Parthenium hysterophorus: understanding the invasion and potential controls

Abstract

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. Although herbicides are effective in controlling P. hysterophorus, they are costly and unsustainable for large-scale or long-term management. Furthermore, concerns surround the likelihood of herbicide resistance in the future as CO2 levels rise. To explore this a multi-generational Conviron study was conducted, finding that increases in atmospheric CO2are likely to decrease the susceptibility of P.hysterophorus to commercially used glyphosate herbicides. The restoration of landscapesinvaded by P. hysterophorus remains an understudied but valuable avenue of research for many South African savannas and their rangelands. Using a randomised block design, the feasibility of restoring pastures invaded by P. hysterophorus within a savanna reserve was explored. The sowing of native grasses, with and without the clearing of P. hysterophorus, was found to promote basal grass cover as well as the establishment of perennial grasses. Moreover, these increases in grass cover enhanced the competitiveness of pastures, gradually suppressing the P. hysterophorus invasion within the trial plots over a three year period. Overall, the research presented in this thesis contributes significantly towards the understanding of P. hysterophorus invasion and the controls utilised in South Africa. Although biological control, the use of herbicides, good land management practices and restoration using native grasses all hold promise, these will need to be formally integrated to attain successful control over P. hysterophorus. At present, South Africa must work towards enhancing and integrating its current controls and limiting the spread of the weed and its environmental and socio-economic impacts. This will be a challenging and long-term endeavour but one which must be supported by government, research and the public