Saturday, June 4, 2011

Dorms for Aculeate Bachelors

Longhorn Bees, Melissodes paroselae
The males of several solitary wasp and bee species come together at dusk to sleep in dense aggregations of bodies. The phenomenon has been described repeatedly - Eric Eaton had two blog chapters last summer and John Alcock in his book 'In a Desert Garden' also devoted several chapters (and many evening hours) to sleeping bees.
The bees or wasps cluster conspicuously on free standing plant parts, and it is difficult to see the advantage that this behavior entails.  I was tempted to call it 'bachelor parties' in the title, but yesterday at the Santa Cruz River in Marana I watched a group of Sand Wasp males come to rest on a twig of Burro Weed: No partying going on.

Steniolia sp.

I found the sleeping place by following several slow-flying wasps that caught the evening light very nicely. They led me to clump of wasps that were already settled. The newcomers picked a place, shuffled and pushed their neighbors a little, sometimes had to fly up again and find a new spot, but then immediately became very quiet and non-responsive. To collect one, I simply pushed him into a vial. No protest from him or his neighbors. Alcock  had observed the same passivity towards a Bee Assassin Bug who just came and collected his victims while the others slept on.

Bee Assassin, Apiomerus flaviventris 
Alcock also observed the bees' unwavering drive to settle on their chosen branch and nowhere else. They would come back to the same twig for several nights, even after he experimentally moved it to a slightly different location, but they did not accept a similar one that he placed in the original location. Chemical traces of last night's gathering seemed to play a role in this preference. I remember seeing the long horn bees several times in the same spot last year, but since neither they nor the sand wasps are in my own backyard I didn't have a chance to really follow up in detail.

Yesterday I filmed the landing of several new arrivals in the sand wasp group and noticed in the video that at least two stuck out their proboscis repeatedly. They didn't seem to actually lick anything, so what were they doing? I am aware that the chemical sense of insects is not limited to their mouth-parts but is located on the antennae and even the pads of their feet (flies). But why did these arriving males stick out their tongues like tasting the air around them? Check out the video on you tube and judge for yourself.

Tarantula Hawks, Pepsis sp.
On a cool morning in May of 2008 I found more than fifty Tarantula Hawk males resting on a knee-high herbaceous plant. There were no flowers offering pollen or nectar - the wasps had probably spent the night there together and were just getting active. This clustering behavior is in strong contrast to their day-time attitude: Tarantula Hawk males are known to stake out territories centering around hilltops or tall bushes.

Chlorion sp. Cricket Hunter
In June of several years  I watched  large numbers of wasps in the late afternoon in a row of Desert Broom (Baccharis sarothroides) bushes along an irrigated cotton field of the Avra Valley. They were not all of the same species, not even the same family. The majority were in the Sphecid  genus Chlorion. Dozens of those big dark metallic wasps were running around on and then settling  to sleep among the slender Desert Broom Branches.

Cicada Killer, Sphecius convallis
Velvet Ant, Timulla sp
unidentified Eumeninae (Potter and Mason Wasps)

Paper Wasp, Polistes dorsalis
There regularly were also males of two species of Cicada Killers (Western and Pacific), Tarantula Hawks, Velvet Ants, Pottery Wasps and even two species of Paper Wasps (flavus and dorsalis). Although there was a long hedge of similar looking bushes, the wasps seemed to frequent just a few of those. The observations were made in June of three years when the Desert Broom was not yet oozing any juices, and the wasps did not seem to be chewing to release any plant juices as they do later in the season. Taking into account  Alcock's branch-displacement experiments, one could speculate that there was a chemical signal emitted by the wasps that drew more wasps, in this case even across species limits.

Three is a crowd
So the  tendency among male wasps to seek each others company for the night seems to be widespread, but depending on the species this can range from loose aggregations (which can be multi-species) in the same bush to dense clusters on the same branch. Densely packed groups seem to always consist of just one species.

The evolutionary advantage of this behavior is not clear. To me it seems that the loose aggregations maybe the more primitive version which excludes group-thermoregulation as a primary incentive. All these males are without stingers, so they can't deter predators in a group-response. Actually, they are so passive, they don't even seem to profit from the increased vigilance of a multitude of  eyes. Alcock concludes that the aggregation may result in a dilution of risk for the individual, but one could argue that such an accumulation of rather sluggish prey could also draw predators? If I can learn how to find the wasps reliably and repeatedly, shouldn't generations of predators have figured it out as well?


  1. These are beautiful pictures. Love this

  2. They are swarming like... bugs. Gorgeous pictures.

  3. What you wrote is very interesting!

  4. That proboscis action could be just grooming, some maintenance after a day of usage.

  5. Yes, that could be. Looking at the video again, that's probably it. Still, Alcocks assumptions about chemical (pheromones?)residues on the twigs seem rather conclusive.
    Thanks, Beatrice!

  6. Bernhard Jacobi on flickr said:
    extending (streching) mouthparts is a common element of preening behaviours of roosting long-tongued hymenoptera (especially in bees), the basitarsus of the left front leg is shown brushing down the tongue in your photo, supporting the idea further.
    In Steniolia (as in other Bembecines) the tongue is rigid and is simply folded back under the thorax when not in use. In this way it is easily smothered in the daily doings of males. Just as we brush our teeth, they need a good tongue swipe at bedtime! :)

  7. Fantastic!! I was thinking of the group as 'appearing' larger than life as a confusing 'signal' like 'tiny fish schools?? But yet it seems like bee hunting insects aren't fooled. Really fascinating post and fantastic photos!! I so enjoy your blog!

  8. Maryl, the group seeming larger than an individual would be a visual deception and maybe good to confuse birds, but it's mostly dark while these guys are huddled together. No idea whether it would confuse bat sonar, though. (by the way, the fish school seems to confuse the predator more than scare him away)

  9. I often see someone different hobbies, including small animals in loving, very interesting... Due care needs to some kind of expertise at a time, not only has the soul of art, patience and experience of many things...
    I always salute about this!

  10. I just came across your site a couple days ago and am fascinated - thanks for all the fantastic info! I've lived in Northern AZ a couple times and am moving back within the year. It's great to have such a great resource for the creatures I see when I'm out and about.
    - Debbie

  11. Are nighthawks the predators of these creatures? Would they be confused by a large mass as Maryl suggests?
    Guessing can be a lot of fun.

  12. As I was standing at the wasp cluster filming Night Hawks flew right into my face when I looked up. They have huge mouths and I thought they might devour my head. Seriously, though: I think they catch more airborne prey and pretty much ignored the wasps.
    Beatrice, I sent you a fb message - more guessing...

  13. Such wonderful Captures.
    Loved them all.

  14. Iam no scientist...but could the behaviour not be a way of 'tasting' the group ...reinforcing identification of others ?...or sensing changes within the group ?...knowing your competition?

  15. This is very interesting. I love reading about new discoveries and ideas.

  16. Awesome photos... That bee assassin bug is really neat, I've never seen one of those here.

  17. Wow!so stunning,i like it.

    Thanks for sharing.

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  19. Wow super super interesting!

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