Thermal Tolerance of the Larval Stadia of Two Forensically Important Blow Fly Species, Chrysomya rufifacies (Macquart), and Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae)

Abstract

The growth and development of insects are heavily temperature dependent. Generally, development time decreases as temperature increases, up to an optimal temperature. Beyond the optimal temperature, development and performance slow until a knockdown temperature or critical thermal maximum (CTvmax) is reached, or enough time is spent at stressful temperatures, resulting in death. Blow fly dependent ecological processes and forensic entomology rely on this temperature-development relationship and are impacted by knockdown and lethality at critical thermal limits. This thesis focuses on two forensically important blow flies, Chrysomya rufifacies and Cochliomyia macellaria (Diptera: Calliphoridae). Both are currently found in Texas; C. macellaria is native, while C. rufifacies is an invasive species from the Eastern Hemisphere. Temperatures in Texas can exceed 40˚C, which often results in maggot deaths. Knowledge of a species’ thermal tolerance is important to forensics as a heatwave could easily disrupt estimates of the time of colonization. The purpose of this research was to observe the thermal tolerance using the ramping and the static method, for all larval instars of C. rufifacies and C. macellaria.

Using the ramping method to determine CTvmax and rates of survival, larvae were heated on a metal plate until knockdown was observed. Larvae were kept at room temperature with food for a 24-hour recovery period. It was found that C. rufifacies had slightly higher CTvmax than C. macellaria. Average knockdown temperatures increased with instar for both species. Cochliomyia macellaria tended to have slightly higher rates of survival than C. rufifacies. Using the static method to determine likelihood of knockdown and rates of survival, larvae with and without food were placed in incubators set at 25, 35, 45 and 50˚C for half an hour, one or two hours. After knockdown was recorded, all larvae were placed with food into an incubator set at 25˚C for a 24-hour recovery period. Older larvae were able to withstand warmer temperatures for longer durations of time and tended to have higher rates of survival. Access to food greatly improved performance at higher temperatures for longer periods. Cochliomyia macellaria tended to perform better than C. rufifacies after longer exposure periods