Cone flies (Strobilomyia spp.) lay eggs on coniferous cones, and larvae must complete development within the cone on which the eggs are laid. Previous field surveys showed that egg distributions of several species are uniform, suggesting that females avoid ovipositing on cones with conspecific eggs or larvae. In both the field and laboratory, S. neanthracina females walked around cones following oviposition, touching their mouthparts to the cone as they walked. In laboratory bioassays, where females were presented with a cone on which they or another female had oviposited and touched with mouthparts, or one without an egg, females laid preferentially on cones without eggs. However, females laid randomly when presented with a cone on which a female had oviposited but been prevented from touching with her mouthparts following oviposition or another cone without an egg. This indicates that females deposit a host-marking pheromone with their mouthparts following oviposition and that this deters further oviposition on marked hosts. Laboratory and field bioassays indicated that the host-marking pheromone is located in the head and thorax of female flies.