Pollen eaters

Much has been written about the benefits of the intimate relationships between plants and their pollinators.  Plants provide the nectar (sugar) reward for the services of an animal that moves pollen from one flower to another.  Nectar isn’t the only reward, however; animals collect that pollen for their own uses.

You don't really need that much pollen to fertilize the neighboring flowers.

You don’t really need that much pollen to fertilize the neighboring flowers.  This little syrphid fly will ingest the pollen as it cleans itself.

hoverfly eating pollenUnlike the solitary syrphid flies (hoverflies) that feed directly on flower pollen, bees collect pollen to take back to their hive to feed their larvae and for storage for future consumption.

The rear pair of legs have specialized hairs that form a "pollen basket".  Full baskets make the bee look like it is wearing pantaloons.

The rear pair of legs have specialized hairs that form a “pollen basket”.  Full baskets make the bee look like it is wearing pantaloons.  A famous bee researcher (Karl vonFrisch) once measured the time it took to fill a honey bee’s pollen basket — just three minutes in some cases.

A small bee on a big anther must fill its baskets in one stop.

A small bee on a big anther must fill its baskets in just one stop.

So many flowers, so much pollen, it sounds like an easy way to gather food.  But nothing in nature is that easy.

Grains of pollen from many plant species differ in size and structure, but have one thing in common -- they  are too tough to break open.

Grains of pollen from many plant species differ in size and structure, but have one thing in common — they are really tough to break open.  From Wikipedia:  http://en.wikipedia.org/wiki/File:Misc_pollen.jpg

The pollen coat is so tough that it can resist decay for millions of years in bog and sediment deposits.  It cannot be broken down by freezing, grinding, or boiling in acid. So how does a mere animal with a relatively soft, chitinous exoskeleton manage to break into the rich interior of a pollen grain?

By tricking the pollen grain into opening its germination pore, just as it would if it landed on the sticky surface of a flower stigma.

Pollen grains of horse chesnut (buckeye) split open around the germination pore when bathed in a sugar solution.

Pollen grains of horse chesnut (buckeye) split open around the germination pore when bathed in a sugar solution.  

Immersing pollen in a sugar solution in an insect’s gut is all that is needed to initiate the germination process, which normally would result in the growth of a tube from where the pollen grain landed on the flower stigma down into the ovary.  In another chamber of the insect’s (or bat’s or bird’s) digestive tract, then, digestive enzymes can attack the interior of the pollen grain through the germination pore, and release the carbohydrate, lipid, and protein stored there.  It turns out this pseudo-germination trick is used by most pollen eaters.  (Read more about this here.)

But Heliconius butterflies exhibit a different strategy.

Heliconius butterflies sipping nectar, taken at the Insectarium in New Orleans, LA.

Heliconius butterflies sipping nectar, taken at the Insectarium in New Orleans, LA.

Butterfly mouth parts are adapted for sucking nectar not ingesting pollen. To get around this potential disadvantage, Heliconius butterflies collect pollen on the tip of their proboscis, secrete some of the nectar they have sipped onto the dry pollen mass, and then agitate the mass with their proboscis for many minutes, while the germination process begins.  The volume of nectar that is eventually re-ingested then contains amino acids and fatty acids that the butterflies need for egg production. (Read more about this here.)

Perhaps other butterfly species do this as well.  If a curious young grad student hadn’t noticed that Heliconius butterflies sometimes spent 10 minutes at a flower instead of their normal 3 seconds, this amazing behavior might have gone unreported.

7 thoughts on “Pollen eaters

  1. This is very interesting information, with great photos. I never knew about the indestructibility of pollen grains. I guess that I’ve been too busy trying to convince people that it is ragweed and not goldenrod pollen making them sneeze to read about it.

    • Pollen was a pretty amazing invention of vascular plants, although the design seems a bit of overkill. I mean, why would you need pollen to last more than one season? So much effort goes into the construction to avoid its potential destruction.

  2. Wonderful images and truly fascinating information. I have photographed plenty of bees covered in pollen and have noticed the way they collect it, but I have never wondered what comes next. Although it seems like a pretty complicated process, you’ve explained it in such a way that I think I actually understand it. The information on the Heliconius butterflies, though, defies explanation. What would cause the butterfly to agitate a solution for many minutes?

    • How smart are those bees?! To feed their larvae, they must mix some nectar with the pollen from the hive stores, and let it incubate for a while before the larvae can digest it. As for the butterfly, I would guess that a lazy butterfly that “played with its food” managed to produce more and better provisioned eggs that led to healthier and faster growing larvae?? And so the trait was passed on…

  3. Fascinating post, I never realised pollen grains were so well protected. Likewise for the protein extracting butterfly. I’ve saved the paper to read properly after the children and grandchildren go home.

    • I know what you mean– the grandkids put life on a different schedule, don’t they? Happy reading, I hope it’s worth the time you invest.

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