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University of the Witwatersrand (2020)

Parthenium hysterophorus : understanding the invasion and potential controls

Cowie, Blair William

Titre : Parthenium hysterophorus : understanding the invasion and potential controls

Auteur : Cowie, Blair William

Université de soutenance : University of the Witwatersrand

Grade : Doctor of Philosophy (PhD) 2020

Native to the neotropics, the noxious annual herb Parthenium hysterophorus L.(Asteraceae), has become a troublesome invader in more than 50 countries globally. In South Africa, P. hysterophorus remains one of the most damaging invasive weeds, threatening food security, native biodiversity as well as human livelihoods and well-being. Given the ongoing threats posed by P. hysterophorus, it has become imperative to assess the invasive ecology of the weed in order to aid management and restoration endeavours. The extent of the Parthenium hysterophorus invasion is believed to be governed largely by the availability of water (soil moisture). Simulating a natural drought in an ambient conditions greenhouse, it was found that the weed is capable of enduring periods of prolonged water stress by employing a drought avoidance strategy. Both the adult and juvenile plants, displayed morphological and physiological adaptations to reduce water stress, suggesting that all but the driest parts of South Africa are susceptible to invasion by P. hysterophorus. Within these susceptible regions, P. hysterophorus invasions were found to be influenced by the land, more specifically grazer and pasture, management. Long-term monitoring showed that “poorly-managed”, particularly disturbed or overgrazed, areas experience exacerbation in both P. hysterophorus invasion and impact. Whereas, comparatively “well-managed” areas showed lower levels of P. hysterophorus invasion, which gradually decreased overtime, allowing for the recovery of native species (richness and diversity). In terms of management, South Africa is largely reliant upon biological control to combat P. hysterophorus invasions at a national level. However, biocontrol is yet to reach its full potential, with the most promising biocontrol agent, Zygogramma bicolorata, failing to deliver any notable impact since its release in 2013. Ecophysiological studies reaffirmed that leaf-feeding by Z. bicolorata is highly damaging to P. hysterophorus, metabolically impairing leaves and reducing their photosynthetic ability. This suggested that despite an abundance of plants available in the field, Z. bicolorata populations are constrained. Thus, investigations into the influence of plant quality on the development and performance of Z. bicolorata was undertaken, finding faster development with larger beetles and more fecund females occurred on plants of higher quality. In regions where biocontrol alone is ineffective or unsuitable, chemical control is often employed as the alternative management strategy


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