Effects of early decomposition time on blow fly (Diptera: Calliphoridae) oviposition

When blow flies land on the corpse and decide to lay eggs and colonize, it also starts their biological life cycle which will begin and finish on the corpse. People could use those vital information to estimate the post-mortem interval (PMI) in criminal investigations. The process of decomposition can be divided into five phases, from the death occurs to the finish of life cycle of blow flies. Those phases of exposure phase, detection phase and acceptance phase, belong to the pre-colonization stage; consumption phase and dispersal phase belong to the post-colonization stage. Most previous studies focused on the post-colonization stage, which are the study of effects from different factors (e.g. temperature, humidity…etc.) on the development of larvae or adults. However, the pre-colonization stage is also important, but related research is scarce. In this study we focus on the acceptance phase of Chrysomya megacephala, which was defined as the period between the corpse finding and the decision making of oviposition of blow flies. Longer time period caused by the delay of oviposition will further increase the bias in PMI estimation. In this study, we assumed that different degrees of decay of pork liver may affect the oviposition decision of the blow flies C. megacephala. In field experiment, our results showed significant differences on egg number among groups of using different decay-aged liver as oviposition media (fresh to 8-day-old). In preference test, the results showed significant differences on egg number among groups of using different decay-aged liver (fresh, 2-d old, 4-d old, 6-d old, 8-d old). Our results indicated different decomposition levels do affect the blow fly oviposition; and when we provide media of different decomposition level individually to test the incidence of oviposition, the results also show significant difference among different treatment. However, in the test of duration of acceptance phase, the results show the durations from media discovery to oviposition have no significant different for different decay-aged media. In conclusion, different decay-aged media have significant effects on the oviposition of blow fly C. megacephala, but no effect on the duration of acceptance phase. However, the delay of egg laying could be up to 9.83 1.92 hours due to media of different decomposition level and should be considered in the future study. Hopefully, these data could be helpful to the adjustment of PMI estimation in the near future.