Backyard pollinators in March

Our patio garden - a mix of local and exotic early bloomers

It's March, and the 'garden' part of our property is erupting in color. We plant a few exotics like aloes close to the house.  They are succulent low water-users from Africa, so they do well here in Arizona. Hummingbirds, Gila Woodpeckers and Verdins love them, but as most imported flowers, they are rather useless for our endemic insects.

Anthophora sp. female at Penstemon flowers, their specialty. Photo Terry Stone

Anthophora sp. males visit the flowers rarely, and always early in the morning

In our backyard the exotics are mixed with locals like Penstemon, Brittle Bush and Creosote Bushes that are all in full bloom too. Each species is visited by a distinctly different group of endemic bees and flies.

Only Honey Bees go wherever they find pollen and nectar. They are imported from Europe and the Middle East, also from Africa, so no wonder that they like Aloes flowers

Andrena male (right) approaching a presumed female

On the yellow Brittle Bush discs, several species in the mining bee genus Andrena are extremely active. They were  nearly completely missing last year after a severe freeze in late February. I am not saying that the bees had not survived, but their usual synchronization with the Brittle Bush bloom was totally off. Instead, last year we saw an abundance of Sand Wasps, that are usually much rarer.

Sand Wasp Steniolia sp.

 After this very warm winter the Andernas are back in their usual high numbers. Especially the males contribute to the buzz around the bushes. They try to mate with everything of the right size. Pairs in copula are nearly impossible to photograph because most mating attempts end in an undignified tumble off the flower.

mating Andrena pair

Mating flies are much easier to capture, even though the next example is a tiny fruit fly. There are several species in this group, like the Sunflower Seed Maggot Fly, that lay their eggs into the developing seeds of composite flowers.

Euarestoides acutangulus

Several species in the syrphid genus Copestylum were feeding on  Brittle Bush nectar, among them the huge Mexican Cactus Fly that is a convincing Carpenter Bee mimic.

Palpada furcata,                   Copestylum mexicanum (Mexican Cactus Fly),                     Eristalis sp.

 Most Copestylum species in Arizona lay their eggs in dead or injured cacti so their huge maggots can develop in the unlimited resources of for example a decaying saguaro.

Copestylum avidum

 The pair in the photo was actually flying around together and probably wasn't planning to use the perfectly healthy little Christmas Cholla as a larval host. 

Eupeodes volucris (Bird Hover Fly)

Here is another syrphid fly. While the adults of Eupeodes volucris also like nectar, they are also drawn by aphid populations on the flower stems. Aphids are the main food of the larvae.

Nomada sp

Any place where many insects that collect and store food for their offspring regularly appear, like here Mining and Honey Bees, attracts also brood predators that want their kids to go along for the ride. These may lay eggs on the flowers or on captured males or females of the target species, or the parasite follows them home to then throws her own eggs into the open nest entrance. They are often related bees like the Nomada species above (Cleptoparasites of Andrena and others).

Eupompha elegans

Lytta auriculata (Red-eared Blister Beetle)

Blister beetles also lay their eggs on the underside of flowers hoping that their hatchling larvae will find their way into a bee's nest by hitching a ride on a flower-visiting adult. Lytta auriculata showed up for the first time on our Brittle Bush.

Anthrax vierecki

Hemipenthes sp.

Bee fly females can often be seen hovering over the entrance of a ground bee nest, tossing their eggs into it in a characteristic forward thrust.

Mecaphesa sp. Flower Crab Spider

Of course the flower visitors are also assaulted by a number of direct predators that are waiting close to the nectar sources. I would have completely overlooked the flower crab spider in its camouflage if I hadn't seen the contrasting spec of its prey.

Zelus renardii (Leafhopper Assassin Bug)

Ambush bugs may be much better hidden on flowers by color and shape than this assassin bug Zelus renardii, but her leafgreen color and stealthy approach give her a wider hunting range.

So I am looking forward to our Brittle Bush bloom every spring, and I don't mind at all that we find more young plants every year. They only grow on the artificial patio elevation anyway, I suspect that the caliche layer under the undisturbed soil does not agree with them. They pretty much dry up during between April and mid July and come alive again during the monsoon.

