Interesting but difficult question! Bees forage optimally, it is thus important what the bees get in relation to handling time of flowers and costs in terms of reduction of flight time, resp. life time. Concentration of sugars in nectar and proteins in pollen may play a role as low concentrations increase costs for carrying.

Your question it a bit misleading, because eusocial bees collect both nectar and pollen and store it in their nests; although nectar is consumed mostly by the adults it is nevertheless transported into the nest.

Important to me is also that eusocial bees eat and regurgitate nectar and also pollen and by this way the collected subszances are chemically processed by enzymes from  the bees that might help to detoxify nectar as well as pollen ingredients (better as in solitary bees).

For me it is most important how the bees know about toxic substances: all nectar is licked and thus all potentially toxic substances are sensed by the bee; pollen is groomed into the scopa and thus the bees´ chance to sense toxic substances is much smaller.

From a plant´s perspective, the pollen should be transferred to other conspecific flowers and not be eaten or collected by bees in contrast to nectar that is procuced by the plant to attract bees but also to to repel ants and other antagonists.

 In response to Klaus's last point, there are of course plants that produce pollen BOTH as a reward and as a means to reproduction, e.g. buzz pollinated species such as many Solanum spp.

In response to Jeff's clarification, those Solanum plants which achieve more pollen to pollination and less to reward will be successful. Many other plants evolved heteranthery to separate and better control to amount of pollen spent for reward and the amount of pollen spent for pollination.

Klaus is correct of course, but there are certainly other plants that are less "strategic" about their production of rewarding pollen, and dispense vast amounts (I'm thinking of some Papaveraceae in particular).  

Has anyone tried cost-benefit modeling of what an optimum ratio of reproductive:rewarding pollen actually is under different circumstances?  Perhaps Papaver-like over-production of pollen occurs when pollinators are less efficient than bees at collecting/consuming pollen (some Mediterranean Papaver spp. are beetle pollinated).

Further to my answer above, this is the study I was thinking about in relation to Mediterranean Papaver (and other genera):  https://www.researchgate.net/profile/Amots_Dafni/publication/265529755_Red_bowl-shaped_flowers_convergence_for_beetle_pollination_in_the_Mediterranean_region/links/54709d780cf2d67fc03148ad.pdf 

Article Red bowl-shaped flowers: convergence for beetle pollination ...

Sure. since different plant species are targeted  for different purposes including either for collecting pollens and  nectar, or for collecting one of them. Several articles showed that plant species might be targeted for collecting nectar or pollens or both

a. Most flowering plants produce a vast excess of pollen grains, and the ratio p/o (pollen grains to ovule numbers) reflects the pollination efficiency of the species. However, flower species that use only pollen as the pollinator reward (papaver, anemone and many others) may carry a bigger excess.

b. The nectar of Epipactis helleborine was found to possess few narcotic compounds, that were proposed to both attract and stun the visitor insects for improve pollination. See an excellent paper in http://www.ecology.kee.hu/pdf/0302_029038.pdf

I don't like the statement about vast excess of pollen grains that is produced by many flowering plants. Probably the number of pollen grains produced is a trait under natural seceltion. There are costs for the production of pollen grains and flowering plants have to allocate their energy for the production of pollen grains, number of flowers, size of corolla, amount of reward for pollinators, etc. The statement of "vast excess of pollen grains" suggests that a reduction in the number of pollen grains would have no effect of the reproductive success, which is highly questionable.

I think that flowering plants have more problems of offer pollen in portions. In contrast to nectar and nectaries emptied anther cannot be refilled. Sequentional ripening of stamens within a flower, sequentional release of pollen grains from anthers are not so common. Sequentional opening of disc florets in Asteraceae might be linked to release of pollen in numerous portions.

I agree, Klaus that "excess" is not the appropriate term for pollen grains' number. May be "surplus" is a better one, since we deal with number of pollen grains per flower that is much bigger than needed to reach the stigma for fertilization. Avocado flower carries 5000-10000 pollen grains. It has only one ovule, and only 20 pollen grains should reach a stigma to ensure fertilization. The main pollinators of the flower are social bees, stingless bees in Central America, and honeybees elsewhere. The visiting bee collects the pollen as food source, and only a small fraction of pro mille is available for pollination. Therefore, the vast surplus pollen grains are not aimed for fertilization, though are important for ensuring the pollination process.

