Trapping the flies that cause myiasis: their responses to host-stimuli.

A critical phase in the life cycle of the fly species that cause myiasis is host location by the adult females, whether by direct location of and contact with the host, or by indirect location through the selection of sites frequented by hosts. The stimuli to which these flies respond in host location, principally visual and olfactory, are discussed here in the context of their use to enhance the performance of trapping systems used in research, monitoring and control. The three major families of flies encountered in myiasis can be divided into two groups for comparison of host-location strategies: (1) Oestridae; and (2) Calliphoridae and Sacrophagidae. The former are all obligate parasites, the latter include obligate and primary facultative parasites. In the Oestridae, the majority of Cuterebrinae deposit their eggs at sites likely to be visited by the host, whereas the majority of Oestrinae, Hypodermatinae and Gasterophilinae deposit their eggs or larvae directly onto the host. It is presumed that most host location is, like that of haematophagous insects, by response to olfactory and visual cues released by healthy hosts, but the subject is very under-researched. In the Calliphoridae and Sacrophagidae, most species that cause myiasis deposit their eggs or larvae directly onto the host at some predisposing site, such as those caused by wounding, necrosis and/or bacterial contamination. Healthy hosts are much less attractive, even for obligate species such as Wohlfahrtia magnifica, which can invade undamaged tissues. The last decade has seen a renewed interest in the potential for using traps in control of myiasis, leading to significant advances in trap and bait efficiency. However, there is great potential for further research and development, particularly on the general responses of Oestridae to hosts and in the precise identification of the olfactory cues for Calliphoridae and Sarcophagidae.