Pyrazines Attract Catocheilus Thynnine Wasps.

Bjorn Bohman,Rod Peakall

doi:10.3390/insects5020474

Abstract

Five previously identified semiochemicals from the sexually deceptive Western Australian hammer orchid Drakaea livida, all showing electrophysiological activity in gas chromatography–electroantennogram detection (EAD) studies, were tested in field bioassays as attractants for a Catocheilus thynnine wasp. Two of these compounds, (3,5,6-trimethylpyrazin-2-yl)methyl 3-methylbutanoate and 2-(3-methylbutyl)-3,5,6-trimethylpyrazine, were attractive to male wasps. Additionally, the semiochemical 3-(3-methylbutyl)-2,5-dimethylpyrazine, a close analogue to 2-(3-methylbutyl)-3,5,6-trimethylpyrazine, identified in five other species of thynnine wasps, was equally active. The three remaining compounds from D. livida, which were EAD-active against Catocheilus, did not attract the insects in field trials. It is interesting that two structurally similar compounds induce similar behaviours in field experiments, yet only one of these compounds is present in the orchid flower. Our findings suggest the possibility that despite the high specificity normally characterising sex pheromone systems, the evolution of sexual deception may not be entirely constrained by the need to precisely match the sex pheromone constituents and blends. Such evolutionary flexibility may be particularly important during the early stages of speciation.

Highlights

Orchids are well known to adopt fraudulent strategies to achieve pollination, with an estimated one third of the 25,000+ species believed to be pollinated by deception [1]
3-(3-methylbutyl)-2,5-dimethylpyrazine (6), which we reported earlier in the dragon orchid Caladenia barbarossa [17]
'calling' female wasps of other genera (Table 1). The finding that this alkylpyrazine is produced by females of multiple thynnine species, together with its structural similarity with compound 5 and similar biologically active alkylpyrazines in the Drakaea glyptodon/Zaspilothynnus trilobatus pollination system [15], prompted us to include compound 6 in this study

Summary

Introduction

Orchids are well known to adopt fraudulent strategies to achieve pollination, with an estimated one third of the 25,000+ species believed to be pollinated by deception [1]. Among the most unusual of these deceptive pollination strategies is sexual deception, whereby male insects are attracted to flowers by semiochemicals that mimic the pheromones released by conspecific female insects during courtship [2]. Several hundred Australian terrestrial orchids, representing multiple genera, secure pollination by the sexual deception of thynnine wasps [3,5,6]. Thynnine wasps represent a diverse and conspicuous component of the Australian insect fauna [7]. Male thynnines make patrolling flights to locate the ‘calling’ females that perch on low vegetation where they release sex pheromones. With the successful male picking up a female in flight before carrying her, in copula, to a nectar source for feeding [3]

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Conclusion