Physiological and ecological responses of the larger grain borer, Prostephanus truncatus (Horn), to changing environments: implications for its population dynamics and management

Abstract

The rapid change in environments owing to anthropogenic activities and climate change has reshaped the geographical range and status of different insect pests of economic importance. Further, the mechanisms by which conspecific and allospecific species phenotypes vary and how that influences species adaptation in novel environments are poorly understood. The larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), is one of the quarantine pests which poses serious threats to stored maize and dried cassava tubers and has continued to expand its ranges in Africa. However, physiological and ecological data required to map the dispersal pathways and factors limiting geographical distribution of such pests in diverse environments are largely understudied. A combination of literature reviews, field (questionnaire survey and pest monitoring through pheromone baited traps) and laboratory studies were conducted to determine (i) the presence and geographical range of P. truncatus in Botswana (Chapter 2), (ii) local farmers’ knowledge, practices and perceptions on cereal postharvest management including the efficacy of current storage technologies against P. truncatus (Chapter 3), (iii) the drivers and implications of P. truncatus and Spodoptera frugiperda biological invasions in Africa (Chapter 4), (iv) effects of increasing temperatures on the transgenerational (Chapter 5) and intergenerational (Chapter 6) responses of P. truncatus, (v) feeding rates and effects of maternal host preferences on progeny fitness (Chapter 7) and (vi) effects of intra- an interspecific competition on P. truncatus’ physiological and ecological performance (Chapter 8). Standardised physiological (critical thermal maxima [CTmax], critical thermal minima [CTmin], heat knockdown time [HKDT], upper lethal temperatures [ULTs]) and ecological performance (% maize grain damage, % grain weight loss, progeny production, % insect feeding dust) traits were measured under controlled temperature and relative humidity conditions in climate chambers. Results confirmed, for the first time, the presence of P. truncatus in Botswana, but its distribution is still patchy. Although the pest seems not yet well-established inland, the current farmer practices dominated by use of botanical pesticides makes them vulnerable if P. truncatus was to become dominant. Lack of information and awareness of the dangers that P. truncatus pose and lack of improved grain storage facilities further expose local smallholder farmers to this threat. Laboratory assays showed behaviourally plastic responses to increased temperatures and varying xvii hosts. First, trait-dependent thermal plasticity responses are passed on to offspring through transgenerational and intergenerational plastic physiological responses which mediate P. truncatus progeny fitness. Alongside, maternal experiences or decisions by offspring through transgenerational plasticity act as signals for adaptation to a host. Ecological performance of P. truncatus was high at 25 and 30℃ in response to temperature suggesting high pest activity under elevated temperature conditions. This may mean that P. truncatus poses serious threats to food security with increasing temperatures under climate change. However, damage was highly suppressed at higher suboptimal temperatures of 35℃. Prostephanus truncatus failed to survive on mopane wood suggesting that the wood may not be a suitable host for the pest. The study shows P. truncatus physiological adaptative responses to changes in the environment and climate and calls for concerted efforts to monitor the pest inland, forewarn farmers of the dangers of the pest and train them on appropriate efficacious grain storage technologies to manage the pest. This work contributes insights into the ecology and physiology of P. truncatus, which are important in developing models for early warning and control, as well as integrated national and regional pest management strategies against the pest.