Abstract:
Lepidopteran stemborers are key destructive insect pests of cereal crops in sub-Saharan Africa with the indigenous noctuids Busseola fusca (Fuller) and Sesamia calamistis Hampson and the exotic crambid Chilo partellus Swinhoe accounting for ~5-75% of potential cereal crop yield losses. The main larval endoparasitoids comprising, indigenous Cotesia sesamiae and exotic Cotesia flavipes Cameron contribute to a significant extent in managing these economic herbivorous insect pests. However, efficacy of biological control under global change is currently under threat, and factors limiting trophic level abundance, distribution and population phenologies of these insect species under climate change remain unclear. Against this background, this study provides an understanding of the thermal ecophysiology of laboratory reared B. fusca, S. calamistis and C. partellus and their larval parasitoids, C. sesamiae and C. flavipes under variable climatic conditions and how their population phenologies are likely to be shaped by variable thermal regimes under global change scenarios. This study reports implications of these results thereof for biological control efficacy under climate change. Firstly, a comparative assessment of C. partellus and C. sesamiae thermal tolerance using dynamic and static protocols showed developmental stage differences in C. partellus thermal tolerance (with respect to lethal temperatures and critical thermal limits) and a compromised temperature tolerance of C. sesamiae relative to its host. This indicates potential asynchrony between host-parasitoid population phenology and consequently biocontrol efficacy under global change. Secondly, I investigated the short to medium term phenotypic plasticity of thermal tolerance of C. partellus developmental stages (larvae, pupae and adults) and its larval parasitoid C. flavipes (adults). Rapid cold hardening (RCH) and rapid heat hardening (RHH) effects in C. partellus larvae, pupae and adults and C. flavipes adults were highly significant (P ˂ 0.001). In addition, high temperature acclimation (33°C) improved critical thermal limits [CTLs (CTmin and CTmax)] and heat knock-down time (HKDT) for C. partellus larvae and C. flavipes adults (P ˂ 0.0001) respectively while low temperature (23°C) acclimation enhanced supercooling point (SCP) for C. flavipes and chill-coma recovery time (CCRT) for both C. partellus larvae and C. flavipes adults (P ˂ 0.0001) suggesting that the host is more plastic than the parasitoid. Thirdly, the study explored basal thermal tolerance of larval endoparasitoids, C. sesamiae and C. flavipes and howthis may influence their geographical distribution and biological conservation. Cotesia flavipes showed a higher basal thermal tolerance (with respect to traits of temperature tolerance, CTLs, HKDT and CCRT) than C. sesamiae indicating future non overlapping thermal environments of these species under climate change. Furthermore, since C. flavipes is exotic, its physiological advantage over indigenous C. sesamiae may likely mean exotic species may likely drive local species out of their native ranges, with significant consequences on biodiversity conservation and synergistic biocontrol efficacy. Lastly, the study investigated the effects of heterogeneous stressful environments on thermal tolerance vis a vis hardening (RCH and RHH), thermal (low, optimum and high), starvation and desiccation acclimation on thermal tolerance traits, measured as critical thermal limits on laboratory reared stemborers B. fusca, S. calamistis and C. partellus using dynamic (ramping) protocols. Thermal acclimation to low, optimum and high temperatures was highly significant for CTmin (P ˂ 0.001) across all species. Similarly, across all species, RCH, RHH, starvation and desiccation acclimation were significant (P ˂ 0.001) for CTmin. However, RHH, starvation and desiccation were not significant (P > 0.05) for CTmax across all species. These findings show differential thermal tolerances following exposure to heterogeneous environmental stress habitats. However, Chilo partellus was more plastic and more heat tolerant following starvation acclimation while B. fusca was more cold tolerant following RCH, starvation and desiccation acclimation. In summary, this work revealed asymmetrical variation in thermal ecophysiology across test species, appearing to be enhanced in C. partellus relative to S. calamistis and B. fusca and in C. flavipes relative to C. sesamiae. Therefore with projected climate change, C. partellus is highly likely to extend its geographical distribution relative to S. calamistis and B. fusca ultimately displacing these indigenous stemborer species. In addition, the larval parasitoids may not cope with the geographical expansion of their hosts due to variability in thermal tolerance, albeit indigenous C. sesamiae looks more vulnerable than exotic C. flavipes. This may have broad implications on biological control efficacy leading to more pest outbreaks. These results are useful in designing pest management options for invasive species and conducting pest risks assessments.