How promptly do blowflies colonise fresh carcasses? A study comparing indoor with outdoor locations.

We investigated the time taken by blowflies to find and oviposit on fresh carcasses placed outdoors and indoors. Paired dead piglets, one in the open and the other in a nearby room (on the first floor of an occupied, detached, suburban house near Cologne, Germany, with a window opened 9 cm) were exposed simultaneously on nine occasions. The species visiting both locations and the number of egg batches deposited by blowflies between both locations were monitored 2, 8, 24 and 48 h after exposure. In all cases the indoor piglet carcass was exclusively infested by Calliphora vicina; only in one case, on a very hot day after a 48-h exposure did Lucilia sericata infest an indoor carcass. The outdoor piglets were infested by a variety of common corpse-visiting species: L. sericata, L. caesar, L. illustris, C. vicina and C. vomitoria. A significant difference in the number of egg batches was detected between indoors and outdoors. Furthermore, in only two of nine runs did oviposition occur within the first 24h of exposure indoors. Ambient temperature, daylength and rainfall had no significant effect on the number of egg batches. Moreover, we observed fewer larvae on indoor piglets, too few to form maggot masses. This might result in slower larval development than in the case of outdoor piglets. We conclude that post-mortem interval (PMI) estimation for corpses found indoors must be handled carefully as oviposition might have taken place with a delay up to 24h.