Sex pheromone and visual trap interactions in mate location strategies and aggregation by host-alternating aphids in the field

Field observations were made on the responses of males and gynoparae of three host-alternating aphid species, the blackberry-cereal aphid, Sitobion fragariae (Walker), the bird cherry-oat aphid, Rhopalosiphum padi (L.) and the damson-hop aphid, Phouodon humuli (Schrank) to species-specific sex pheromones released from transparent and coloured water traps. Pheromone traps caught significantly more males than did control traps without pheromone, whereas transparent, light green, yellow and orange traps caught most insects. Measurements of the distance over which sex pheromones function indicated that male P. humuli detect the pheromone 2-6 m from the source and can fly upwind to a source in wind speeds of less than or equal to 0.7 m s(-1). In all three species significantly more gynoparae were caught in pheromone traps than in control traps, suggesting that pheromone released by adult sexual females may assist late-flying gynoparae to locate a suitable host plant on which to deposit their progeny. The response is relatively stronger for males than gynoparae, but the pheromones appear to act as both sex and aggregation pheromones.