By the time the Brittle Bush flowers are drying up and the gold finches are beginning to investigate them for seeds, the first cacti are blooming, usually Beavertail Prickly Pear and Arizona Hedgehog.

Augochlorella sp. on
Echinocereus pentalophus

But this year the Lady Finger Cactus (Echinocereus pentalophus) has beaten them by a couple of days. Little green sweat bees, Augochlorella sp. are buzzing the flowers

 Even after the flowers are all wilted, Brittle Bush still offers something to the local bee population: Megachile Leafcutter Bees are cutting the tapestry for their nest chambers from the leafs.

Centris Bees, the Bean Tree pollinators

Foothills Palo Verdes bloom along Picture Rocks Road in the Tucson Mountains, Arizona

Legume Trees are blooming all over the Sonoran Desert. Those hunger specialists with nitrogen-binding symbionts attached to their root system dominate habitats with poor sandy soils from Texas to California.

Ironwood Tree in Saguaro National Park West

Ironwood flowers are built like bean or pea flowers

While Acacias and Mesquites expose their pollen in very reduced flowers to any willing pollinator and even the wind, PaloVerde and even more so Ironwood and the  western Smoke Tree have retained most of the typical leguminose  flower architecture that requires insects with a short proboscis but a sturdy body to to push their way inside towards the nectar. This triggers an upward movement of the flower's stylus which then rubs along the furry underside of the bee to ensure pollen transfer to the stigma.

Centris pallida at Ironwood flower

The main pollinator of Palo Verde and Ironwood Trees is the Digger Bee Centris pallida. As soon as the chilly desert night gives way to the warmth of an early May morning, the droning sound of the big bees fills the air around every blooming tree. Females zap from flower to flower, visiting each only for seconds but quickly filling the pronounced pollen baskets on their hind legs.

A white-faced Centris pallida male

In our blooming Ironwood, I watched males stake out little territories that they patrol and mark with pheromones (Frankie et al. 1989), zigzagging back and forth,  hoping to find an unmated receptive female. It seems that a more successful strategy would be to wait for females directly where they emerge from the nest burrows, and indeed, in many Centris species, including C. pallida, metandric forms occur. These males are larger than the territorial males and appear to patrol nest sites where they wait for females and to mate as they emerge or to even attempt to dig them up to mate (Alcock et al.1976a, 1977, Chemsak 1985, Toro et al. 1991).

Both strategies, searching for and digging up emerging females, and releasing pheromones and defending territories, have been shown to be energy intensive (Vinson et al. 1982). During my attempts to photograph them, I found that males often take brakes from their vigil, usually on the inner branches of the tree. There they fall for several minutes into a torpor like state. I actually coaxed one onto my finger where he kept sleeping peacefully. This was the chance to shoot close-up portraits showing clearly the male's white clypeus (the female has a dark face).

Female collecting nectar and Pollen

The females are constantly on the move and difficult to photograph. They have to collect food for themselves and the brood provisions, plus fluids to bind the pollen, plaster the cell walls and seal the brood chamber.

Female digging a brood chamber and distributing debris

The females digg their nesting chambers in loose sand, often into a slight slope. The debris is carried off and distributed rather carefully and doesn't attract attention to the entrance hole. Cleptoparasitic bees, who would position their own offspring to steal the Centris Bee's provisions, are probably her main concern.

A tunnel and brood cells are constructed, and soil and collected fluids are mixed to stabilize the walls. Pollen, nectar and plant oils are collected as provisions for the brood. Female Centris Bees have huge corbicula (pollen baskets covered in special hair)on their hind legs, and both pairs of front legs are used to push the pollen tightly onto these receptive areas.

Centris artripes

Centris rhodopus

Besides Centris pallida, several other species of Centris occur in the Sonoran Desert. The two females above were drinking at a seep in the Ironwood National Monument.

Literature: Chemical Ecology of Bees of the Genus Centris SB Radleigh, V Inson, I Gordon, WF Rankie, HJ Williams 1996