Electrophysiological and Behavioral Responses of Sorghum Shoot Fly, Atherigona soccata, to Sorghum Volatiles

The sorghum shoot fly, Atherigona soccata, is an economically important pest of sorghum in Asia, Mediterranean Europe and Africa. Field observations have suggested that shoot fly susceptible sorghum varieties emit attractive volatiles, but the compounds involved were unknown. The objective of the present study was to identify plant-derived attractants for A. soccata. Headspace samples were collected from the susceptible cultivar 'Swanta,' and when female A. soccata were exposed to the volatiles in an olfactometer bioassay, a strong positive behavioral response was observed. Coupled GC-EAG with female A. soccata revealed eight compounds that elicited an EAG response, which were identified by coupled GC-MS and GC peak enhancement on two GC columns of different polarity as (Z)-3-hexen-1-yl acetate, (-)-alpha-pinene, (-)-(E)-caryophyllene, methyl salicylate, octanal, decanal, 6-methyl-5-hepten-2-one and nonanal. When an eight-component synthetic blend of the EAG active compounds, at the same concentration (2.64 mu g) and ratio as in the natural headspace sample, was tested, A. soccata spent more time in the treated region of the olfactometer than controls (P=0.001). Furthermore, when this synthetic blend and the natural headspace sample were tested in a choice test, the shoot flies did not show any preference for either of the two treatments, demonstrating that the synthetic blend had similar activity to the natural sample. Results are discussed in relation to breeding sorghum varieties less attractive to this pest.