Pheromone-based Mating and Aggregation in the Sorghum Chafer, Pachnoda interrupta

Adults of the sorghum chafer, Pachnoda interrupta Olivier (Coleoptera: Scarabaeidae: Cetoniinae), form aggregations during the mating period in July, but also in October. The beetles aggregate on food sources, e.g., Acacia spp. trees or sorghum with ripe seeds, to feed and mate. During the mating season, field trapping experiments with live beetles as bait demonstrated attraction of males to unmated females, but not to mated females or males, indicating the presence of a female-emitted sex pheromone. Unmated females combined with banana (food source) attracted significantly more males and females than did unmated females alone. Other combinations of beetles with banana were not more attractive than banana alone. Thus, aggregation behavior appears to be guided by a combination of pheromone and host volatiles. Females and males were extracted with hexane during the mating period, and the extracts were compared by using GC-MS. In a field trapping experiment, 19 compounds found only in females were tested, both singly and in a mixture. Traps baited with one of the female-associated compounds, phenylacetaldehyde, caught significantly more beetles than any other treatment. However, the sex ratio of beetles caught in these traps did not differ from that of control traps, and it is possible that other components may be involved in the sex pheromone signal. Furthermore, traps baited with a mixture of all 19 compounds attracted significantly fewer beetles than did phenylacetaldehyde alone.