Selecting for blow fly development: Forensically important Cochliomyia macellaria

Blow flies (Diptera family: Calliphoridae) are commonly used to infer Post-mortem interval (PMI) in forensic investigations. This application is aided through the use of previously published data on the development rates of Dipteran species, which can help develop an estimate regarding the timing of insect colonization of remains. In the southern United States Cochliomyia macellaria(Secondary screwworm) is a common primary colonizer of carrion and is encountered in forensic entomology casework.

Recent studies of genetic variation in a number of species, including C. macellaria, indicate that genetic variation may contribute to error in PMI estimates. To evaluate the nature and extent of natural variation in immature development time in the wild, an artificial selection experiment conducted with three Texas wild type populations. Over 23 generations, a culmination of 18 months of selective breeding, immature C. macellaria were reared at 25^OC, selecting for the fastest and slowest development possible from the first generation.

The first generation of the three populations showed an average development speed from egg to adult in 270 hours. By the 23^rdgeneration, fast lines developed one day faster, while slower lines took an additional 3 days. This four-day range demonstrates that PMI estimates from a Secondary screwworm could vary by 4 days due to genetic factors alone and that wild flies are under natural pressure to develop quickly.

For the final three generations, replicate colony lines were reared at additional temperatures, 20^OC and 30^OC, to evaluate effects of the selected alleles in differing thermal regimes. As expected, blow flies at lower temperatures took longer to develop, while the warmer temperatures showed faster development speeds, a consistent pattern among the fast and slow selected lines. However, survival rate was lowest in the slow lines at cooler temperatures; indicating “slow” alleles are deleterious, especially at low temperatures.

These results indicate and clarify effects of wild genetic variation on C. macellaria development.  They provide an estimate regarding the extent of error that may occur in a PMI prediction due to genetic factors.  In addition, it is clear that wild alleles are skewed toward fast development, with slow development alleles interacting with the thermal environment.  Forensic entomology casework should be conducted with consideration to these factors.