preferences of N. arizonensis. The rate of oviposition in N. arizonensis peaked between the third and eighth day after emergence from pods and was highest during the first hour when females that had been deprived of pods for at least three days, were provided with pods. Each N. arizonensis female laid an average of about 80 eggs during her lifetime, which was about 3 5 days on average. An event-recording computer programme was developed to quantify the oviposition behaviour of N. arizonensis when females were provided with one of four types of Prosopis pods for one hour: (a) pristine pods, (b) pods with conspecific eggs, (c) pods with A. prosopis eggs deposited within slits, and (d) pods with egg-free slits. Analyses of time budgets indicated that pod type had no significant effect on behaviour, although some activities differed significantly with the type of pod provided, but probably only as a result of the greater number of eggs laid on some pod types. Certain activities i.e. inspection of the pod surface, remaining stationary, and scraping of the ovipositor across the surface prior to egg deposition, occupied significantly more time on pods than other activities. Pod type did not affect the total time females spent on pods, nor the frequency of visits to pods. The availability of clean seeds did not affect the quantity of eggs deposited and the rate of oviposition did not differ significantly on pods of different types.