The influence of winter temperature on fly colonization and developmental rate during a field experiment in Northern Italy

In the years 2006 and 2008, two pig (Sus scrofa) carcasses were exposed in an open field in the beginning of November and left out until they reached complete skeletonization; both pigs were placed in the same open field located in the surroundings of Pavia, a town in northern Italy. The purpose of this study was to observe the entomological species involved in the colonization of the carcasses during the winter season as well as their developmental time, focusing on the potential temperature-related differences. Both carcasses weighed about 40kg and they were both exposed in direct sunlight protected by a metallic cage. Pitfall traps were distributed around the carcasses and air traps were hung from a tree not far from the experimental area. Pictures of the carcasses were taken once a day while entomological samples were taken twice a day during the first two weeks and then once a day or less, depending on weather conditions. Immature stages were in part preserved in EtOH and in part reared on minced meat. Environmental data were recorded and compared to the ARPA weather station data. Winter 2006 and winter 2008 were significantly different in terms of temperature and meteorological conditions: the former was mild and wet while the latter was rigid and characterized by significant amounts of snow. The pig carcass exposed in 2006 was almost immediately colonized by Diptera in the primary sites; the flies continued to lay eggs repeatedly because of the frequent episodes of rain and the mild temperatures. The most abundant species in the first phases of decomposition were Calliphora vicina, Calliphora vomitoria, Lucilia Caesar and Lucilia sericata, while in the most advanced phases of decomposition, we observed members of the genera Fannia and Piophila. Post-mortem consecutive phenomena were observed during the first two weeks of the trial. After that period, very moderate chromatic changes were observed in the head of the carcass, which was also the first place to be colonized by Diptera and the first part of the carcass to become skeletonized. The pig exposed in 2008 was colonized almost exclusively by a single species, Calliphora vicina, which started oviposition in the head; in the spring, this species was replaced by Calliphora vomitoria and Piophila casei. During the first weeks of the trial, the temperature was still mild; after that, temperatures started to decrease and from the end of the month until the middle of February they remained very low. Moreover, this period was also characterized by significant quantities of snow that accumulated around the carcass and on the cage, but not directly on the pig. Because of these temperatures, the first thanatological changes were observed only 20 days after exposure; the bloating phase was never observed and the chromatic phase was not particularly marked and was characterized by a weak gray coloration of the abdomen. The flies continued oviposition until late November, when it began to snow, and interrupted their activity until March when the temperature rose again. Until this moment, the carcass was still in good condition. The maggots born from the eggs laid in November, continued to slowly develop inside the carcass; due to the extremely rigid environmental conditions, maggot development was so slowed down to be synchronous. Due to these circumstances, it was possible to observe the collective maggot migration from the carcass: masses of post-feeding maggots were in fact collected in the pitfall traps around the carcass and their pupation sites were very easily identified. The simultaneous emergence of young adult flies from their underground puparia was also observed.