(2R,7S)-Diacetoxytridecane: Sex Pheromone of the Aphidophagous Gall Midge, Aphidoletes aphidimyza

Abstract

In a recent study, evidence was presented that females of the aphidophagous midge Aphidoletes aphidimyza (Rondi) (Diptera: Cecidomyiidae) release a sex pheromone to attract mates. Our objectives were to identify and bioassay the pheromone. Coupled gas chromatographic-electroantennographic detection(GC-EAD) analyses of untreated and hydrogenated pheromone extract on three fused-silica columns (DB-5, DB-23, DB-210) revealed a single compound that elicited responses from male antennae. Retention index calculations of this candidate pheromone (CP) suggested that it was a di-acetate. Considering that most of the presently identified cecidomyiid pheromones consist of a 13-carbon chain with (at least) one acetate group in C2, we synthesized 2,6-, 2,7-, 2,8-, 2,9-, 2,10-, 2,11-, and 2,12-diacetoxytridecane. In GC analyses of these compounds, only 2,7-diacetoxytridecane cochomatographed with CP on all columns. In laboratory two-choice experiments with stereospecifically synthesized stereoisomers, only (2R,7S)-diacetoxytridecane elicited significant anemotatic responses by male A. aphidimyza. In trapping experiments in greenhouse compartments, only traps baited with (2R,7S)-diacetoxytridecane captured significant numbers of male A. aphidimyza, clearly revealing the absolute configuration of the pheromone. Failure of the stereoisomeric mixture (containing all four stereoisomers including the pheromone) to attract males is due to inhibitory characteristics of the (2R,7R)- and (2S,7R)-stereoisomers. The pheromone of zoophagous A. aphidimyza resembles those from phytophagous cecidomyiid midges, suggesting a common, diet-independent pathway for pheromone biosyntheses.

The Chemical Ecology of Cecidomyiid Midges (Diptera: Cecidomyiidae)

“…For example, in C. oregonensis (Gries et al, 2002) and the aphidophagous gall midge, Aphidoletes aphidimyza (Choi et al, 2004), pheromone in extracts was undetectable by a mass spectrometer. Gland extracts of C. pisi contained a few picograms per female (Hillbur et al, 1999).…”

“…However, in the case of the raspberry cane midge, Resseliella theobaldi, air entrainment from 200 females yielded 1.5 μg of pheromone (i.e., 7.5 ng per female; Hall et al, 2009). Analysis by coupled gas chromatographic-electroantennographic detection (GC-EAD) has proved effective at detecting pheromone components in collections from female Cecidomyiidae, with the male antenna capable of detecting components below a GC threshold of detection (Hillbur et al, 1999;Gries et al, 2002;Choi et al, 2004). Specially designed Plexiglas holders have been used for the antennal preparation (Hillbur et al, 2001(Hillbur et al, , 2005Andersson et al, 2009a), while Gries et al (2002) used a severed head and antenna suspended between two glass electrodes.…”

“…This procedure can be used directly on appropriate terminal epoxides in the synthesis of pheromone components of Cecidomyiidae. For synthesis of (2R,7S)-2,7-diacetoxytridecane, the pheromone of the aphidophagous gall midge, Aphidoletes aphidimyza, the 2R functionality was generated by reaction of the Grignard reagent from 1-bromo-4-pentene with (R)-propylene oxide (Choi et al, 2004). The terminal alkene was epoxidized, and the epoxide reacted with (R,R)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diaminecobalt(II) (Jacobsen's reagent) in aqueous tetrahydrofuran, which selectively hydrolyzed the S-epoxide to the corresponding diol, leaving the R-epoxide.…”

“…In C. pyrivora (Table 1, entry 7), field tests suggest that the first-eluting isomer of 2,7-diacetoxyundecane from the chiral HPLC column is attractive to males, while the second-eluting isomer is responsible for the lack of attractiveness of the racemate (Amarawardana, 2009). Choi et al (2004) reported that (2R,7S)-2,7-diacetoxytridecane was attractive to male A. aphidimyza (Table 1, entry 9). Addition of the 2R,7R-or 2S,7R-isomers reduced attractiveness, but addition of the 2S,7S-isomer had little effect.…”

“…Filter papers, cotton wicks, and dental rolls were used as pheromone dispensers in some initial field studies (e.g., Hillbur et al, 2000Hillbur et al, , 2005Choi et al, 2004;Gries et al, 2005). However, rubber septa and polyethylene vials or caps are used as dispensers in field applications, as they give longer persistence and more uniform release rates ( Table 2).…”