Hawkmoth pollination of aerangoid orchids in Kenya, with special reference to nectar sugar concentration gradients in the floral spurs

Abstract

The African orchid flora has a high proportion of species with long-spurred white flowers. Few data exist to test the prediction that this floral syndrome pattern reflects an important role for hawkmoth pollination in the evolution and ecology of these orchids. The pollination biology of five aerangoid orchid species (Rangaeris amaniensis, Aerangis brachycarpa, A. confusa, A. thomsonii, and A. kotschyana) was investigated in Kenya. Four of these have long spurs (>10 cm) and were pollinated by Agrius convolvuli and Coelonia fulvinotata. Aerangis confusa, which has relatively short spurs (ca. 4 cm), was pollinated by the short-tongued hawkmoths Hippotion celerio and Daphnis nerii. Nectar frequently filled the entire spur in some of the study species, even at anthesis. Sugar concentration of the nectar of four species was found to vary from ca. 1% at the mouth of the spur to 20% at the tip. Gradients were expressed more strongly in species with long, straight spurs. Species with spirally twisted spurs showed both steep and shallow nectar gradients. These gradients, previously unknown in plants, may function as a "sugar trail," enticing long-tongued hawkmoths to probe deeply into spurs without incurring the cost of filling an entire spur with concentrated nectar. In addition, the most concentrated nectar is kept out of reach of short-tongued pollinators.

Biologia reprodutiva de Campylocentrum micranthum (Orchidaceae, Angraecinae)

“…Na subtribo Angraecinae a autocompatibilidade é uma característica comum, e tem sido documentada em várias outras espécies desse grupo (e.g., Nilsson 1988;Dressler 1993;Luyt & Johnson 2001;Micheneau et al 2006Micheneau et al , 2008aMartins & Johnson 2007). Já as flores submetidas ao tratamento de emasculação ou à autopolinização espontânea não desenvolveram frutos.…”

When is an asymmetric plant-pollinator relationship symmetrical?

“…an example of the asymmetrical orchid-pollinator relationship is the hawkmoth (Panogena lingens), which was found to pollinate at least 5 species of long-spurred angraecoid orchids (nilsson et al 1987). such behaviour has also recently been recorded in other species of angraecoid orchids (martins and Johnson 2007). in these cases the pollinator may only have adapted to access the nectar resource of long spurred orchids (nilsson 1988).…”

Diel rhythms and sex differences in the locomotor activity of hawkmoths

“…S2), suggestive of a similar phenomenon in this species. Furthermore, the extensive temporal differences in the behavior of male and female H. lineata raise the possibility that the two sexes may exploit different floral resources while foraging, as floral rewards also are subject to species-specific diel rhythms (Stone et al, 1998;Martins and Johnson, 2007).…”

“…Many night-blooming hawkmoth-pollinated flowers exhibit clear rhythms of flowering, scent emission and nectar secretion that coincide with periods of moth activity (Haber and Frankie, 1989;Hoballah et al, 2005); however, it is unknown whether this pattern of moth activity is under endogenous control or if moths become active and begin foraging in response to exogenous cues, such as the onset of darkness. Endogenous regulation of locomotor activity could confer a significant advantage if peaks of activity coincide with floral rhythms by allowing an insect to arrive at newly opened flowers, often the time of highest nectar availability (Gregory, 1963;Martins and Johnson, 2007), thus increasing profitability (caloric intake per unit foraging time or effort) while reducing flight costs incurred by foraging at times when resource availability is low. This type of endogenous control of locomotor activity has been shown for honeybees and other hymenopteran pollinators (Spangler, 1972;Stelzer et al, 2010) but endogenous control of adult locomotor activity remains undetermined for the majority of the Lepidoptera (Merlin and Reppert, 2010).…”