An experimental investigation into the colonization of concealed cadavers by necrophagous blowflies

Abstract

We used seven baited boxes with different combinations of access holes and odor diffusion surfaces to study the arrival of necrophagous flies. During laboratory experiments, 30 gravid Lucilia sericata females were kept in a chamber with one of the boxes. The box with the largest odor diffusion surface (99 cm(2)) combined with the lowest accessibility (one 1 cm(2) entrance hole) was entered least (5 ± 3.7 flies per run). In contrast, the most frequently entered box (one 9 cm(2) entrance hole with no additional odor diffusion surface) caught a mean of 24.6 ± 3.4 flies per run. These results indicate that 1) L. sericata entered nearly inaccessible places and 2) both odor diffusion and accessibility impacted the number of flies caught. During field experiments, the seven boxes were placed together outdoors. The box with the most entrances (ten 9-cm(2) holes) caught the most flies (55.6-99.4% of the total). Only a few flies entered the other boxes. Access to the less accessible boxes (poor odor diffusion and small entrances) was also delayed. The major conclusions of the field experiments are that 1) boxes with low accessibility took longer to be accessed; 2) larger odor diffusion surfaces were more attractive to flies; and 3) flies accessed boxes more readily through larger holes than through an equivalent surface area made up of smaller holes. With these conclusions in mind, attempts to quantify the preappearance interval or to interpret the number of flies observed in indoor forensic entomology cases should be approached with caution.

Keywords: Lucilia sericata (Meigen 1826); body concealment; delayed colonization; indoor forensic entomology; postmortem interval.

Fig. 1.

Location of the field experiment boxes (symbolized by rectangles). Experiments were performed in a 25 by 12-m green space located near the forensic institute of Lille, France. For each trial, the traps were simultaneously placed on the ground for 3 d, and box positions were randomized between each trial. Bold lines indicate a building wall; dashed lines depict open access; and crossed circles indicate bushes or small trees. The access holes in the boxes are indicated by an arrow and the meshed surfaces with an asterisk.

Fig. 2.

The number of flies caught in each box during the laboratory experiments. Thirty gravid females were confined for 53 h (33 h of day and 20 h of night) inside a cupboard containing one of the baited boxes. Each point corresponds to the total number of flies caught in a given box during a single trial, and bold horizontal lines represent means. Only box C differed significantly from the others (Kruskal–Wallis: K = 19.9, P value = 0.003).

Fig. 3.

The number of flies caught in boxes (y axis) during the field experiments with (left) and without (right) box G. Each point corresponds to the total number of flies caught in a box during a single trial, and asterisks indicate statistical significance (Friedman test: ***P value < 0.0001).

Fig. 4.

The species composition observed during the field experiments. Boxes are reported on the x axis, and the contribution (%) of each taxon to the total number of flies caught in each box is reported on the y axis. The number of males includes all the Calliphoridae species. The numbers on top of the columns indicate the total number of flies caught in the box.

Fig. 5.

Comparison of the precolonization time (delay before the first insect was observed) between the boxes. The most accessible box (box G for the first set of experiments and box F for the experiments without G) was used as the reference; for each replication, the delay before insect arrival was calculated as the time of the first catch in a box minus the time of the first catch in the reference box (G or F). Bars correspond to trials. Black bars indicate the delay in colonization, and the gray bars represent any colonization, i.e., a delay in fly arrival extending over 56 h (the duration of one run). No bar indicates no difference, i.e., colonization at the same time as the reference box.