Analysis of 2,4,6-nonatrienal geometrical isomers from male flea beetles, Epitrix hirtipennis and E. fuscula

Geometrical isomers of 2,4,6-nonatrienal have been reported from a variety of food- and insect-related sources. It was discovered recently that the eggplant flea beetle, Epitrix fuscula, uses the (2E,4E,6Z) and (2E,4E,6E) isomers as,components of its male-produced aggregation pheromone. Here, we learned that the related species, E. hirtipennis, also emits a blend of 2,4,6-nonatrienals, including isomers not previously characterized. Patterns in emission and response suggest a pheromonal function. In an effort to acquire standards to aid in identification, we found that exposing (2E,4E,6E)-2,4,6-nonatrienal (or other available 2,4,6-nonatrienals) to light readily generated a mixture of six geometrical isomers. Configurations of these were determined by NMR, and chromatographic properties (GC and HPLC) were documented. On the basis of chromatographic comparison to these standards, the most abundant, new compound from E hirtipennis was concluded to be (2E,4Z,6Z)-2,4,6-nonatrienal. Minor components from both E hirtipennis and E fuscula were also characterized. The analytical approach given here would also be of use in the food industry, where 2,4,6-nonatrienals are important as aroma compounds.