Relative response of male Bactrocera frauenfeldi (Diptera: Tephritidae) to phenylbutanoid phytochemicals: implications for fruit fly control and plant–insect interactions

The interactions between Dacini male fruit flies and phytochemical male lures are unique. Lure response, fate and its effects after consumption on fruit fly mating behaviour are species- and lure-specific. Bactrocera frauenfeldi is known to respond to the phenylbutanoids raspberry ketone (RK) and cue lure (CL), anisyl acetone (AA), and zingerone (ZN), which are produced by some rainforest orchids. Here we compared the relative field responses of B. frauenfeldi males to these phenylbutanoids in two selected locations to determine the most attractive lure for this species. We also performed gas chromatographic-mass spectral analyses of male rectal pheromone glands to understand the fate of the ingested compounds. Results showed that B. frauenfeldi males were most responsive to CL, equally to RK and AA, while poorly to ZN in Cairns, a site with high population density. No significant difference was observed in Lockhart River which has a low population density of B. frauenfeldi. Chemical analyses showed that most of the ingested phenylbutanoids were sequestered into rectal glands, either unchanged or with minimal structural changes except for AA, which is converted to RK via a demethylation of the methoxy- to a hydroxy-moiety and reduced to 4-(4-methoxyphenyl)-2-butanol via the keto-moiety. This study provides both practical and ecological implications: it identified the most attractive lure, which is important for monitoring and management of B. frauenfeldi; and based on the relative responses, conversion and retention rates by B. frauenfeldi males, revealed the ecological significance of these phytochemical lures in plant-fruit fly co-evolution.