On the function of sex pheromone components in Periplaneta americana : improved odour source localization with periplanone‐A

ABSTRACT. The female American cockroach, Periplaneta americana L., produces two sex pheromone components: periplanone‐A (PA) and periplanone‐B (PB). PB induces male locomotion and upwind orientation. PA, when presented alone, has the same effect but with a higher threshold concentration. When added to PB in the natural ratio, however, PA reduces rather than increases locomotory activity. To examine whether PA affects male orientation in the natural blend, a pheromone source was suspended above the centre of a circular arena. Efforts were made to exclude air movements so that pheromone concentration declined centrifugally. Males walking in a PB gradient spent more time near the source than in the peripheral areas when low doses were presented. At emission rates approximately equivalent to those of a female, however, they were not able to locate the pheromone source as precisely as at lower rates. In contrast, males walking in a PA gradient spent more time in the centre of the arena when the pheromone dosage was increased. When PA was added to natural levels of PB, males walked more slowly and stopped more frequently near the source than with PB alone. Thus PA improves chemo‐orientation of males when presented in a natural ratio with PB. A reduction of linear velocity near the pheromone source (an orthokinetic effect) is involved in this process.