Bimodal cue complex signifies suitable oviposition sites to gravid females of the common green bottle fly

Gravid females of the common green bottle fly, Lucilia sericata Meigen (Diptera: Calliphoridae), readily locate recently deceased vertebrates as ovipostion sites particularly when these animals had been injured. We investigated semiochemical and visual cues that mediate attraction of gravid females to fresh rat carrion.  Female flies were more strongly attracted to incised rat carrion than to intact carrion. They were also attracted to Porapak Q head space volatile (HSV) extract of incised rat carrion.  Analyzing aliquots of Porapak Q HSV extract by gas chromatographic-electroantennographic detection revealed nine components [phenol, para-cresol, meta-cresol, guaiacol, dimethyl trisulfide (DMTS), phenylacetaldehyde, (E)-2-octenal, nonanal, tetramethyl pyrazine] that consistently elicited responses from blow fly antennae. In laboratory experiments, a synthetic blend of these nine components was as attractive as Porapak Q HSV extract to gravid females, but blend attractiveness was due entirely to DMTS. In both laboratory and field experiments, increasing doses of DMTS attracted increasingly more flies.  Coupled with DMTS, carrion-type color cues (dark red, black) were more effective than bright color cues (white, yellow) in attracting flies. In field experiments, dark-colored traps baited with DMTS captured a total of 214 calliphorid flies (200 L. sericata, 10 Lucilia illustris, 3 Calliphora vicinia, 1 Calliphora vomitoria), all of which were gravid females. These results support the conclusion that DMTS and dark color represent a bimodal cue complex that signifies suitable oviposition sites to gravid calliphorid females, particularly L. sericata.