Global insect pest invasions have dramatically increased over the past decades, posing significant biosecurity threats to natural and managed ecosystems. As such, the mechanisms of spread and rapid establishment of invasive species have been a key issue for agricultural stakeholders. Similarly, while availability of suitable plant hosts is key to establishment, the eco-physiological mechanisms facilitating invasion amongst most invasive species are unclear. The South American tomato pinworm, Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae) is a devastating invasive global insect pest of tomato, Solanum lycopersicum L. (Solanaceae) accounting for ~80- 100% yield losses. It has aggressively invaded the African continent since its first detection in North Africa in Morocco and Tunisia in 2008. Despite its economic importance and global pest status, little is known on eco-physiological mechanisms likely aiding its rapid establishment and spread. In addition, although diapause has not been reported in the pest’s Mediterranean invaded areas, no studies have looked at its overwintering survival in sub-Saharan Africa. Against this background, this study assessed the role of eco-physiology in explaining part of the observed invasion success of this insect. Specifically, I investigated (i) the natural and low risk T. absoluta control strategies including scoping the native natural enemy guild of the pest insect in Botswana (Chapter 2) (ii) short- and long- term plastic responses to temperature (Chapter 3), (iii) basal cold hardiness and made inferences on overwintering strategy (Chapter 4), (iv) basal water balance and desiccation resistance (Chapter 5), and (v) integrated stress resistance (cross tolerance) to various abiotic co-occurring stressors as a potential survival mechanisms for this species (Chapter 6). Natural substances (NSs) vis pesticidal plant extracts, naturally occurring antagonists and related substances (e.g., biological control agents) for T. absoluta were reviewed and considered more environmentally friendly, bio-based and sustainable alternatives for management that can be used in combination with other low risk substances in a holistic way for successful pest control. Focus was also taken on the enabling and limiting factors that influence farmers in embracing the use of these NSs in an integrated approach. A scoping of the local natural enemy guild for T. absoluta showed a limited range of natural enemies that can be used for biocontrol of T. absoluta in integrated management systems. Chapter 2 also revealed that (i) different physiological mechanisms facilitating population establishment of T. absoluta in novel environments under rapidly changing environments and (ii) different natural- and low-risk substances and local natural enemies can be used to sustainably manage T. absoluta in agroecosystems. Results for the potential plastic responses to both short- and long-term acclimation showed that larvae are more thermally plastic than adults and can shift their thermal tolerance in short- and long- timescales. Larval plasticity advantage over adults reported here suggest asymmetrical ecological role of the larvae relative to adults in facilitating T. absoluta invasion. Empirical study on T. absoluta’s cold tolerance showed that larval lower lethal temperatures ranged from -1˚C to -17˚C for 0.5-4h durations and were more cold hardy than adults. Adults showed lower temperature activity limits than larvae albeit freeze strategy experiments showed neither life stages survived internal freezing indicating that both larvae and adults were chill-susceptible. In addition, fasting and dehydration pre-treatment depressed supercooling points (SCPs), although asymmetrically, conferring more negative SCPs for larvae while ramping rates and inoculative freezing also affected SCPs (Chapter 4). An investigation of water balance and desiccation tolerance showed body water content (BWC), body lipid content (BLC) and water loss rates (WLRs) significantly varied across life stages. Second instars recorded the lowest while 4th instars exhibited the highest BWC and BLC while they also had lowest WLRs. Desiccation resistance assays were consistent, showing that 2nd instars were the most vulnerable while 4th instar larvae and adults were the most desiccation xvii tolerant. This showed that T. absoluta 4 th instar larvae were the most resilient developmental stage and potentially contribute the most to the invasion success of the pest in arid and semi-arid environments (Chapter 5). Integrated resistance assays showed context dependent (type of stress x developmental stage) integrated stress resistance that may heighten invasiveness in T. absoluta. Indeed, fasted individuals had a significantly higher desiccation resistance while desiccation acclimation improved starvation resistance. This cross tolerance indicates potential shared co evolutionary mechanisms across divergent stressors and may help facilitate survival for invasive species under heterogeneous but stressful environments (Chapter 6). Unravelling such factors associated with successful invasion provide a basis for mitigating the introduction, establishment and spread of the pest species, especially when key life stages e.g., larvae and adults are targeted for experiments that inform early warning systems. This information is also important in modelling pest risk status, allows temporal life-stage specific targeting of management strategies and for developing early-warning system predictive models. Incorporation of these species-specific eco physiological traits in predictive models can help refine invasive species potential spread and may help predict biogeographic patterns under changing climates

Thesis (PhD in Biological Sciences) --Botswana International University of Science and Technology, 2021