No abstract

Thrips are direct pests as well as vectors of important viruses infecting crop plants. One of the major constraints in studying the relationship between thrips vectors and tospoviruses is the difficulty of identifying the vector species because of high intraspecific variation among thrips populations. Molecular approaches have been used to identify species differences. In this study, partial cytochrome oxidase I (COI) sequences were used to understand the phylogenetic relationship among thrips populations, and assess their usefulness to identify and classify unknown thrips species collected from different crops. In total, 29 COI variants were obtained while examining the sequence polymorphisms in COI of 182 insects analyzed in this study, which were collected from six countries on tomato, chilli, onion, cabbage, cucumber, watermelon, Ethiopian mustard, French bean, and peanut. The phylogenetic analysis showed that the insects used in this study clustered with five distinct species-groups designated as Thrips palmi group, T. tabaci group, Frankliniella occidentalis group, Scirtothrips dorsalis group and an unclassified group. Higher intraspecific genetic variation was observed in S. dorsalis and T. palmi followed by T. tabaci and F. occidentalis. Thus, it was confirmed that the COI gene could be useful in grouping different thrips species and genera that coexist in a particular cropping system.

Aggregations of the striped flea beetle Phyllotreta striolata on their crucifer host plants are mediated by volatiles emitted from feeding males. The male-specific sesquiterpene, (6R,7S)-himachala-9,11-diene (compound A), was shown previously to be physiologically and behaviorally active, but compound A was attractive only when combined with unnaturally high doses of the host plant volatile allyl isothiocyanate (AITC) in field trapping experiments. This indicated that our understanding of the chemical communication in this species is incomplete. Another male-specific sesquiterpenoid, (3S,9R,9aS)-3-hydroxy-3,5,5,9-tetramethyl-5,6,7,8,9,9a-hexahydro-1H-benzo[7]annulen-2(3H)-one (compound G), has been reported from an American P. striolata population. We confirmed the presence of compound G, and investigated its interaction with compound A and AITC in a P. striolata population in Taiwan. Compound G was attractive to Taiwanese P. striolata in laboratory bioassays, but significantly more beetles were attracted to a blend of compounds A and G. Under the same conditions, P. striolata showed no preference for the blend of A and G combined with a range of doses of AITC over the sesquiterpenoid blend alone. The sesquiterpenoid blend was tested further in field trapping experiments and attracted significantly more beetles than traps baited with compound A and ecologically relevant amounts of AITC. We conclude that A and G are components of the male-specific aggregation pheromone of P. striolata in Taiwan, and that the attractiveness of the pheromone is not reliant on the presence of AITC. Our results further indicate that the male-specific sesquiterpenoid blends differ qualitatively between the Taiwanese and American populations of P. striolata.