Effect of thermal acclimation on the tolerance of the peach fruit fly (Bactrocera zonata: Tephritidae) to heat and cold stress

Understanding the thermal biology of insects is of increasing importance for predicting their geographic distribution, particularly in light of current and future global temperature increases. Within the limits set by genetic makeup, thermal tolerance is affected by the physiological conditioning of individuals (e.g., through acclimation). Considering this phenotypic plasticity may add to accurately estimating changes to the distribution of insects under a changing climate. We studied the effect of thermal acclimation on cold and heat tolerance of the peach fruit fly (Bactrocera zonata) – an invasive, polyphagous pest that is currently expanding through Africa and the Middle East. Females and males were acclimated at 20, 25 and 30 °C for up to 19 days following adult emergence. The critical thermal minimum (CTₘᵢₙ) and maximum (CTₘₐₓ) were subsequently recorded as well adult survival following acute exposure to chilling (0 or −3 °C for 2 h). Additionally, we determined the survival of pupae subjected for 2 h to temperatures ranging from −12 °C to 5 °C. We demonstrate that acclimation at 30 °C resulted in significantly higher CTₘₐₓ and CTₘᵢₙ values (higher heat resistance and lower cold resistance, respectively). Additionally, adult recovery following exposure to −3 °C was significantly reduced following acclimation at 30 °C, and this effect was significantly higher for females. Pupal mortality increased with the decrease in temperature, reaching LT₅₀ and LT₉₅ values following exposure to -0.32 °C and −6.88 °C, respectively. Finally, we found that the survival of pupae subjected to 0 and 2 °C steadily increased with pupal age. Our findings substantiate a physiological foundation for understanding the current geographic range of B. zonata. We assume that acclimation at 30 °C affected the thermal tolerance of the flies partly through modulating feeding and metabolism. Tolerance to chilling during the pupal stage probably changed according to temperature-sensitive processes occurring during metamorphosis, rendering younger pupae more sensitive to chilling.