The flowers of the bluebonnet are visited by several different native bees. I went through The Hymenoptera in America North of Mexico and searched for Texas bees that visited Lupinus and compiled a pretty long list. As the bluebonnets begin their annual bloom, I’ll be on the lookout for these genera of bees (and others, of course).
Hylaeus, Andrena, Anthidium, Hoplitis, Osmia, Megachile, Chalicodoma, Melissoides, Anthophora, Centris, Ceratina, Xylocopa, Apis, and Bombus
I find it helpful to look at photos of these genera before I go bee watching on bluebonnets. Of course, there’s a lot of variation, but it does help me recognize common characteristics.
The Bluebonnet is an interesting plant. If you look at the photo above, you will see a white or pale yellow spot on some flowers and a purple spot on other flowers. Ever wondered why there were different spots on one plant? Me, too.
Apparently, all the flowers start out with a white or pale yellow spot. By the fifth day after the flower opens, the spot begins to turn pinkish and by the sixth day, the spot is completely purple.
Bees will choose flowers based on the color of the spot. In one study, 96% of bee visits were to white-spotted flowers. It appears that bees will limit their visits almost exclusively to white-spotted flowers. Wonder why?
Pollen from new white-spotted flowers is sticky and makes a nice ball. As the pollen ages, it is less sticky and gets powdery–and the flower spot begins to darken. It turns out that the color of the spot is a pretty good indicator for the bees. Scientists (Schaal and Leverich 1980) determined that a bee can gather much more pollen from white-spotted flowers–as much as 150 times more!–than from purple-spotted flowers. So, if a bee is looking to get the most pollen for its effort, the bee is going to pretty much ignore the purple-spotted–older–flowers and just forage on the white-spotted fresh flowers.
This is good for the bee because it can be a more efficient pollen-gatherer–and it is good for the bluebonnet because the white spot directs bees to flowers with good, fertile pollen and the transfer of this pollen is likely to result in successful cross-pollination for the bluebonnet.
I have often heard that the purple spot is the result of pollination. I will hear that flowers with white spots are not yet pollinated while those with purple spots are already pollinated. I’m still wondering a bit about this. In the Schall and Leverich paper, they said that the spot color change was not related to pollination or fertilization.
But . . . several papers from UC–Davis (the hotspot for studies on flower preservation) indicate that an increase in ethylene production can be triggered by pollination.
So I think this means that the color change would occur anyway, regardless of whether or not pollination occurred. But if pollination does occur, it increases the amount of ethylene and the color change would happen faster.
Ethylene is a chemical compound produced by plants. It is responsible for many plant processes, but you are probably most familiar with its role in fruit ripening. Have you noticed that when a bunch of bananas ripen, it happens to the whole bunch at the same time? That’s because of the release of ethylene.
Heard the old saying about “one bad apple can spoil the whole barrel?” Again, that’s because of ethylene production. Once one apple starts the ripening process, it releases ethylene gas and ends up starting the ripening process in the whole barrel–or plastic bag. Growers will ship green fruits to a distribution center where they will be gassed with ethylene to start the ripening process before being sent to your neighborhood store.
That is probably more than you wanted to learn about a bee’s visit to a bluebonnet, but I’m going to keep an eye out this spring and watch which flowers the bees visit. And I’m going to pick out the bad apples quickly–and maybe I’ll separate my bananas too.
Pollination and Banner Markings in Lupinus texensis (Leguminosae), Barbara A. Schaal and Wesley J. Leverich, The Southwestern Naturalist, Vol. 25, No. 2 (May 15, 1980), pp. 280-282
The Effect of Pollination and Ethylene on the Colour Change of the Banner Spot ofLupinus albifrons (Bentham) Flowers. A. D. Stead and M.S. Reid. Ann Bot (1990) 66(6): 655-663
Pollination ecology and the significance of floral color change in Lupinus pilosus L. (Fabaceae)
Ne'Eman, G | Nesher, R; Israel Journal of Plant Sciences [ISR. J. PLANT SCI.]. Vol. 43, no. 2, pp. 135-145. 1995.