Tuesday March 27, 2012
When Evidence Crawls Away
A common practice among crime scene investigators may result in compromised or erroneous evidence, according to new research.
A common practice among crime scene investigators may result in flawed evidence, according to new research from the University of Dayton.
In a study published in the Journal of Medical Entomology, University of Dayton biology professor Eric Benbow and graduate student Andrew Lewis found that blow fly larvae sometimes travel significantly farther after feeding on a carcass than the current recommended search radius of 10 meters.
Blow fly larvae are often the first insects to colonize on decomposing remains, usually within hours or even minutes after death, Benbow said. Based on environmental surroundings, forensic entomologists are able to calculate the age of blow fly larvae and thus determine the time of initial colonization, resulting in an estimated time of death.
"It is therefore essential that investigators locate the oldest larvae at a crime scene, otherwise the interpretation of the insect data can be compromised and erroneous," Benbow said.
Previous studies have shown that blow fly larvae typically disperse from a carcass as individuals or in small groups in a 360-degree pattern, burrowing within two to 10 meters of the carcass. But based on his findings, Benbow recommends forensic entomologists increase their search radius for larvae to a distance of 20 to 25 meters.
"Our study suggests that in a forensic case with insect evidence, there would be a one in three chance that more than 90 percent of the dispersing larvae would not be located and collected if the search stayed within the current recommendation of 10 meters," Benbow said.
In the spring of 2009 and again late summer, Benbow and his team placed six pig carcasses in a wooded area. The researchers observed typical blow fly larvae colonization and dispersal in the spring experiment. On five of the six carcasses in the summer experiment, however, larvae dispersed en masse (defined as more than 90 percent of larvae moving at once and in the same direction). Two of these larvae masses moved beyond the standard 10-meter distance, with one reaching 14 meters and the other 26 meters.
Benbow suggests three hypotheses for the en masse dispersals and the great distances, which include the effect of recent rainfall, rapid decomposition of the carcasses saturating the soil with fluids and byproducts, and the presence of predators.
More study is needed to test these hypotheses, Benbow said, but the fact that long-distance dispersal does happen is sufficient to recommend expanding the search area in crime scene investigations.
Benbow's research, "When Entomological Evidence Crawls Away: Phormia regina en masse Laval Dispersal," is available online at the related link.
The research is motivated in part by heavy criticism of the forensic sciences in recent years. The National Research Council in 2009 issued a report calling attention to a lack of sound scientific research in the forensic sciences and making several recommendations to improve it.
Benbow is among a team of researchers who are at the forefront of responding to the National Research Council report, with two articles published in 2011 on the future of forensic science research.
He is currently working with a grant from the National Institute of Justice, which awarded $476,348 to a team of researchers that includes Benbow; Jeffery Tomberlin and Aaron Tarone of Texas A&M University, Department of Entomology; and Tawni Crippen of the USDA-Agricultural Research Service.
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An en masse dispersal of larvae from a pig carcass in Eric Benbow's experiment, Summer 2009.