Comparative analysis of steam distilled floral oils of cacao cultivars (Theobroma cacao L., Sterculiaceae) and attraction of flying insects: Implications for aTheobroma pollination syndrome

Abstract

Steam-distilled floral fragrance oils from nine distinctive cultivars ofTheobroma cacao L. (Sterculiaceae) in Costa Rica were examined with GC-MS to determine whether or not major differences existed among these cultivars for volatile constituents comprising 50% or more of the samples. The cultivars selected for floral oil analyses were chosen to represent diverse cultivars having supposedly different genetic backgrounds and histories of artificial selection for agronomic purposes. Cluster analysis revealed two major groupings of cultivars: those with higher molecular weight dominant compounds, and those having lower molecular weight compounds. Additionally, one cultivar, Rim-100, selected from criollo or ancestral-type cacao in Mexico and resembling criollo in the appearance of flowers and fruits, formed an extreme group having the highest molecular weight profile for major volatile compounds. Based upon these analyses, bioassays using McPhail traps were performed in an abandoned cacao plantation in northeastern Costa Rica during rainy and dry seasons to determine the relative attraction of these oils to flying insects. Bioassays revealed that the Rim-100 cultivar attracted by far the greatest numbers of cacao-associated midges (Diptera: Ceratopogonidae and Cecidomyiidae), as well as stingless bees (Hymenoptera: Apidae: Meliponinae), suggesting that a floral fragrance having high-molecular-weight volatiles is more potent as an attractant to flying insects than floral oils having lower-molecular-weight compounds. It is suggested that Rim-100 more closely resembles an ancestral or wild-type cacao than the other cultivars examined, and therefore it is more effective in attracting opportunistic dipteran floral visitors and pollinators than other cultivars in plantation settings. Several of the major volatile compounds found in the floral oils ofT. cacao and other species ofTheobroma occur in mandibular and other exocrine glands in various bees, including stingless bees and halictids, known visitors ofTheobroma flowers. These compounds are particularly present in noncultivated species ofTheobroma and have much more noticeable fragrances than the seemingly scentless flowers of cultivatedT. cacao selected for agriculture. It is hypothesized that the floral attraction system of ancestral or wild (noncultivated)T. cacao and other species ofTheobroma may have evolved to attract certain bees as their principal pollinators in natural habitats in the Neotropics, with a floral reward hypothesized as being sociochemicals needed by bees for mating, foraging, territorial defense, etc. Because of the many generations of extensive selection by cloning for desired cultivars,T. cacao might have lost much of its original floral attraction system for bees, instead being pollinated opportunistically by dipterans in plantation habitats. This may help to explain why natural pollination in cultivatedT. cacao is generally very poor relative to observed levels of fruit-set in wild or noncultivated species ofTheobroma.