A closer look at the thermal tolerance of the larval stadia of two forensically important blow fly species, Chrysomya rufifacies and Cochliomyia macellaria (Diptera: Calliphoridae)

As ectotherms, insects rely on temperature for growth and development.  Generally, development time decreases as temperature increases, up to an optimal temperature.  Once optimal temperature is exceeded, development slows until a lethal temperature is reached, or enough time is spent at stressful temperatures, resulting in death.  The field of forensic entomology relies on this temperature-development relationship for estimating forensically important timelines.  This study focuses on two blow flies, Chrysomya rufifacies and Cochliomyia macellaria (Diptera: Calliphoridae), both found in Texas.  Cochliomyia macellaria is native while C. rufifacies is invasive, originating from the Eastern Hemisphere.  Temperatures in Texas can exceed 40˚C, which may result in maggot die offs.  Knowledge of a species’ thermal tolerance is important to forensics as high temperatures could disrupt estimates of the post-colonization interval.  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.  As expected, temperature and duration of exposure had the strongest effect on knockdown, but species by temperature is also significant.  Generally, C. macellaria was found to be more thermally tolerant, particularly in the feeding 3^rd instar.  Tolerance to temperature and duration increased as larvae aged.  In the post-feeding 3^rd instar, when larvae are preparing for pupariation, thermal tolerance dramatically decreased in C. macellaria.  Older larvae of both species were able to withstand higher temperatures for longer durations of time.  Access to food also improved performance over time at higher temperatures.