School of Animal, Plant and Environmental Sciences (ETDs)

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Subject: search.filters.subject.Biological control

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    Assessing the climatic suitability and environmental responses of Anthonomus morticinus Clark (Coleoptera: Curculionidae), a potential biological control agent of Solanum mauritianum Scopoli (Solanaceae) in South Africa
    (University of the Witwatersrand, Johannesburg, 2024-10) Keizan, Yaron Yakov; Byrne, Marcus; Venter, Nic
    Insect biology is greatly influenced by the surrounding climate. Desiccation, owing to low relative humidity, poses a significant threat to insect populations, especially those with small body sizes. To avoid exposure to low relative humidity (RH) levels, insects actively seek out nearby microenvironments with raised RH. The flowerbud feeding weevil Anthonomus morticinus is an optimistic agent to limit the spread of the highly invasive Solanum mauritianum throughout South Africa. However, the restricted geographic range shown by A. santacruzi in South Africa due to climatic unsuitability questions how A. morticinus will respond to climatic variables, particularly humidity. The effect of low temperatures has shown to be a limiting factor by Mkhomazi (2022) on the survival of A. morticinus, but the effects of RH remains untested. This study examined the influence of RH on the survival and oviposition of A. morticinus inhabiting S. mauritianum inflorescences. Furthermore, this study also intended to reveal the influence of lethal humidities on A. santacruzi cultures compared to that of A. morticinus when sheltered by host inflorescences. Anthonomus morticinus colonies inhabiting S. mauritianum bouquets were kept at seven relative humidities, from ~80% to ~20% for a period of 14 days. Survival and oviposition, observed by dissection of buds, were recorded. For comparison, colonies of A. morticinus and A. santacruzi were maintained at 20% and 46% relative humidity for seven days to determine if the species had differing responses at the lower RH levels. A. morticinus survival and oviposition showed no significant differences at the decreasing humidity levels. Moreover, no significant differences were observed in the survival between A. morticinus and A. santacruzi at 46% and 20% RH. Weevils residing within the S. mauritianum flowers and shoots were shielded from the unfavourable environmental humidities owing to higher humidity levels within these microclimates. These results highlight the dependence of biological control agents on their hosts for more than just food and reproduction but also for physiological functionality.
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    The past, present and future of cactus invasions in South Africa in response to rising atmospheric CO2 and climate change
    (University of the Witwatersrand, Johannesburg, 2023) Venter, Nicolaas; Byrne, Marcus
    Cactaceae originate from the Americas and over the past 250 years have been introduced into South Africa and elsewhere for agricultural and horticultural purposes. Numerous species, including useful taxa, have become important invasive weeds that have serious socio-economic and environmental impacts. Fortunately, management strategies, in particular biological control, have been successful in controlling certain species. However, with rising atmospheric CO2 invasive cacti are likely to pose a renewed threat, whereby evidence shows that cactus species are responsive to CO2 fertilisation, which is likely to increase their vigour, mainly through improved water use efficiency (WUE). Additionally, studies show that plant quality declines with increasing CO2 which in general has negative effects on their arthropod herbivores. This study sought to determine the effect of CO2 on two functionally different invasive cactus species and how they may respond to predicted increases in atmospheric CO2. Opuntia stricta (a succulent shrub) is an obligate CAM photosynthetic species that invades grasslands and savannas across semi-arid to subtropical environments whereas Pereskia aculeata (a scrambling woody vine with well-developed leaves) is a CAM-cycling photosynthetic species that invades forest type habits across subtropical environments. Plants were grown at three CO2 concentrations that represented pre-industrial (sub-ambient - 250 ppm), current (ambient - 400 ppm) and future (elevated - 600 ppm) atmospheric CO2 conditions. Plants were also subject to suboptimal and optimal watering treatments for the duration of the experiments to determine the ameliorative effect of CO2 on productivity in response soil moisture deficits. In addition, an experiment was conducted on O. stricta to test