Changes in volatile compounds of Dittrichia viscosa caused by the attack of the gall-forming dipteran Myopites stylatus

The false yellowhead Dittrichia viscosa (L.) Aiton (Asteraceae) is a spontaneous plant occurring in south-ern Europe. A tritrophic relationship is observed in the aerial parts of this plant since flowers are attackedby the gall-forming dipteran Myopites stylatus (Fabricius) which, in turn, is parasitized by hymenopteranspecies. This relationship may be mediated by volatile organic compounds (VOCs) released by the aerialparts of the plant. Thus, VOCs exhaled by leaves, flowers and flower galls of D. viscosa were characterizedby headspace solid-phase microextraction (HS-SPME) and gas chromatography-ion trap-mass spectrom-etry (GC-IT-MS). A total of 46 compounds were identified, including alcohols, aldehydes, esters, ketones,phenol derivatives, terpenes and sesquiterpenes. Volatile fractions registered from the different aerialparts of D. viscosa were quantitative and qualitatively different. Leaves were composed by 31 volatile com-pounds, flowers by 34 and galls by 29. Sesquiterpenes represented about 76.8% of the volatiles exhaledfrom leaves; flowers were composed mainly by sesquiterpenes (49.1%) and terpenes (34.6%) such as -pinene, limonene and -pinene and flower galls exhaled phenol derivatives (68.7%) such as anethol andestragole, and terpenes (25.1%) such as eucalyptol and limonene. Several sesquiterpene like compoundswere exclusive from leaves, while esters (e.g. methyl-2-propylhexanoate) were exclusive from flowersand the terpene eucalyptol from flower galls. The potential effect of volatiles exhaled by the differentaerial parts of D. viscosa on the behaviour of insects implicated in the relationship with D. viscosa isdiscussed.The authors are grateful to the Portuguese Foundation of Sci-ence and Technology for financial support through the projectEXCL/AGR-PRO/0591/2012 “Olive crop protection in sustainableproduction under global climatic changes: linking ecological infras-tructures to ecosystem functions”