Bumblebee Dynamics

by Carl Strang

In my last update on bumblebees  at Mayslake Forest Preserve I described the 5 species I had observed to that point. In the subsequent few weeks there have been a couple of changes to report. First is the apparent addition of a 6th species.

Bombus pennsylvanicus 1b

I believe this to be Bombus pennsylvanicus. At first glance it is very close to Bombus auricomus, which I described earlier. Both are relatively large bumblebees, both have lots of black on the thorax (especially on the sides), and both have segments 2 and 3 of the abdomen all yellow. However, auricomus has a cleaner, sharper look. All the yellow areas are bright, pure yellow. The black areas are unblemished by hairs of other colors. Both black and yellow areas are sharply defined, and correspond to the edges of body segments. If I am correct in my identification, pennsylvanicus has a muddier look. The yellow of the dorsal abdomen spills a little onto the first segment as you can just see in the above photo. Also, the yellow area of the posterior dorsal thorax has some black hairs mixed in, and the black on the sides of the thorax has some yellow hairs mixed in.

The more astonishing change is the nearly total disappearance of Bombus bimaculatus over the past three weeks. Through July, this was the most common bumblebee at Mayslake, with numbers exceeding those of all other species combined. I was away a week for Roger Raccoon Club , and returned to find the bimaculatus nearly gone. Now I see at most one or two a day. The other common small species, Bombus impatiens, continues essentially unchanged.

Bombus impatiens worker b

This is my first year of following the bumblebees at Mayslake. I don’t know if this disappearance is usual, and I don’t know what it means. I doubt that the species has a limited season, as bumblebees generally continue through the summer. Some of the native, solitary bees are specific to a single flower species and thus have defined seasons, but bumblebees are generalists. They are known to monitor changes in flower abundance and to travel miles, and so may have abandoned Mayslake for an abundant appearance of other flowers elsewhere. Disease might have impacted one colony, but for all the local colonies to be affected with no obvious change in other species seems unlikely (though the uncommon Bombus griseocollis also has been absent in August).

One day earlier this week I saw a couple odd looking bumblebees that seemed clumsy on the flowers.

Bombus bimaculatus or griseocollis male b

Their markings were like those of griseocollis or bimaculatus, not precisely fitting either. They had enormous eyes, and were larger than most workers of either species. Photos showed them also to have unusually long antennae. These are males. What, if anything, their sudden appearance has to do with the absence of their species’ workers is another piece of the puzzle that needs fitting.

3 Comments

  1. September 25, 2009 at 6:29 am

    […] a summer in which I made good progress in my knowledge of Illinois bumblebees, I was interested to find that in that part of the U.P., as back home, only one common species of […]

  2. Mark said,

    August 7, 2010 at 2:06 am

    Bombus bimaculatus, vagans, perplexus, and griseocollis all end their colony life cycles in July. The colonies of those species aren’t to large. But bombus impatiens lives all through the year till fall and their colonies can get to be hundreds of workers. So all the other species have sent their queens out and have found hiding places.

    • natureinquiries said,

      August 9, 2010 at 5:50 am

      Thanks, Mark,
      Certainly my observations in 2010 bear this out.
      Regards,
      Carl


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