The responses of the saw-toothed grain beetle Oryzaephilus surinamensis to pheromones and food odours
The responses of the saw-toothed grain beetle, 0ryzaephilus surinamensis to food odours and pheromones were investigated at both sensory and behavioural levels to give an integrated picture of olfaction, and the importance of odours in this species. The sensory capability of the antenna was assessed by a study of the structure, distribution and function of sensilla using SEM and TEM. Seven different sensilla types, including 3 with an olfactory function, were identified, although the total number of olfactory sensilla was low compared with those for mechanoreception. Antennal responses to odours were studied using both the electroantennogram (EAG) technique and single-cell recordings, whilst a 2-choice pitfall assay and a single-insect arena test were used to investigate behavioural responses. Food-produced volatiles from carob pods were found to produce large amplitude antennal responses and attraction in behavioural assays. Analysis of such volatiles by coupled gas chromatography-electroantennography (GC-EAG) demonstrated at least 3 active components, of which one was identified as hexanoic acid. Attraction to food odour was found to vary with insect age, and this correlated with similar variation in the EAG response, suggesting a role for peripheral receptor sensitivity in modulating insect behaviour. Beetle-produced volatiles also caused antennal responses and attraction of conspecific individuals of both sexes, thus acting as an aggregation pheromone. The active components were identified by GC-EAG as (Z,Z)-3,6-dodecadien-11-olide, (Z,Z)-3,6-dodecadienolide and (Z,Z)-5,8-tetradecadien-13-olide. These act as a multicomponent pheromone, with separate antennal receptors for each component. Although no sexual differences in antennal responses were found, consistently more females than males were attracted to the pheromone. This suggested a partial sex pheromone function, as the pheromone was shown to be male-produced. However, the ratio of females:males attracted was found to vary with the blend ratio tested. Thus if males are able to alter the blend they produce, the pheromone may act at different times as either a sex pheromone or an aggregation pheromone. Finally, the functional and evolutionary importance of odours to this insect are discussed.