Search

Drosophila mojavensis wrigleyiand D. mojavensis sonorensisare geographically separated races of cactophilic fruit flies. D. mojavensis sonorensisinhabits the Sonoran Desert and utilizes necrotic rots of Stenocereus thurberiEngelm. as a food source and to oviposit while D. mojavensis wrigleyiinhabits Santa Catalina Island, California and utilizes the necrotic rots of Opuntia littoralis(Engelm.) Cockerell. The objectives of this study were to determine the volatile compositions of the necrotic cacti and to determine if the volatile components show either selective toxicity or attraction toward the fruit flies. The volatile chemical compositions of field-rot specimens of both necrotic cacti were obtained by dynamic headspace (purge-and-trap) and hydrodistillation techniques and analyzed by gas chromatography – mass spectrometry. The volatile fraction of necrotic S. thurberiearly rot was dominated by carboxylic acids (84.8%) and the late rot by p-cresol (32.6% in the dynamic headspace sample and 55.9% in the hydrodistilled sample). O. littoralisvolatiles were dominated by carboxylic acids (86% in the dynamic headspace sample and 89.1% in the hydrodistilled sample). Fifteen compounds that were identified in the necrotic rot volatiles were used to test insecticidal activity and olfactory preference on the cactophilic Drosophilaspecies, as well as D. melanogaster. Differences in toxicity and olfactory preference were observed between the different taxa. Both races of D. mojavensisexhibited toxicity to benzaldehyde and 2-nonanone, while butanoic acid and palmitic acid were tolerated at high concentrations. D. m. wrigleyidemonstrated a greater olfactory preference for anisole, butanoic acid, 2-heptanone, and palmitic acid than did D. m. sonorensis, while D. m. sonorensisdemonstrated a greater preference for hexadecane, octanoic acid, and oleic acid than did D. m. wrigleyi.

Four different cactus species growing in the United States, Stenocereus thurberigrowing in Organ Pipe Cactus National Monument, Arizona, Opuntia littoralisand Opuntia ficus-indica, growing on Santa Catalina Island, California, and Opuntia stricta, growing in northern Alabama, were examined for the presence of silica bodies (opaline phytoliths). Silica bodies were found in all four of these cactus species, parallelepiped-shaped crystals in S. thurberi, and starburst-shaped crystalline structures in the three Opuntiaspecies. In addition, the essential oils of the four cactus species were obtained by hydrodistillation and analyzed by gas chromatography – mass spectrometry. To our surprise, S. thurberi, O. littoralis, and O. ficus-indica(but not O. stricta) essential oils contained cyclic oligosiloxanes. To our knowledge, cyclic oligosiloxanes have not been previously found as essential oil components.