Browsing by Author "Mukwevho, Vuledzani Oral"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    The Harlequin ladybeetle Harmonia axyridis (Coleoptera: Coccinellidae) in the Western Cape Province, South Africa: effects on arthropods in urban, agricultural and natural ecosystems
    (Stellenbosch : Stellenbosch University, 2015-12) Mukwevho, Vuledzani Oral; Pryke, James S.; Roets, Francois; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Invasive arthropod predators are one of the largest and most diverse groups of invasive insects in the world. Many are generalist predators, with cosmopolitan distributions due to their use as biological control agents in agriculture. Harmonia axyridis (Coleoptera: Coccinellidae), an invasive arthropod predator species native to Asia, which now has a world-wide distribution. It is considered one of the most successful biological control predator species and is generally considered to be economically beneficial. However, negative effects have recently emerged in agricultural and natural systems. Harmonia axyridis poses a threat to biodiversity as it outcompetes native species for food resources. It can also feed directly on native predatory arthropods that disrupt natural ecosystem processes. Their movement in-and-out of agricultural landscapes may depend on food availability with natural vegetation alongside agricultural areas often utilised for refuge and alternative food resources. This beetle has also been recorded in urban areas. The aim of this study was to determine how the invasive H. axyridis beetle uses the local landscape in the Western Cape province, South Africa, and to determine its threat to native species. I sampled urban landscapes, vineyards, natural vegetation/vineyard edge zones and pristine natural areas for arthropods every second month using a suction sampler. Data collected included the abundance and diversity of H. axyridis, herbivores, local predators and non-Harmonia ladybeetles. Most H. axyridis were collected in urban areas during all sampling periods. Highest abundance was recorded in May and July (winter). This indicates that urban areas were the preferred landscape feature and that these act as ovipositing areas, particularly as larval H. axyridis were also only collected in urban areas. Significantly, vineyards and natural vegetation had very low abundance of H. axyridis, questioning their value as a biological control agent in this region. Harmonia axyridis had a negative effect on the overall local arthropod community, as well as the predator and herbivore guilds, although it was positively correlated with the abundance of non-Harmonia ladybeetles. This suggests that H. axyridis and non-Harmonia ladybeetles are responding to the same resources in these landscapes. A negative correlation found between H. axyridis and the abundance of predators is most likely due to competition for the same resources (e.g. prey items). These negative impacts, along with their negligible value as biological control agents in agriculture, suggest that a programme should be implemented to control this invasive species. More specifically, control should be aimed in urban areas during winter when and where the species aggregates and when larvae are present.
  • Thumbnail Image
    Item
    The role of competition and mutualism in shaping microbial communities in Protea flowers
    (Stellenbosch : Stellenbosch University, 2019-12) Mukwevho, Vuledzani Oral; Roets, Francois; Dreyer, L. L.; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Protea is a keystone member of the Cape floristic region and a crucial part of the ecological functioning of the fynbos ecosystem. These plants structurally dominate fynbos vegetation and they maintain large numbers of phytophagous organisms and pollinators. Their iconic inflorescences form the basis of a thriving cut flower industry, but this is under threat from pests and pathogens. Protea inflorescences and infructescences are also colonised by saprobic fungi that are of phytosanitary concern. These are dominated by ophiostomatoid fungi in the genera Knoxdaviesia and Sporothrix that form complex, often mutualistic, interactions with mites, pollinating insects and pollinating birds. How these fungi affect their host plants are not currently known. Also, it is unknown how they are able to dominate fungal communities within an environment optimal also for the dominance of common contaminant saprobic fungi. The fourteen described species of ophiostomatoid fungi from Protea inflorescences have well-defined host ranges and may even be associated with specific tissue types. Here I test various hypotheses related to fungal competitive abilities to explain patterns of association between ophiostomatoid fungal species, ‗environmentally acquired‘ fungal taxa, and their hosts. I showed that host chemistry partially explain host exclusivity of ophiostomatoid fungi, but that differences in the actions of spore vectors may be more important. I found that without ophiostomatoid fungi, infructuscences are dominated by ‗environmentally acquired‘ fungi such as Penicillium, Cladosporium and Fusarium. Even though the ophiostomatoid fungi are comparatively weak competitors, they are able to defend captured space against these when they colonise structures early and when they grow on their usual hosts. Although ophiostomatoid fungi do not increase numbers of viable seeds, they prevent seed release when recruitment will be suboptimal. This is because infructuscences containing ophiostomatoid fungi persist longer on plants. There is therefore mutual benefit for the association between Protea and ophiostomatoid fungi. I also uncovered complex interactions between different ophiostomatoid fungi within individual infructescences. Some species are neutral competitors and they can occupy the same tissue types within individual infructescences, while others are strong competitors on specific tissue types and can exclude competing species. Again the actions of spore vectors likely explain the persistence of weaker competitors in this scenario, but the actions of possible bacterial mutualists or other microbes should not be ignored in future studies. In this work I demonstrated the use of fungal competition studies for investigations into host relations and dispersal ecology of microbes in an atypical ecosystem, but these same techniques can be adapted to investigate associations between microbes in multiple other systems.