Dear Klaus,

as shown in this video we're finding quite a few cases of pollination with "scopa" in several nectar robbers bumblebees.

In many plants with aggregate flowers like occurs in Anthyllis vulneraria (Fabaceae), when bumblebees "walk" clumsily over the inflorescence, they pollinate effectively with legs and ventral abdomen..

Something similar occurs in several other systems of short tubular robbed flowers visited by bumblebees (as we represent in the video, which is taken from our work in Petrocoptis grandiflora -Caryophyllaceae-).

Do you mean to these kind of cases?

Article Pollination Ecology of Anthyllis vulneraria Subsp. vulgaris ...

Dear Klaus:

To answer this question we must answer another question.  Do ALL bees (regardless of species or family) mix pollen with nectar or water BEFORE it goes into the scopa?  Eusocial apids (Apis, Bombus. Trigona, Melipona?) certainly do before adding grains to their corbicula.  The grains are packed into a pellet and have hydrated.  Hydration starts the germination process so the sperm could die before they ever reach a stigma. It's most unlikely that corbicula deposited pollen will transfer to a receptive stigma anyway due to their pre-hydrated state and the fact that corbicula pellets are now arranged at such an angle that they won't contact the angle of the stigma.

I've examined the scope of bees in the Halictidae and Colletidae over the past 35 years.  In contrast, the grains on their scopa appear loose and dry.  The same should be said for some megachilids in which there is a triangle of hairs at the bases of their abdomens that is also used to store pollen.  It is my impression that some flowers have styles that are so angled that the receptive stigma is most likely to contact one, or both scopa and/or pollen collecting hairs deposited ventrally towards the bases of the abdomen.  This seems most common in some Australian plants especially (not not exclusively) those species offering lots of pollen to bees but they don't secrete nectar.  Persoonia is a most important exception to that rule.  Here are some references.  

Bernhardt, P. 1989. The floral ecology of Australian Acacia. pp. 127-155. In, Advances In Legume Biology, Stirton, C.H. & Zarucchi, J.L. Monographs in Systematic Botany from the Missouri Botanical Garden, St. Louis Mo.

-----. & Walker, K. 1984. Bee foraging on three sympatric species of Australian Acacia. International Journal of Entomology 26: 322-330.

Bernhardt, P. 1984. The pollination biology of Hibbertia stricta (Dilleniaceae). Plant Systematics and Evolution 147: 266-277.

Bernhardt, P. 1986. Bee-pollination of Hibbertia fasciculata (Dilleniaceae). Plant Systematics & Evolution. 152: 231-241.

-----. 1995. The floral ecology of Dianella caerulea var. assera (Phormiaceae). Cunninghamia 4: 1-17.

Bernhardt, P. & Weston, P.H. 1996. The pollination ecology of Persoonia (Proteaceae) in Eastern Australia. Telopea 6: 775-803.   

Peter and Klaus- first, as to Peter's question, carrying dry vs wet pollen varies among the bee taxa...many of the small panurgines (e.g. Perdita) do, but equally small sweat bees (e.g. Dialictus) don't.  For certain, megachilds must transfer scopal pollen to stigmas on those plants whose anthers and stigmas brush against the metasomal (abdominal) venter, such as many Asteraceae.  It is unavoidable, although some Megachile will hold their abdomen cocked up like a banner to try to limit such losses as the pollen load grows large.  Other plants I've worked with (legumes such as Hedysarum) jam their stigma into the  scopal venter even as the bees are removing the pollen from the adjacent anthers.  I can send pictures and even some video if you like, or wait to see it i a few months!  Maybe all of this was obvious to you already, though.  Apologies if that is true.

jim

Dear Klaus,

there is a nice piece of evidence dealing with your question that has just been published by Parker et al. in Arthropod-Plant Interactions (DOI 10.1007/s11829-015-9358-z). You might want to have a look at that.

Take care, Marcin

Dear Luis, Peter, Jim & Macin, thank you for these helpful suggestion. Although I have many personal observations of bees visiting flower, collecting pollen and storing wet or dry pollen in the scopa, your comments were very helpful to think about selective pressures for flowering plants to exclude the visitation of some bees and encouraging the visitation of other bees. Keel flowers like Coluthea for example hide the pollen in a stiff keel that bumblebees and honeybees do not have access to the pollen, wheras Megachile bees anchor themselves with their mandibles at protuberances of the flag and then open the keel to directly touch the stamens and stigma with their ventral scopa. Anthidium, another megachilid bee, collect pollen with the head mainly from Lamiaceae and then store it in the scopa. I want to understand the selective pressures that caused these strange coevolutionary adaptations.

Thanks to all, Klaus

Great question Klaus.  Clearly, as others have noted above, pollen can be transferred from scopae to stigma.  But as to whether the pollen is viable or not, that's a different (and I think unanswered) issue.  Peter mentions hydration starting the germination process; the other factor is how long even dry pollen remains on the bee.  I know that Amots Dafni and others have documented very short pollen viability times for some taxa, so pollen of such species that is packed away on a bee and infrequently has an opportunity to land on a stigma may be inviable even if it's dry.  

