Bionomy and host plant finding in oil collecting bees

The pollination system between oil offering flowers and oil collecting bees is one of the most specialised mutualistic interactions between insects and their host plants. It occurs in four floristic regions with about 1800 plant species and 400 bee species involved. The bees collect the fatty oils to provide their offspring and in some cases also to line the walls of their brood cells. The only oil flowers in the Holarctic are species in the genus Lysimachia. About 40 % of the species in this genus secrete floral oils and these species are almost exclusively pollinated by oil collecting bees of the genus Macropis, whereas non-oil species in this genus are regarded as being pollinated by generalised bees. In the present work, I focused on (i) the bionomics of a flight cage population of Macropis oil bees, (ii) the visual (colour) and olfactory (scent) advertisement in oil and non-oil secreting Lysimachia species, (iii) the relative importance of visual and olfactory cues of L. punctata for their pollinating bees M. fulvipes, and (iv) the specific cues (scent compounds) mediating the interaction between Lysimachia and Macropis. I also tested whether cues mediating the Lysimachia-Macropis interaction may be important for other oil plants and oil bees. The bees started to hatch in the flight cage when a certain temperature sum (temperature five weeks before hatching was most important) was reached, and although the date of emergence varied over the four years, it coincided with the start of the flowering period of L. punctata. The population was protandrous and the sex ratio balanced in three of the four observation years. Not only female but also male bees fed on pollen of their host plants after hatching, and for females seems pollen feeding to be the trigger to search appropriate nesting sites and construct a cell. Following excavating a cell, they collected floral oil to line their cell walls. After finishing the cell lining, they collected oil+pollen for the larval bred, and finally laid an egg. At good weather condition a female bee completed two cells per day. About 460 flowers (10 inflorescences) were needed to complete a cell for one larva. To sustain a viable population of 50-500 bees, 20 000-200 000 flowers (at least 400-4 000 inflorescences) are necessary. Behavioural experiments with decoupled and combined visual and olfactory cues of L. punctata demonstrated that Lysimachia-inexperienced M. fulvipes females prefer olfactory over visual cues and primarily rely on olfactory cues to locate their host plants, whereas for experienced females the importance of visual cues was increased. In male bees visual cues play a more important role independent of experience. Overall, data demonstrate that the relative importance of visual and olfactory cues for locating host plants depends both on sex and experience of M. fulvipes bees. In 17 different Lysimachia species we found 63 flower-specific compounds and 62 compounds were found in the vegetative scent samples. Vegetative and floral scent was species-specific and variability in floral but not vegetative scent was lower in oil compared to non-oil species. Although oil species did not differ in either floral or vegetative scent from non-oil species we did find six floral scent compounds in oil species, which correlated with oil secretion. The petals of most yellow coloured oil secreting species appear green to bees, that of diverse coloured non-oil species appeared UV-blue, UV, UV-green, or blue-green, but never green to bees, whereas leaves in all species were similarly coloured. The bee green color of oil species correlated with oil secretion. Both floral scent compounds and the bee-green colour that correlated with oil secretion are likely selected by Macropis oil bees and may be involved in attracting these bees to the oil flowers. Few compounds in the flower extract of Lysimachia punctata elicited antennal responses in M. fulvipes bees, among them diacetin, which was not known as a natural compound before this study. This compound was a key stimulus for attracting M. fulvipes bees in the bioassays, whereas others were needed to obtain attractiveness comparable to that of natural flower extract. Diacetin not only occurs in scent samples of L. punctata flowers, instead I found it in many oil secreting species around the world. Similar to M. fulvipes, the South African oil bee Rediviva neliana responded to diacetin. In contrast, neither Melitta haemorrhoidalis non-oil bees, nor the honey bee responded in electroantennographic measurements to this compound. These findings point towards diacetin as a ‘private communication channel’ in the Macropis-Lysimachia and possibly also in other oil bee oil plant pollination systems. Diacetin is structurally similar to the floral oils and likely produced by similar metabolic pathways as the non-volatile fatty oils. Therefore it represents a reliable and honest cue for bees in search for oils.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2012

Author dc:creator

• Schäffler, Irmgard

Contributors dc:contributor

• Dötterl, Stefan

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1670/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1670