I (DJ) have many fond memories associated with this species. First and foremost, I got the photo (below) and learned this species on a multi-day trip into Stehekin, NCNP to do a NPS sponsored bio-blitz with some of the rockstars of the native bee world: Jamie Strange, H. Ikerd, Terry Griswold and Chris Looney, to name drop a few. What a nice introduction to some amazing people and fun evenings around the campfire getting to know each other.
Second, the encounter documented above also shows an important reason why native bees in general, and bumble bees in particular, are the best pollinators for the money–from the perspective of the plant. It demonstrates the foraging behavior of constancy.
The general way that constancy works is that a bumble bee leaves her nest colony for the first time as a worker and stumbles across a flower that offers the needed resources: pollen and nectar.
Not really knowing what she is doing, she figures out how to get her reward. Once she figures it out she looks for a similar flower (same plant species) and is able to figure out even faster how to extract the goods and move on. Now she is efficient with this flower type.
At that point she hones in on every flower of that plant species that still has resources to offer (if the flower has just been visited, the nectar is probably gone and will need to refill over some period of time) until she is fully loaded and goes back to the colony to unload, or runs out of untapped flowers of that species. At that point, if her load is not full, she will look for another flower type and repeat the process.
This behavior, going to the same flower type time again and again is called constancy. The reason why it is so important to the plant species involved is that this greatly increases the probability that the pollen will be spread to many other plants of the same species, resulting in successful pollination and increasing the “percent seed set” for the fertilized flower. Win-win for both plant and bee.
Field guides on bumble bees do a good job show the possible range of color combinations a given species may exhibit across its’ entire range. Trying to condense that into a visual that shows the full spectrum is challenging.
I say that because when I compare the individuals in these photos with the color schema’s shown in Bumble Bees of the Western United States, there are some inconsistencies.
These photos show that T1 sometimes shows yellow, not always black. The field guide shows the rear of the thorax (scutellum) to be black where these photos show some yellow present.
This is a good example of two different lessons. First, bee coloration is variable and may not align exactly with the examples in the book. Different than with most birds, for example.
Second, because color is variable, depending on the species, absolute identifications from photos may not be possible. Photographs rarely capture taxonomic level details like cheek shape. A collected specimen can be keyed out later as a voucher specimen of what was photographed. This should rarely be needed as a good working knowledge of our common species is relatively easy to acquire.
Field ID Tips
A primary field mark to distinguish the van Dyke bumble from others is that the van Dyke shows T3 with yellow.
van Dyke bumble bee can be confused with Two form bumble bee mainly because they are fairly similar is size and both striped black and white/yellow. At least, I have been confused and assumed B. bifarius because bifarius is much more common. Part of the learning curve.
The van Dyke bumble has yellow on T3. T3 is black on the Two form bumble bee. The van Dyke bumble bee also lacks a black inverted triangle on the rear of its thorax.
Based on color patterns, van Dyke bumble bee and yellow forms of California bumble bee are similar. From my experience the van Dyke bumble bee is smaller bodied.
When you look at the pollen gathered on her corbiculum (photo above), you can see it is first one color (orange) and then it shifts to a 2nd color (washed out brown). The bumble started with collecting pollen (and nectar) from first one species, flower after flower (orange pollen). Finally, when it became too difficult to find flowers that had not already been visited, the individual bee switched to a 2nd species of flower and is now demonstrating constancy between flower types–as shown by the change in color of the pollen collected.
One might see the photo above and think Yellow head bumble bee, but this individual shows T2 as black.