The effect of habitat management on the impact of liothrips tractabilis mound and pereyra (thysanoptera: phlaeothripidae), on pompom weed in South Africa
Date
2019
Authors
Mudau, Phuluso
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Abstract
Campuloclinium macrocephalum (Less.) DC. (Asteraceae), is an invasive perennial herb, mainly in the grassland biome of South Africa, commonly known as pompom weed. It is native to South and Central America. Since the 1980s it has invaded wetlands, roadsides and grasslands and poses a significant environmental threat, such as displacement of indigenous vegetation, reduction in grazing capacity of farms and game reserves because of its unpalatability to wildlife and livestock. Given the shortcomings of chemical and mechanical control methods, biological control is considered the most sustainable and environmentally friendly method of controlling some invasive plants such as pompom weed.
Liothrips tractabilis Mound and Pereyra (Thysanoptera: Phlaeothripinae) is a stem and leaf deforming biological control agent released against C. macrocephalum in 2013. During laboratory impact studies L. tractabilis was found to significantly reduce the growth of C. macrocephalum. Young and regrowth plants that were inoculated with thrips demonstrated a significant reduction in leaf production, biomass and height compared to the control plants, with a reduced floral production.
Light intensity is one of the factors that has been found to affect plant growth and population structure and consequently that of other living organisms, including insects. Exposure to light alters leaf quality, such as phenolic and nitrogen contents, water content and structural defences. Plants that are exposed to high light are often less palatable, making herbivores more likely to colonise plants in the shade. Many studies have observed increased rates of herbivory on plants in shaded environments, while conversely other studies found increased herbivory on plants in full sun. The first aim of this study was to compare the impact of L. tractabilis on the growth of C. macrocephalum in sunny (areas with short, mown grass) versus shaded environments (areas with long unmown grass).
In this study, the number of adults, nymphs and eggs of L. tractabilis were found to be significantly higher in the sunny environment, which resulted in 78% of C. macrocephalum plants with deformed growth (plants with altered apical shoot tips), as well as a 64% reduction in plant height and a 74% reduction in the proportion of plants with flowers. Therefore, an early season mowing of C. macrocephalum in invaded veld, has the potential to enhance the performance of the biocontrol agent, L. tractabilis in reducing the vigour and reproduction of pompom weed.
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The second aim of this study was to investigate the life stages of L. tractabilis that overwinter and assess their impact on the fleshy rootstocks of C. macrocephalum. In the southern hemisphere, perennial plants such as C. macrocephalum respond to seasonal reductions in rainfall with dieback to below-ground plant tissue during the dry and cold winter from May onwards followed by regrowth from the fleshy rootstocks in spring (October) at the commencement of the new growing (rainy) season. Liothrips tractabilis was found to display a similar pattern by moving to the underground plant parts of C. macrocephalum in winter, only to reappear in spring on the above-ground shoot regrowth. Adults of L. tractabilis were found to be the main life stage that overwinter on the plant roots, with only 4% of L. tractabilis individuals being nymphs. Liothrips tractabilis had no significant effect on the number, mass, thickness or length of the weed’s roots during the dry season. Therefore, L. tractabilis is only effective in controlling C. macrocephalum above-ground growth during the growing season. The effect of habitat management such as mowing enhanced the impact of L. tractabilis on C. macrocephalum. Overwintering L. tractabilis adults were most frequently found at the 4-6 cm depth from the soil surface (52%), which suggests that they could survive the typical Highveld winter fires. Thus, integrated control of C. macrocephalum using both fire and the biocontrol agent Liothrips tractabilis is feasible and should be trialed.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science, Johannesburg, South Africa, May 2019