How Blow Flies Compete (or Not) for Decomposition Duties

Editor’s Note: A photo included in this blog post depicts an entomologist collecting blow fly larvae from a decaying animal corpse.

Blow flies—or, more precisely, the squirmy maggots that are their larvae—are as familiar to fans of crime dramas as Sherlock Holmes or Miss Marple. Best-selling crime novelist Patricia Cornwell even wrote a book titled Blow Fly. By examining maggots doing their thing on a corpse, forensic investigators can find all sorts of clues as to how and when a person became one. Species of maggots can indicate, for example, the time of death, because some adult females arrive and lay eggs minutes after the last breath; others later, in succession. If eggs from a species that prefers a moist environment are found on a corpse in a dry area, chances are the body was moved after death.

A new study by a Florida International University researcher, published this month in Environmental Entomology, supports the idea that timing of egg laying by different species is an adaptation that has evolved so they can avoid competition within the egg laying environment. (Read “environment” as a dead body.) If a warming climate raises seasonal temperatures sufficiently, says author Amber E. MacInnis, the variety and proportion of blow fly species available for sampling by forensic investigators might change, posing a potential issue for investigators.

Only in an extreme case might the variety of blow fly species on a corpse be truly scrambled because of rising temperatures, says MacInnis, “but it is not out of the realm of possibility. What is more likely [is that] the number of a species could decrease [and] might not be collected due to low numbers.”

“This research is a study into the interactions of these flies with other members on the carrion,” says the author. “This is important because carrion is a complex environment with many different species and ages of insects present. I think it is important to understand how the species interact with each other, and the presence of certain species can alter either the species on the carrion or the behavior of the insects there. Also, since the late 1800s, workers on forensic entomology have discussed insect succession on carrion, but evidence for competition that might drive such succession has been lacking.”

A new study on three species of blow flies that commonly factor into forensic entomology investigations examined what drives the timing of their arrival and how they interact. To study those interactions, researchers placed multiple ratios of the different species of blow fly maggots in containers (pictured here) with a mouse corpse on which to feed, and the containers were then placed in a temperature-regulating growth chamber. (Photo courtesy of Amber E. MacInnis)

In forensic entomology investigations, a variety of blow fly species commonly arrive to feed on a corpse, and they often arrive in a predictable order. Entomologists examine these patterns in experiments with animals to help investigators better understand what conclusions can be inferred from the presence of insects on a corpse. (Photo courtesy of Amber E. MacInnis)

Her paper is based on observations of three species of blow flies: Phormia regina, Lucilia sericata, and Chrysomya rufifacies. The maggots were reared in a growth chamber: an incubator with a fan to circulate the air that holds a constant temperature. It was lighted to mimic the day/night cycles that the insects would experience in the wild.

Lucilia sericata lays eggs immediately, avoiding competition with P. regina, which delays oviposition. Enter C. rufifacies, also known as the hairy maggot blow fly, which brings another element into the game. If C. rufifacies cannot get enough nourishment out of the corpse, it eats other maggots. It is a nasty customer indeed, wrapping around its prey and then sinking in its mouth hooks. It does not chew its victim, but rather secretes digestive enzymes that liquefy tissues so they can be consumed in a pre-digested slurry. When MacInnis mixed the two maggot species, C. rufifacies killed all P. regina present.

Currently, the two species seldom come into contact within the United States because P. regina, also known as the black blow fly, is a spring and fall fly, avoiding the hotter and colder times of the year, and C. rufifacies is a summer fly. Moreover, C. rufifacies is an introduced Australian native now ensconced in Southern California, Arizona, Texas, Louisiana, and Florida. “However, given global warming,” writes the author, “C. rufifacies will likely overlap more with P. regina in the future.”

MacInnis opines that if C. rufifacies is as predatory on P. regina in nature as in her experiments, P. regina would either have to evolve to deal with this competition or become locally extinct. For example, selection saving more cold-tolerant P. regina could reduce or eliminate seasonal overlap with C. rufifacies. Alternatively, selection for greater delays in oviposition by P. regina might also provide a means to avoid C. rufifacies, although it would leave P. regina open to competition from other later-occurring species. “Irrespective of the possible evolution of P. regina, our results imply that as seasons are extended with global warming, or as it continues to spread north even in the cooler months, C. rufifacies seems likely to become more common, possibly displacing P. regina in much of its range,” writes the author.

Temperatures also separate L. sericata, which prefers the warmer summer months, from autumn-active P. regina, so the species seldom coexist. When they do, the different timing of egg laying may enable each to exploit the food source without competition. “I think it is important to understand how the species interact with each other, and the presence of certain species can alter either the species on the carrion or the behavior of the insects there,” says the author.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.