Don't try this at home

In the summer, when it is warm and the flowers are plentiful, the vast majority of bumble bees out and about are female workers.  Instead of reproducing themselves, these workers collect nectar and pollen to feed to their mother (the queen) and their many younger siblings.  However, the workers are not all completely sterile.  After the queen dies towards the end of the season, some of the workers may begin laying eggs of their own.  Even though these eggs are unfertilized (since the workers never mated), they are still viable -- and they will all develop into males.

A male orange-belted bumble bee (Bombus ternarius) visiting a flower.

In bumble bees, as well as other Hymenoptera, the females are diploid (having two sets of chromosomes and developing from fertilized eggs) and the males are haploid (having only one set of chromosomes and developing from unfertilized eggs).  A consequence is that as bumble bee colonies begin to decline in the fall, more and more of the bumble bees visible on flowers will be males.

A male common eastern bumble bee (Bombus impatiens) visiting a cosmos inflorescence.

Male bumble bees differ from females in the number of their antennal and abdominal segments, the structure of their hind legs, and, in some species, the size of their eyes and the length and color of their hairs (especially on the head).  For example, notice the yellow fuzz on the face of the male bee below.  In the females of this species, the hairs on the face are black.

A very fuzzy male Bombus ternarius resting on a rock.

An important, and potentially less subtle, difference between male and female bumble bees is that only the females sting.  Bee stingers are modified ovipositors, which bees' ancestors used to lay eggs.  Since only the females had ovipositors, only female bees now have stingers.  Not only are male bumble bees unable to sting, they are also quite docile; many of the male bumble bees I found would tolerate being petted gently without flying away.

The bee was very fuzzy indeed!

Please note that I do not recommend trying this yourself (unless you have also worked extensively with bumble bees and are confident that (a) you can correctly identify the males and (b) you are not allergic to bumble bee stings).  Although less common in the fall, female bumble bees are still present and they may sting -- repeatedly -- with little provocation.

A male (above) and much larger female (below) Bombus impatiens.

Explore some more: Bumble bee identification and see more pictures of male bumble bees at the Natural Current Events Facebook Page.