The avocado male-phase flower releases pollen in two stages: first from the upper stamen's valves, a after an hour from the lower ones. Furthermore, fritillaria persica flower (Liliaceae) has 6 days of male activity. On each day it opens one of its six stamens for pollen releasing.

Interesting Gad, I agree that a surplus of pollen grains is good for maximization of fertilization, and particularly helpful for "female" choice among pollen grains that have been deposited on the sigma at the same time.

Dear Andre,

The bees needs the nectar primarily as an energy resource. This is especially true for higher eusocial bees which able to incubate offspring. It is also known that even species having a very limited trophical links (i.e. monolectic or narrow oligolectic bees that collect pollen only from certain plant species) are able to visit a much wider range of plants to collect the nectar. See: Radchenko, Pesenko, 1994. Biology of bees (Hymenoptera, Apoidea).

Best regards,

Vladimir

Dear Klaus and Gad,

With regard to the ratio of the amount allocated and the necessary pollen for pollinators, this issue was considered in detail on the example of two species of cruciferous plant, and bees that pollinate them:

Pesenko Yu.A., Radchenko V.G., Kaygorodova M.S. Ecology of pollination in Strigosella grandiflora and Erysimum badghysi (Brassicaceae) by bees (Hymenoptera, Apoidea) in Badkhyz: estimation of the pressure of competitive relationships. Entomological Review. 01/1980; 59(4): 58–73.:

https://www.researchgate.net/publication/268279604_Ecology_of_pollination_in_Strigosella_grandiflora_and_Erysimum_badghysi_%28Brassicaceae%29_by_bees_%28Hymenoptera_Apoidea%29_in_Badkhyz_estimation_of_the_pressure_of_competitive_relationships

Kind regards,

Vladimir

Article Ecology of pollination in Strigosella grandiflora and Erysim...

There seem to be two parts to your question.  One is whether bees visit two types of flower on a foraging trip and the second on why they have preferences between the flowers. 

A solitary species cannot tank up on honey before leaving the nest so, when they visit flowers that do not offer nectar, they alternate between a species with only pollen and one with nectar (see:- Raw, A.  1974.  Pollen preferences of three Osmia species (Hymenoptera).  Oikos 25: 54-60. 

For the second part, you should check publications on the use of Osmia species to pollinate apple and almond, and why Apis dislikes these flowers. 

Dear all, thank you for the wonderful comments! I really enjoy the high level debate we have here. Just to point out, I´m particularly interested in Eusocial bees (Apis and Stingless bees). I like we have included also solitary bees in our debate but I understand they have different biologies and sometimes the predicted behaviour would be different for each group...

Dear Andre:

Why not follow the literature on the pollination of flowers that never offer nectar to their pollinators and are pollinated almost exclusively by eusocial apids?  This would include the flowers that have porose-poricidal, vibrate anthers like the bumblebee-pollinated Solanum, Senna (Cassia) and Echeandia species.  See which authors collected the bees and examined their corbiculae to count the number of grains that came from co-blooming nectar-secreting species.  How about studies on flowers that offer lots of anthers (polyandrous) but never make nectar (Rosa, Pappaver, Nymphaea).  Then go to the orchids pollinated by female apids that make no reward at all.

Unfortunately, most of my research on  bee-pollination in "nectarless" flowers has been on species that are pollinated primarily by solitary bees (Australian Acacia, Cypripedium montanum, Dianella, Hibbertia, Thelymitra) but I always show how many pollen "species" are carried by visiting bees to the nectarless host species.  See my old paper on bumblebee-pollination of Echeandia macrocarpa in Brittonia. 

Good ideas Peter! Why not also follow the causes for visiting various sources of pollen? Peters answer is somewhat restricted to pollen mixing during a single foaging trip, which might be caused by the rareness of some flowering plant species or signal standarsization (formerly Mullerian mimicry) in these plants. Recently Eckhardt M, Haider M, Dorn S & Müller A published an article termed Pollen mixing in pollen generalist solitary bees: a possible strategy to complement or mitigate unfavourable pollen properties? which arreared in J Anim Ecol. (2014) 83(3):588-97. doi: 10.1111/1365-2656.12168. showing that pollen mixing might be caused by toxic pollen. Other caused might be rareness of amino acids and other compounds in pollen of some plants. In these cases solitary bees are more affected than eusocial bees, the latter of which exhibit pollen mixing at the colony level.