Of maggots and murder - UF researcher says sleuths need new way to estimate time of death
GAINESVILLE, Fla. — Blow fly maggots are one of the best tools forensic entomologists have for estimating when a person died—but a University of Florida researcher says current methods should be revised to account for a phenomenon that can skew results.
Maggot size and maturity can indicate when adult blow flies first encountered a corpse. But unlike many insect larvae, blow fly maggots generate heat when they’re grouped in large masses, which can speed their development, said Susan Gruner, an entomology doctoral student with UF’s Institute of Food and Agricultural Sciences.
Faster maggot development can make it harder for experts to determine when the flies arrived, Gruner said. In the worst-case scenario, a murder suspect may be wrongly accused or exonerated due to inaccurate time-of-death estimates.
Experts have known about this phenomenon for decades but have no standard method for dealing with it, she said. There’s a well-known model for predicting insect development based on air temperature, but when it’s applied to blow flies, experts must rely on their own judgment to account for maggot-generated heat.
“They’re not wrong, they’re just doing things the best way that they know how,” Gruner said. “We have to come up with a standard protocol that everybody can use.”
Gruner and Dan Slone, an ecologist with the U.S. Geological Survey’s Florida Integrated Science Center, conducted the largest-ever field study of heat generated by blow fly maggots. The results were published in the May issue of the Journal of Medical Entomology.
For more than two years, Gruner placed almost 70 pig carcasses in a wooded area in North Central Florida, monitoring blow fly and maggot activity. A similar three-month study was conducted in Indiana by forensic entomology pioneer Neal Haskell, an associate professor of biology with Saint Joseph’s College in Rensselaer, Ind.
The Florida results showed that when maggots formed a mass larger than a golf ball, air temperature had little effect on the mass’s interior temperature—indicating that air temperature may not be a reliable way to gauge maggot development.
The study was funded by the U.S. Department of Justice’s National Institute of Justice Office of Science and Technology, Slone said. He received a grant with Jon Allen, then a UF entomology professor, to study the error rates of a computer model previously developed by Allen and UF colleague Jason Byrd to calculate the development time of blow fly maggots.
Slone and Gruner are now refining the model. Gruner’s doctoral research is aimed at providing another needed item — data on the effects of temperature on the time needed for two common blow fly species to grow from egg to mature fly. Gruner is rearing the insects in a laboratory under controlled temperatures.
Blow flies are among the first insects to arrive at a corpse outdoors. Their larvae feed on dead or rotting flesh, so adult females are attracted to decay odors. In North America, about 50 blow fly species are used to estimate time of death.
“Forensic entomologists had very little quantified data on the effects of maggot mass temperature prior to this study for implementation with case work,” said Tomberlin. “Sue’s work will allow greater precision in estimating the period of insect activity for blow fly larvae collected from human remains.”
The study may be important to lawyers as well, said James Cooney III, a criminal-defense attorney in Charlotte, N.C., who has dealt with forensic entomology evidence in several cases, including one where he freed a man from death row.
“What flies do and maggots do is open a new door in the quest for the Holy Grail, which is trying to find how long it’s been since someone died,” he said.
- Tom Nordlie, firstname.lastname@example.org, 352-392-0400