Pollinators and Robbers (and Squirrels)

The last tulips in my garden this morning…

It is finally flower season! How I love it! However the happiness of the season is constrained somewhat by the gray squirrels, which have systematically beheaded my tulips (despite the fact that I have been simultaneously trying to ward the pests away with foul chemical sprays and appease them with nuts).  Alas, most of my tulips now lie sad and beheaded beneath the cherry blossoms.

My (ineffective) struggles to protect my beloved tulips remind me of the struggles of wild flowers which face a similar arms race.  The tulips I plant are propagated by big nurseries, and the squirrels don’t really want to eat the blossoms: they merely tear them apart to see if there is any food inside (and (probably) because the miserable rodents enjoy my suffering).   Flowers are plant reproductive organs which exist to repopulate the species.  In the case of garden tulips this involves a complicated relationship between myself, Lowes, tulip farms, nurserymen, and squirrels.  In the world of wildflowers, the players are fewer and the stakes are much higher.

Buff-tailed Sicklebill (Eutoxeres condamini) by Ernst Haeckel

Flowers and their pollinators have a mutualistic relationship:  the hummingbird –or bee, or moth, or bat, or whatever–gets a meal while the flower directly shares its gametes (in the form of pollen stuck to the beak or fur) with distant members of the same plant species.  Some blossoms coevolve to provide nectar to specialized pollinators as with the famous sicklebill hummingbird (which feeds on the nectar of specialized Centropogon and Heliconia flowers which fit the bird’s beak and produce colors appealing to the hummingbirds).

This whole relationship falls apart sometimes though, thanks to a behavior first reported by Charles Darwin. Some animals are nectar robbers.  Lacking the long proboscis or curved beak or special senses necessary to obtain the sweet nectar which the plant offers as a reward for its reproductive interlocutors, some animals simply cut through the blossoms or rip them apart to take the pollen.  Although this can be beneficial (if a robber ends up pollinating a flower anyway, or forces a legitimate pollinating species to travel over a larger area—and thus provide greater genetic diversity), more often it is destructive.

Um, sure I guess…thanks, art department.

Interestingly, a recent study determined that bumble bees learn how to cut holes in flowers and steal the nectar directly from other bumble bees (you can read about the particulars of the study here).  Bumble bees are not the only pollen robbers–various lepidopterans, bats, and birds are guilty in various ways–but the bumble bee example is the first case to prove Darwin’s thesis that such robbing behavior was learned by insects.

It all begins to make more sense now…

Flowers, though passive, are not helpless.  Over generations, they coevolve with both the robbers and the pollinators—which is how they obtain so many convoluted and fanciful forms (and why there are so many toxicologically and pharmacologically active compounds therein).  It is worth thinking about when you encounter a spring landscape of beautiful flowers—beneath the surface lies a world of sex, appetite, and larceny.

The horror!

Advertisement

Orchid Bees

An Orchid Bee in Nicaragua (from whatsthatbug.com)

Here in New York the weather outside is February gray.  The buildings are gray.  The sky is gray.  The trees are gray.  The people are dressed in gray and black.  Fortunately we can beguile away this monochromatic tedium by contemplating the Euglossini, also known as the orchid bees!

Despite their Latin name, the Euglossini are not uniformly eusocial.  This means that most species of orchid bees live solitary lives (in marked contrast to honeybees–which live in vast hives more ordered than the strictest totalitarian state).  The orchid bees live in Central and South America, apart from one species which ranges into North America.  They are notable for their brilliant iridescent blue and green coloring.  The females build nests out of mud and resin.

Museum specimens of orchid bees

The most remarkable aspect of Euglossini behavior is the male bee’s obsession which the aromatic compounds produced by various tropical orchids.  Male orchid bees have a rarified ability to sense these fragrances even in small quantities (like many heady floral/fruit scents the chemicals produced by the orchids are usually complex esters).  The bees harvest the molecules with front legs specially modified to resemble little brushes (and in doing so they generally pollinate the orchids, which are wholly dependent on the bees).  Astonishingly, the male bees store the chemicals in a cavity on their back leg which is sealed off and protected by waxy hairs.

English: An orchid bee, Euglossa viridissima sleeping on a leaf. Miramar Florida (by Efram Goldberg)

The male bees appear to use these compounds when trying to attract a mate but no female attraction to the odors has been proved.  On the other hand, many Stanhopeinae and Catasetinae orchids are absolutely dependent on the male bees to reproduce.  Different species of these orchids rely on specific species of orchid bees to successfully pollinate far-away partners in the rainforest.  Charles Darwin wrote about this pollination system after observing it in the wild and later referred to the highly specialized orchids as proof of the ways in which species adapt to their environments.

Euglossa mixta (from The University of Arizona’s Center for Insect Science Website)