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.

Introducing Wasp mimicry to school kids

Today a group of 5th graders from Sells, AZ  visited the University of Arizona Insect collection. Our 'Bugman' Carl Olson wasn't here to entertain them, so I had to come up with something interesting, fun, but educational.

The kids and parents were all Tohono O'odham tribal members, and a great group to talk 'Bugs' to. They admired our pinned specimens of especially pretty  insects. Arizona has so many carismatic species...

By borrowing from other flickr members and using my own quite extensive digital insect collection I prepared a slide show of insects from divers orders that mimic stinging wasps and hornets by displaying yellow-dark banding.
I myself was surprised at some of the striking examples that I found.

Add caption

Add caption

Wasp mimicry (Yellow/dark), a set on Flickr. Click on the link to see all.

The kids went cheerfully along with my flickr slide show calling out the  different insect orders that they recognized.

They loved to handle my two life Agave Weevils. Of course, they also enjoyed hunting one of them down when it flew around the class room - after I had introduced it as a flight-less species.

A big Dynastes male with its wings spread open was everyones favorite. I wished the class had visited in summer, so I could have had some life Dynastes and Oxbeetles and maybe some Chrysinas. All of them occur on Reservation land, but I doubt that many of the kids get to see them.

The teacher Kathryn Killmer gave a great introduction to the path to college and university, and kids and parents were obviously fascinated with our subject matter, so one can hope that some of them  will eventually return to the U of A as students.

I hope they had as much fun as I did.

Birds, Bugs, and Herps in the western Santa Rita Mountains

Eric Eaton and I took a trip to the Santa Rita Mountains (a last one before Eric moves to Colorado or the mountains get eaten up by the planned Rosemont Copper Mine? I hope not).

We went to Madera Canyon Lodge first. Still lots of bird activity, we especially enjoyed the Black-headed Grossbeaks. Caught a glimpse of a Lazuli Bunting, mistook him for a Bluebird, didn't get a photo.

Bombyliid Paravilla sp.

Blooming Velvet-pod Mesquite Trees would have been interesting enough to spend the day. But we were headed for yet another canyon.

Acrobat Ants, Crematogaster, tending to Membracid Vanduzea laeta

At Florida Canyon, a short stop at the creek revealed no water, but Eric spotted and orange flash among the rocks and dry oak leaves: the short distance flight of the incredibly cryptic Oak Leaf Grasshopper.

Oak Leaf Grasshopper,Tomonotus ferruginosus

Butterflies often patrol creeks and roads in the mountains. As it was rather windy, a Spotted Purple found a perch above my head and wouldn't leave, even when I pulled him down to eye level. An imitator of the bad-tasting (so I was told) Pipevine Swallowtail, he had some people fooled.

 Red-Spotted Purple, Limenitis arthemis astyanax

Ironically, we ran into another Pipevine Swallowtail mimic, a Black Swallowtail, and got some good photos while he was clinging to his Arizona Thistle head, but we got no shots of the usually much more common Pipevine Swallowtail itself.

Black Swallowtail, Papilio polyxenes

Those Thistles yielded scores of interesting bees that I'm saving for another blog and some enormous Sand Wasps - for scale, just consider that the Didasia on the right is about the same size as a Honey Bee.

A nice Ambush Bug also showed up on the thistle. This species is always recognizable by the arrow head on its back.

Macrocephalus dorannae

The canyon is full of Sotols, Yuccas and Agaves with their own fauna of bugs and beetles. Since the tiny Mirids defied our photo skills,  here is a photo of the very cute, cooperative agave weevil.

Peltophorus polymitus seminiveus

Prickly Pear Cacti were not blooming yet at this elevation, but the pads were coming to life and oozing juices.
 We found all the True Bugs that we expected and more. Also Soldier flies and many different species of wasps that were drawn to the sugary leaks.

Zarhipis integripennis

When all these insects tumble around among those sharp spines, accidents happen. I often find impaled flies or bees. This time it was a male Glow Worm. According to Eric, the males fight during courtship which could have contributed to his predicament.  We got him off the thorn.

Lycus sanguineus

Several very bright red male Lycus sanguineus were impossible to overlook. Much lighter and equipped with larger wings than the flightless females, they usually allowed only a few shots before taking off.

 

 One more vertebrate on the way down:  I found this snake after I'd already stepped over him with one foot. For a Sonoran Whipsnake he was quite patient, but once he got going, he disappeared in a flash.

Sonoran Whipsnake Masticophis bilineatus

Microcosmos Cactus Flower

Engelmann's Prickly Pear, Opuntia engelmanni 

Engelmann's Prickly Pear is the most widespread and common Opuntia around Tucson, and it's just starting to bloom. The flowers are beacons for photographers and pollen and nectar seeking insects. They are flat and easily approachable for bees with short proboscis and even unspecialized beetles.

 Carpophilus  sp.

 In the morning, when fresh flowers open, countless  Nitidulids of the genus  Carpophilus  arrive. They 'drop out of the sky', land and slip between the petals, where they congregate, feed, and find their mates. 

I used to think that they are specialized on cacti, but I this week I found clusters of them in Apache Plume (Rosaceae) and in Arizona Thistle (Asteraceae) in Florida Canyon where the cactus flowers were just budding.

Lasioglossum sp.

 Bees of all sizes are visiting. This small, partly metallic one will hopefully be identified by Monday. 

P. s Thanks to John Asher on Bug Guide: he identified the genus as Lasioglossum.

Cactus Bees,  Diadasia sp.

Cactus Bees, genus Diadasia hectically buzz from flower to flower and dive deeply between the stamina. Males refuel with nectar, females collect pollen for the brood in addition. Often, before they fly off again, they seem to hesitate and wait motionless. That's the moment when males will suddenly pounce on the females.

So this is how the Diadasia couples meet. They share their dating grounds with many other hopefulls. Just check out the depth of the flower behind them, the red and black wing pattern belongs to one of the beetles below

Clerid beetles Trichodes ornatus

Clerid beetles Trichodes ornatus come, like the cactus flowers, in red and yellow. But they don't have to match their back ground - the red ones look deceptively like a blister beetle that flies at the same time, the yellow ones imitate wasps.

The two color morphs occur equally among the larger females and the small males, and there seems to be no mating preference related to color. Still, three is a crowd.

Bruchid Mimosestes amicus

Bean Weevils can be found in cactus flowers but they may just be resting while searching for leguminose flowers where they lay their eggs.

Listrus, a dasytin Melyrid

Dasytinae forage wherever there is a lot of pollen. With their dense hairy coats they make good pollinators.

Where so many insects come together predators will also hang around. Here is one predator stalking another. But the little jumping spider did not attack the tiny Attalus, his aposematic colors  may have warned her off.

Staghorn Cholla Trichopuntia versicolor

Prickly Pear Cacti (Opuntiae) and their close relatives, the Chollas (Cylindropuntia or in older literature still also Opuntia) dominate our Sonoran desert with an abundance of species. Every day seems to bring new color schemes. But the flowers are all very similar in their simple architecture and they are visited and pollinated by the same army of insects.