Comparative study of the headspace (HS) composition from olive leaves, olives of varying degrees of ripeness, and crushed olives revealed significant quantitative and qualitative differences. Three different methods were employed to test the effect of the substances identified on the behavior ofDacus oleae gravid females. Attractive/repellent features were tested by presenting the insects with olfactory choices of a water solution of the test chemicals examined, water (blank), and a highly attractive (control) solution of olive maceration water (MW). Toluene and ethylbenzene, present in leaf and half-ripe olive HIS proved attractive, while (E)-2-hexenal emitted by the crushed olives was found to be decidedly repellent. An oviposition stimulation/deterrence test was carried out on olives placed in proximity to test chemicals under examination in water solution, with olives from the same crop serving as control. In the oviposition testα-pinene, mostly emitted by the leaves and half-ripe olives, emerged clearly as an activant,p-xylene, myrcenone, ethylbenzene,n-octane ando-xylene as weak activants. (E)-2-hexenal and hexanal, both emitted by the crushed olives, displayed an oviposition deterrent effect. The highly repellent property of (E)-2-hexenal was again confirmed in this test.
The egg dispersion strategy of the olive fruit flyDacus oleae, which is dependent on chemicals from the fruit, was investigated. In particular, the exact role ofo-diphenolic compounds, such as the typical olive glucosides, oleuropein and demethyloleuropein, and their derivatives was clarified. It appears that the strong chemotactile repulsive effect exerted by the water fraction of crushed olives is due mainly to (E)-2-hexenal. Several compounds, such asβ-3,4-dihydroxyphenylethanol and other oleuropein derivatives, which exert a strong chemotactile repulsion, were newly identified or confirmed either in fresh olive juice or in olive mill waste water. This result confirms that the small droplets of olive juice, regurgitated just after egg laying by theD. oleae female all around the oviposition hole, actually prevent other females from ovipositing on the same fruit.
An interpretation is given of a number of observations on the chemiotropic behavior ofBactrocera oleae in connection with olive maceration water and the fly's return to the olive groves after the first summer rains. To this end, the headspace of both maceration water and leaf leaching water, simulating rainfall, were examined. In both cases, the presence of ammonia, which is generally known to attract fruit flies (Diptera, Tephritidae), was detected and, for the first time, in addition to other compounds that are inert for the fly, the presence of styrene was also detected. This aromatic hydrocarbon was found to be a strong attractant. It is shown that both ammonia and styrene are products of the metabolism of microbial flora present on the olive and leaf surface.
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