Invasive alien species (IAS) are a major biosecurity threat affecting globalisation and the international trade of agricultural products and natural ecosystems. In recent decades, for example, field crop and postharvest grain insect pests have independently accounted for a significant decline in food quantity and quality. Nevertheless, how their interaction and cumulative effects along the ever-evolving field production to postharvest continuum contribute towards food insecurity remain scant in the literature. To address this within the context of Africa, we focus on the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), and the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), two of the most important field and postharvest IAS, respectively, that have invaded Africa. Both insect pests have shown high invasion success, managing to establish themselves in >50% of the African continent within a decade post-introduction. The successive and summative nature of field and postharvest damage by invasive insect pests on the same crop along its value chain results in exacerbated food losses. This systematic review assesses the drivers, impacts and management of the fall armyworm and larger grain borer and their effects on food systems in Africa. Interrogating these issues is important in early warning systems, holistic management of IAS, maintenance of integral food systems in Africa and the development of effective management strategies.

The fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) is a global invasive pest of cereals. Although this pest uses maize and sorghum as its main hosts, it is associated with a wide range of host plants due to its polyphagous nature. Despite the FAW's polyphagy being widely reported in literature, few studies have investigated the effects of the non-preferred conditions or forms (e.g., drought-stressed forms) of this pest’s hosts on its physiological and ecological fitness. Thus, the interactive effects of biotic and abiotic stresses on FAW fitness costs or benefits have not been specifically investigated. We therefore assessed the effects of host plant quality on the developmental rates and thermal tolerance of the FAW. Specifically, we reared FAW neonates on three hosts (maize, cowpeas, and pearl millet) under two treatments per host plant [unstressed (well watered) and stressed (water deprived)] until the adult stage. Larval growth rates and pupal weights were determined. Thermal tolerance traits viz critical thermal maxima (CT_max), critical thermal minima (CT_min), heat knockdown time (HKDT), chill-coma recovery time (CCRT), and supercooling points (SCPs) were measured for the emerging adults from each treatment. The results showed that suboptimal diets significantly prolonged the developmental time of FAW larvae and reduced their growth rates and ultimate body weights, but did not impair their full development. Suboptimal diets (comprising non-cereal plants and drought-stressed cereal plants) increased the number of larval instars to eight compared to six for optimal natural diets (unstressed maize and pearl millet). Apart from direct effects, in all cases, suboptimal diets significantly reduced the heat tolerance of FAWs, but their effect on cold tolerance was recorded only in select cases (e.g., SCP). These results suggest host plant effects on the physical and thermal fitness of FAW, indicating a considerable degree of resilience against multiple stressors. This pest’s resilience can present major drawbacks to its cultural management using suboptimal hosts (in crop rotations or intercrops) through its ability to survive on most host plants despite their water stress condition and gains in thermal fitness. The fate of FAW population persistence under multivariate environmental stresses is therefore not entirely subject to prior environmental host plant history or quality.

Acclimation induces physiological and/or behavioural plasticity that promotes survival to environmental stress spatio‐temporally in insects. As such, the time‐course of acclimation and reversal may shape survival in insect pests under stress. The fall armyworm, Spodoptera frugiperda , elicits plastic responses that improve survival under temperature stress. However, its time‐course for attainment and reversal of temperature acclimation is unknown. We investigated whether acclimation temperature and duration could influence changes in critical thermal limits (CTLs), that is, critical thermal maxima (CT _max ) and critical thermal minima (CT _min ). If affirmative, how much time would be required for the reversal of the induced changes in CTLs (plasticity)? Third‐instar fall armyworm larvae were acclimated at high (33°C, 38°C) and low (23°C, 18°C) temperatures for 15 min to 48 h before recording CT _max and CT _min . To assess reversal, CT _max or CT _min was measured from 15 min to 120 h post‐acclimation. Fall armyworm rapidly acclimated to 38°C (15 min), than (33°C) (2 h). Durable heat tolerance gains followed prolonged acclimation (1–12 h) at 38°C. Acclimation gains were reversed faster at 33°C than at 38°C (24 vs. 72 h, respectively). Overall, prolonged acclimation slowed reversal of gains. In contrast, cold acclimation did not confer any measurable benefits, underscoring the pest's resilience to high‐temperature environments. These findings demonstrate that the fall armyworm's physiological high‐temperature plasticity explains its persistence under high‐temperature stress and that low temperature may limit the pest's persistence in cold environments. Acclimation time‐courses may dictate ecological resilience by shaping stress responses, persistence, and geographical range dynamics, informing early warning and strategies.