A newly described lonchaeid from Venezuela, Dasiops caustonae Norrbom and McAlpine, has been recommended as a biological control agent of Passiflora mollissima (H.B.K) Bailey, an aggressive exotic weed of Hawaiian rainforests. In this study, elementary biological information was collected in the field in order to determine the appropriateness of using D. caustonae as a biological control agent against this important weed. Field observations suggested that the host range of D. caustonae is limited to species of the subgenus Tacsonia . Female flies appear to mark flowers with an oviposition deterrent pheromone. This is possibly the first report of marking behaviour outside Tephritidae. The geographical range of this species is wide and comparable with that of the target weed. Life‐cycle studies demonstrated that D. caustonae is multivoltine with an estimated life‐cycle length of 3 to 4 months. Survival to adult was limited by premature flower fall, drought and interspecific competition by a bud and flower feeding Lepidoptera, Pyrausta perelegans Hampson. Additional ecological studies and host range testing of D. caustonae are recommended to determine its impact on the target weed and its safety as a biological control agent.