Busy little bees

Posted on June 16, 2011

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This will certainly be an interesting summer; I have landed a summer internship with a postdoctoral researcher who specializes in the behavior and ecology of solitary bees.  The bee we will be studying is the genus Osmia, and the population of interest to us is located near Lake Tahoe at the Sagehen Creek Field Station.  I will be spending approximately 10 weeks of the summer studying these little fellas, and this will hopefully go a long way toward strengthening my résumé.  Alright, and now about the bees!

Here she is:

As you’ll notice, these bees are darker than the typical bee you’re probably used to seeing.  They’re a lovely metallic bluish-green, and a little smaller than the common honeybee.  What makes these bees so unique is that they are completely solitary, they do not nest in hives.  This may seem strange, because we are so used to the association of bees with hives and honey, but there are actually many more species of solitary bee than there are hive-nesting bees.  The evolution of eusociality (social, hive-nesting bees) is actually a derived character, with the solitary lifestyle being basal–the ancestral condition of bees.  In other words, solitary bees don’t represent hive-nesting bees that have gone rogue, but rather hive-nesting bees represent a coming together of solitary bees.  There are many ideas as to how this has come about, one being that intense parasitism drove the evolution of eusociality as a way to minimize the time nests are unattended, and hence vulnerable to parasitic wasps or other bees.

Osmia, the particular genus we are studying, builds nests in a very interesting and effective way.  For one, it doesn’t build its nest the way some bees do, but instead looks for preexisting cavities in which to nest.  Common nest sites include bored out, straw-sized holes in felled trees or tree stumps carved out by wood-dwelling beetles.  Once Osmia has found a suitable hole, she begins renovation.  First, she collects leaf matter with which she will line the back of the hole.  Once this leaf matter is laid down, she then begins the ponderous task of collecting pollen which she accumulates into a large ball in the back of her nest.  The pollen ball, having reached a suitable size by her standards, is now ready to acquire an egg.  The egg is laid on top of, underneath, or inside of the pollen ball depending on the species of Osmia.  This pollen + egg stash is then sealed off by a partition of leaf matter–the same kind used to line the back of the nest.  The cell is now complete and, depending on the depth of the hole, she will add on more cells to the first in linear fashion.  She lays eggs that will develop into females in the first cells, toward the back, and males in the terminal cells.  Amazingly, the males emerge first despite being laid last.  These males then await the emergence of the females, who then emerge in the reverse order in which they were laid.  Truly astonishing!

Above you can see the individual cells separated by the green leafy partitions, each with a stash of pollen and live pupa.

As mentioned above, these bees nest in preexisting holes in tree stumps and logs–they do not make their own nests.  This implies that the availability of nesting sites is a limiting resource, since appropriate holes for nesting are finite.  No matter how many flowers may be in bloom, the population at any locale is constrained by the number of potential nesting holes.  No nesting holes = no bees.  This may seem like a no-brainer, but this phenomenon has yet to be experimentally demonstrated.  Our research, then, is designed to focus on the precise nature of the relationship between nest-hole abundance to population size.  Our experimental procedure is pretty straightforward–add nesting sites and measure the response in population size to the manipulation, relative to control sites where no nests are added.  Here is an example of our nests.

I have yet to take an actual picture of our nests, but the above image is nearly identical to the ones we are using.  The holes act as nesting cavities for Osmia.  By adding these to an area, we are increasing the number of available nesting sites–a limiting resource–and in doing so should observe population numbers increase as a response.  You’ll also notice that each hole is lines with a paper tube.  This tube is very important because it allows us to remove each tube and observe the progress of the nest, or whether or not a nest has become parasitized.

In order to monitor population sizes, we have to undergo the tedious task of a method known as mark-and-recapture.  Basically, we catch a bee, place a dot of special paint on its abdomen, release it, and keep track of whether or not we recapture the same bee.  The rate of recapture is inversely proportional to the population size.  If the population is large, we would expect to catch very few previously captured bees.  Smaller populations equate to a larger frequency of recapture.  This is difficult, especially with small, fast-flying bees, and as a result is prone to error.  As long as we don’t get better (or worse!) at capturing bees, the data obtained should at least be somewhat conclusive.  If this method fails to yield results, there are numerous other “Plan B” (Plan Bee?) projects we have lined up, such as the influence of cleptoparasites (parasites that lay their eggs on the bee’s pollen, which hatch, kill the bee larva, and live off the pollen stash) on nest sites.  There are also several aspects of bee behavior (beehavior?) we can investigate.

It’s an interesting project, and I am happy to be a part of it.  Stay tuned for more updates, and actual in-field photos of our nest boxes and experiments.  Also stay tuned for more horrendous “bee” puns.

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Posted in: Science