Thermal resilience of Prostephanus truncatus (Horn): Can we derive optimum temperature-time combinations for commodity treatment?

Abstract

Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), is a wood-boring, destructive quarantine insect pest of stored cereal grains and tuber crops. Current disinfestation methods against this pest mainly include fumigants, whose usage in some countries has been contested and discontinued owing to increasing pesticide resistance, public health risks and environmental hazards. Grain temperature treatments thus, offer a sustainable non-chemical and near universally acceptable form of disinfestation for international commodity movement. Currently, blanket temperature treatments are applied regardless of as-yet-unknown P. truncatus developmental stage thermal mortality thresholds that simultaneously optimise grain quality. Here, we used established static and dynamic protocols to determine the low and high thermal profile of P. truncatus larvae and adults measured as critical thermal minima (CTmin), lower lethal temperatures (LLT0), chill coma recovery time (CCRT), supercooling points (SCPs), critical thermal maxima (CTmax), upper lethal temperatures (ULT0) and heat knock-down time (HKDT). We tested the adult ULT-time matrices on maize and sorghum grain quality (germination %) to determine the most effective temperature-time combination(s) retaining optimum grain germination quality. Our results showed adults had higher basal heat (CTmax and HKDT), cold (CTmin, CCRT and SCP) and potential thermal plasticity than larvae (P < 0.05). The LLTs and ULTs ranged −1 to −15 °C and 41–49 °C respectively. Using LLT0 and ULT0, our results showed that for heat treatment, moderate temperature × long duration matrix; i.e. either 45.5 °C × 4 h or 47 °C × 2 h were more efficacious while retaining commodity quality. Similarly, for cold treatment; −9 °C × 4 h, −11 °C × 2 h, −13 °C × 1 h and −15 °C × 0.5 h were effective for complete mortality. These temperature-time combinations may be a sustainable alternative to fumigants in phytosanitary grain disinfestation against P. truncatus or related pests. Such pest- and commodity -specific thermal profiling is critical for development of effective standardised grain disinfestation protocols.