Plan “Bee”

Posted on June 26, 2011


I told you the bee puns would bee back.

So, I’m into my first week of my internship working on solitary bees about ten miles north of Truckee.  Jessica and I spent the first couple of days setting up our bee nesting blocks in hopes that the bees would find them irresistible.

Jessica setting up a nesting block.

At each site we have chosen five trees to be fitted with two nest blocks each.  One nest block is placed at 0.5 m from the ground, and the other at 1.5 m.  Some evidence suggests that Osmia may be vertically selective of their nesting sites, and we are hoping to quantify at least some of this selectivity by observing the ratio of active nests in the top block relative to the bottom.  Each block has 14 holes drilled in them of three different radii, fitted with a specialized paper tube that allows for the removal of individual nesting holes.

The nesting season got off to an abysmally slow start.  The bees seemed to be out, yet many of their host plants were either just beginning to bloom, or were not blooming at all.  The phenology was completely off.  We decided that we had to wait at least another week for the plants to catch up to the bees in hope that serious nesting would then be underway.  Sadly, a week later, and for reasons unknown, the bees decided that our nesting blocks did not suit their tastes, despite the abundance of host plants in bloom.  The nesting block project would have to be abandoned.

All was not lost, however, because Jessica had a Plan “bee.”  Instead of estimating population sizes based on nesting block activity, we would now try to obtain similar data by mark-and-recapture.  This method has only been used on bumblebees (Apidae) in the past, but never before on the Megachilids (the family to which Osmia belongs).  Here’s how it works.

Sites are chosen based on their floral abundance, ranging from scant to prolific.  This allows for comparison between the availability of floral resources and the size of the population making use of them.  Each site is patrolled by two people (Jessica and me) for two hours (4 person hours).  Bees are caught on the flower, marked with fast-drying, colored paint (we use regular hobby paint–the kind used to paint model cars), and then released.  The site is revisited several times, and the ratio of new bees to old, previously marked bees caught gives an estimation of the population size.  How on earth do we manage to mark the bees with paint, you might ask?  I give you the “Bee Squeezer.”

Bee Squeezer

This nifty contraption consists of only two parts.  The first is a plastic cylinder with a fine-meshed cheese cloth hot glued to the top of it.  The second part is a cylinder of smaller diameter fashioned with a piece of sponge which slides into the first cylinder, sponge-end first.  Basically, when a bee is caught in an insect net, we maneuver the larger-diametered cylinder with the cheese cloth into the net and over the bee, trapping the bee in the cylinder.  Next, the second, spongy cylinder is quickly placed into the open end of the first cylinder, now confining the bee to the space between the cheese cloth and sponge.  When the bee crawls along the sponge, we then quickly push the sponge against the cheese cloth, effectively trapping the bee in the ideal orientation for marking.  A small dot of paint is placed on the thorax, and the bee is then released to perhaps (and hopefully!) be caught again in the future.

Bee marked with a dot of green paint.

So far the project appears promising; we have recaptured many of our marked bees at some sites.  It appears that these bees do not venture far in their search for pollen, but instead find a suitable area to forage and stick to it.  Of course this is just our initial take on the situation with very limited data thus far.  Other factors resulting in range restriction may be the absence of smaller patches acting as a corridor to other larger patches that would otherwise greatly increase the bee’s range.  In other words, patches of flowers may be like islands, effectively isolating them from other such patches.  There are many such questions that we hope to answer.  Now a word about their host plants.

Golly, these bees are sure picky about practically everything.  They’re picky about the nesting blocks.  Despite the time and effort Jessica put into making them as appealing as possible to these little bastards, they seem to have turned the other mandible at them.  And they are also picky about the type of pollen they gather.  They are not generalists (polylectic) in the least, but are instead specialists (oligolectic), preferring only a few closely related host plants of the Asteraceae–the sunflower family.  The genera they prefer are Wyethia (often referred to as mule’s ears), Balsamorhiza (commonly called balsamroots), and Senecio.  They seem to prefer the first two quite a lot with no obvious preference and, though they also visit Senecio, it appears to be their least favorite.

Left to right: Wyethia, Balsamorhiza, Senecio.

The distribution of these plants is rather unpredictable.  In some sites one of the three plants above clearly dominates the landscape, though sometimes it is a heterogeneous amalgam.  At each site we have set up transects that radiate 100 meters from the center of our site in each of the four cardinal directions.  Along these transects we are attempting to quantify the floral resources at each of our sites.  This quantity consists of the number of individual plants, the number of flowers per plant, and the number of florets per flower.  Of course not every floret of every flower or every plant can be counted, so we will find a solid average per plant.

Setting up transects.

As you can see, we are keeping ourselves quite busy–busy as bees, you might say.  And though the original project having to do with nesting sites appears dead, we haven’t given up all hope.  The possibility of obtaining residents is still a possibility, and Jessica attests that every season seems to start out in despair, only to have things turn around for the better.  Well, I certainly hope that the bees decide to use the nests, but we definitely have our hands full at the moment with our mark-and-recapture project.

Well, that’s it for now.  I will follow this up with a post about the identity of the bees themselves–Osmia californica, and Osmia montana.  There are subtle differences, but once you have trained your eye, the differences are quite profound.

Stay tuned!

Posted in: Science