Seasonality, Longevity, Host Choice, and Infection Efficiency of Salticella fasciata (Diptera: Sciomyzidae), a Candidate for the Biological Control of Pest Helicid Snails

Host specificity and seasonal incidence of parasitism by the sciomyzid fly Salticella fasciata was studied at a site near Montpellier, France, and in the laboratory. In field collections, only adult snails (> 11-mm diameter) of the pest helicid species Theba pisana and Cernuella virgata were found bearing eggs of S. fasciata in their umbilicus. Flies were present in the field from September to June, which coincides with the active breeding period of the host snails. The percentage of adult snails bearing fly eggs increased with time while the adult snail population decreased through mortality after breeding. In choice experiments, both T. pisana and C. virgata were significantly preferred as hosts when maintained in pairwise combinations with the co-occuring pest helicid species Cochlicella barbara, Cochlicella acuta, and Helix aspersa. Surprisingly, fly larvae were found to be inefficient at infecting and killing host snails though they fed readily on the decaying tissues of manually killed snails. The host-specific oviposition habit of the fly and larval killing inefficiency are discussed in relation to both the behavioral evolution of the Sciomyzidae and the inappropriateness of this species as a biological control agent of terrestrial snails.