Brittle Bush Bee Heaven

 Last year a late, very strong freeze kept many Brittle Bushes in the Tucson Mountains from blooming. In March it felt as if all kinds of interesting bees had found their way into our protected backyard habitat. Of course, the distances that these small endemic bees travel are really quite small. But for whatever reason, my BugGuide entries for last March show dozens more species than I'm finding this year.

So far I saw no Colletes (left), few Anthophora (middle) even though the Penstemons are already past their prime, and the long horned Eucera (right) are still missing. While a Leaf-cutter bee in Saguaro National Park volunteered to star in a video, I hardly see any in our backyard. The cactus bee Diadasia will certainly show up once the cacti are in flower.

Lasioglossum sisymbrii (left) is here, and several Agapostemon species (middle and right), some of them very pretty and shiny green. Some Agapostemons had been invading our living room windows throughout the winter to the delight of a resident Kukulcania Spider.

In the early morning hours, Honey Bees seem to be taking advantage of their greater body mass and thermoregulated hives: they visit the Brittle Bushes when it is still rather cool. (this observation is contrary to the info in BugGuide - Maybe the remarks there refer to climate conditions different from Arizona's)

Around noon,  by far the dominant genus in our backyard is Adrena, recognizable by their furry faces (facial fovea). I'm seeing at least 2 morpho-species, a brown one and one with a partly rust-colored abdomen. The females collect pollen into tibial brushes similar to those of honey bees. They will take the pollen to their solitary ground nests. Each egg will be laid on its own ball of pollen and sealed in its own chamber. Our sandy, dry soil is just perfect for these Mining Bees.

Adrena males are smaller and more narrowly built, with the light face that is characteristic for many male bees. They spend their time hovering and zigzagging over the flowers. They rarely land to refuel with nectar. All bees that are quietly collecting attract  their attention. Although  pheromones prevent greater confusion, these eager males zoom in on anything from Agapostemon to Honey Bee. It's quite annoying if you are trying to photograph.

If they find a conspecific female they grab it, and in most cases a struggle ensues which usually makes both bees fall off the flower and out of focus. To get photos I finally just focused on an attractive female and waited for the males to show and that worked like a charm.

The abundance of Adrena females also drew another visitor with a quite different agenda. Today I found my first Nomada Bee and promptly misidentified it as a wasp. Nomadas got their name from the fact that they have no nest of their own. Instead, they are clepto-parasitic cuckoo bees, depositing their eggs into the nests of the other bees, mostly in the genus Adrena.

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?

A Swarm of Solitary Bees?

Yesterday at the Javelina exhibit of the Arizona Desert Museum we watched an amazing spectacle: There was a wooden fence post with old wood-boring beetle holes. A swarm of hundreds of bees was hovering, and individuals seemed to be entering one of the holes. You really have to watch the video to get an impression of how many bees were buzzing around that hole in their frenzied attempts to enter.
Human visitors trying to get a better view of a group of javelinas resting in the shade of a bridge were leaning close to the post, oblivious of the bees. The bees, in turn, ignored the people completely. Weird!

 One clue to the solution was the size of the bees - about 1/3 smaller than Honey Bees. They were also quite gray compared to the more rusty, golden Apis mellifica.
They looked very much like Leafcutter Bees, Megachilinae, to me. Except,  those are solitary and shouldn't appear in swarms? ( Most of the over 20 000 globally-known bee species are solitary. A single mated female builds and provisions a nest, lays one egg per brood cell. Usually there are several brood cells to a nest, and sometimes there are guests, but that is another story). Megachilinae females do sometimes share the entrance to a nest, but inside they take care only of their own.

Back to my 'bee swarm'. My first pictures, rather than direct observation, showed that all the entrance-seeking bees had long antennae which indicated males. Close observation also revealed that only very few were actually getting into the entrance, but there seemed to be at least one bee facing out, blocking the hole.

A male bee from the 'swarm' of suitors, Identified by J. Asher as Megachile subgenus Chelostomoides

So my interpretation of the phenomenon is this:
These were indeed solitary bees, but the number of occupied, mostly still sealed holes, indicates communal nesting. This occurs because the bees requirements for nesting sites are very specific and good sites are rare. So this log with probably optimal hole size, sun exposure, humidity, food source vicinity, and nesting material availability had attracted scores of female bees to build their individual nests and lay their eggs.
(From Gordon's Solitary Bee Page: The largest recorded aggregation is 423,000 female bees in an area of 1300 m2. Aggregations can continue of over 20 years or can become extinct.)

Most solitary bees are protandrial.  This means that males hatch before females (in fact in serial nests with rows of brood cells, their cells are the ones closest to the entrance) So the males hatch - and wait around the nesting site for the females to emerge, often days later. Released pheromones will keep them informed of the progress inside.

Here is another video that I posted to flickr where the quality is better.

Yesterday at the ASDM Javelina exhibit, many males hatched from the aggregation (but not  hive or colony) of nesting sites. Other males may have joined in. They were hanging around, waiting, until the emergence of females from at least one hole began. At that moment, female pheromones whipped the whole waiting gang into a frenzy, resulting in literally hundreds of males hovering in front of that one hole hoping to get lucky. The very short life span of the males probably gives them a 'now-or-never' chance at procreation.

Megachile females collect pollen on the underside of their abdomen. 

 The mated female will live longer. She has to find a suitable nesting hole, cut leaves to build a series of brood chambers. Here is a link to flickr image of a female arriving with a neatly circular cut of a leaf that she will use to cover the walls of her nest and separate the brood cells from each other.

She has to collect pollen which she transports in a thick layer under her belly (honey bees carry it on their hind legs) as provision, lay one egg per chamber with the predetermined males towards the entrance, and seal the nest to give the brood a protected place for its development.
She will not return to actually care for the larvae like the social honey bees do.

 A female is cutting a circular piece of leaf from a bell pepper plant. She's already holding it in a grip that will allow her to fly to her nest with it. It only takes a couple of seconds.

In September the next generation of Leaf-cutter Bees is busy cutting circles from the leafs of pepper plants. So the one in the video may be a daughter of the one in the photo.