As with so much in pollination biology, a lot will depend on the identities of the players involved!

Some years ago there was a paper published in Japan on the re-viable-ization of pollen taken from the corbiculae of honey bees.  If memory serves, this had to do with obtaining pollen for fruit tree pollination.  Perhaps one of the scientists at Tamagawa University may be able to help.  Peter K

Dear Klaus,

According to my unpublished data, the ability of pollen germination decreases sharply after its placement in skopa. After that pollen is much stronger sterilized by bee female under the formation a pollen ball in the cell (probably by adding secretory substances). This prevents further germination of pollen in a cell, for not spoiling the food for larvae.

Best regards,

Vladimir.

Once pollen grains leave the microhabitat of the anther their viability must decline for various reasons as they encounter bee-body antibiotics, UV light, increasing temperatures and other factors.  In the early 1980's Dr R Knox and J. Kenrick did some work hand-pollinating Acacia retinodes in two isolated populations to determine the number of self-incompatibility alleles.  They found that, within two hours after removing grains from the flowers the pollen grain was dead and did not respond to deposition on a wet, receptive stigma.  To increase the longevity of pollen between sites Acacia polyads were placed in capped vials but the vials were stored on dry ice in a styrofoam cooler or "esky" (to use the Australian word).  The type used to keep beer cold on picnics worked fine.  Acacias pollinated from pollen produced over two kilometers away set more seed when they came from vials in the chilled cooler.  

Lumping all bees together may not be justified. In the ventral scopa of some megachilid bees, the pollen is stored dry and not influenced by bee secretions, the pollen load are immense and only the undermost pollen is in contact with the antibiotic substances on the bees surface, due to the orientation of the scopa the pollen is not exposed to direct UV-light, the pollen is not in all cases manipulated by the bee´s legs, but taken direktly by contact of the scopa with the exposed pollen. My guess is: Megachilid bee may be an exception regarding fertility of scopal pollen.

Klaus

What does it  means SCOPA ?

Dear Chantal,

bees have evolved two different pollen transport organs: Corbiculate bees such as honeybees, bumblebees and stingless bees use a so called pollen basket on the hindlegs, whereas most other bees transport pollen in the scopa which is an area of dense and elongated, often brached bristles.

Dear colleague

THANK YOU, it is not my speciality, excuse me

I do not know why i have received this question

Dear collegue,

please check this paper: Pinzauti M., Rondinini T., Niccolini L., Felicioli A. 2002. Investigation of the germination potential of pollen transported by some bees. Ins. Soc. Life 4:107-114.

The authors showed that the germination of the pollen transported in the scopa is 90% and reach 0% only when the bees deposit and compact the pollen inside the nests. From this data it seems that the pollen in the scopa is available for pollination.

Very interesting discussion about bee-floral interactions. These days Im collecting some bees from two plants "Heliotropium bacciferum" and "Trichodesma africanus" of Fam: Boraginaceae. Both species visited by genus Hoplitis with unique bristles on the mouth parts. Andreas Müller mentioned good information about this and mostly the pollen on the scopa is very dry but for the pollen germination I have no idea. The species I collected never visit any other floral resources.  

Dear Mohamed, the Hoplitis females use the bristles on the mouthparts to extract the pollen grains from the flowers, but then reload the pollen into the abdominal scopa for transport into the nest. There are thus two structures which carry masses of pollen grains that may come into contact with the stigma.

Klaus

Dear Prof Klaus, 

Do you think by that behavior the plant pollination is assured by such bee species. you mentioned that the plant could exclude some bees and attract others based on that Hoplitis is so welcomed by such plants more than other bees. By the way if the Hoplitis scopa full with pollen loads that's mean it takes more longer time than other Megachilids takes with other plants  alfalfa, asteraceae or other floral resources. 

Dear Mohamed,

you have the perfect chance to check this.

Klaus

Some of the Boraginaceae have the smallest grains of pollen in the  angiosperms.  Some grains are not much larger than a fat bacterium.  Dr. Shebl should consider measuring the grains of the plants he is studying against the grains of other species in other families carried by his Hoplitis species.  My old professor explained that the size of pollen grains in the Boraginaceae was based on the unique shape of the stigma cells and grains had to be deposited in a very special way for pollen tubes to reach the receptive sites, germinate tubes and penetrate tissue.  It is very possible that grains of these borages have a better chance of deposition on stigmatic surfaces if they are in bristles in the mouth parts vs. the scopa. Consider this.  The bee pushes her head and mouth parts into the narrow floral tube.  The head reaches the stigma BEFORE the scopa.  Does the bee's scopa "fit" into that little floral tube at all?  How high is the stigma in relation to the tube?  Leioproctus filamentosa (Colletidae) can carry pollen grains on its strange, filiform mouth parts when it forages on Persoonia (Proteaceae) based on some of our old photographs (not included in the following paper).

Bernhardt, P. & Walker, K. 1996. Observations on the foraging preferences of Leioproctus (Filiglossa) Rayment (Hymenoptera: Colletidae) in eastern Australia. Pan-Pacific Entomologist 72: 130-137.