the effects of the three CO2 concentrations on plant quality and the subsequent effect on the fitness and efficacy of its insect biological control agent, Dactylopius opuntiae. Growth and productivity of O. stricta and P. aculeata responded positively to increasing CO2, however the responses varied with CO2 concentration. Increasing atmospheric CO2 concentration from date of introduction to present possibly facilitated O. stricta invasion whereas this was less likely for P. aculeata. In both species WUE increased with increasing CO2. Under suboptimal watering, there was partial amelioration of productivity at 600 ppm CO2, but the plant traits that benefitted varied according to species. Plant quality declined for both species, most notably cladode nitrogen (N) content decreased, and carbon/nitrogen ratios (C/N) increased. When D. opuntiae were exposed to O. stricta grown at elevated CO2 (only tested on well-watered plants), D. opuntiae fitness was reduced which in turn reduced the rates of plant mortality due to the insect damage. Using the WUE results from O. stricta, a mechanistic species distribution model (SDM) created here predicted greater increases in its potential distribution in South Africa under climate change relative to the SDM that did not include WUE as a predictor variable. This suggests that improved WUE under future CO2 concentrations can offset the effect of declining rainfall in certain regions of South Africa. Overall, these results suggest that O. stricta and P. aculeata will show more vigorous growth and are also likely to expand their ranges into regions where rainfall currently limits their distribution. This expanded distribution may be further facilitated by reduced biocontrol agent efficacy as host plant quality declines. These findings suggest that management of these two species and other invasive cacti is likely to become more challenging with increasing atmospheric CO2 and climate change.
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    The influence of plant phenolic compounds on the host range of Coniatus tamarisci Fabr. (Coleoptera: Curculionidae): a potential biological control agent of invasive Tamarix L. (Caryophyllales: Tamaricaceae) Taxa in South Africa
    (University of the Witwatersrand, Johannesburg, 2022) Hatile, Sivenathi Luvolwethu; Byrne, Marcus; Mayonde, Samalesu
    Tamarix L. (Caryophyllales: Tamaricaceae) are halophyte species that are native to Eurasia, North Africa, and southwestern Africa. These phreatophyte trees or shrubs have become prominent and widespread invaders in North America, South America, Australia, and South Africa. In South Africa, the Alien and Invasive Species regulations of the National Environmental Management: Biodiversity Act 2014 (NEM:BA) has classified invasive Tamarix as category 1b invader, which require control. Thus, three potential biological control agents have undergone laboratory-based host-specificity trials for the long-term sustainable control of Tamarix. The first two of these biocontrol agents, Diorhabda carinulata (Desbrochers.) (Coleoptera: Chrysomelidae) and Trabutina mannipara (Hemprich & Ehrenberg) (Hemiptera: Pseudococcidae), both failed the laboratory-based host-specificity trials because they completed their life cycle on the indigenous T. usneoides E. Mey ex Bunge. Previously, potential biocontrol agents were selected based on their native distribution and the phylogenetic relatedness of the invasive weed to indigenous nontarget species. However, it has recently been suggested that secondary metabolites also play a major role in insect host selection, and thus should be considered to improve the selection criteria of potential biocontrol agents. The current study is based on the third biocontrol agent that recently underwent laboratory based host-specificity trials in South Africa, Coniatus tamarisci (Fabr.) (Coleoptera: Curculionidae). This was in conjunction with an analysis of the potential influence Tamarix phenolic compounds have on insect host selection. The results show that although C. tamarisci could complete its’ life cycle on T. usneoides, the weevil had a low affiliation/risk associated with the indigenous Tamarix taxon. This advocates for the conduction of open field host-specificity trials, which will allow for a better understanding of C. tamarisci behaviour in a natural setting. Regarding phenolic compounds, three phenolic acids have been identified as being significantly prominent in T. usneoides compared to the invasive Tamarix taxa present in South Africa. These are gallic acid, dehydrodigallic acid, and syringic acid. These compounds are associated with protection from plant herbivory, which could explain the behaviour of C. tamarisci when exposed to and reared on T. usneoides.