Friday, March 18, 2022

Andrena: A Closer Look

Spring is here! And with it, a new season of bees. Honey bees, having spent the winter clustered for warmth and feeding on honey stored from the previous year, are well adapted to take advantage of the earliest of blooms on sunny days, especially with the disconcertingly dry winter we've had in the Rogue Valley. Bumble bee queens are also out and about looking for potential nest sites, and looking for nectar to replenish them after their long winter dormancy. Despite the widespread recognition by the lay observer of the honey bees and bumble bees, the most speciose group of early spring bees are the mining bees in the genus Andrena.

Andrena males on Polemoneum carneum near Pilot Rock (Ashland, OR)
There are roughly 550 species of Andrena in the United States and Canada, with at least 1500 species worldwide. At least 200 species are found in the Pacific Northwest. Andrena are found all around the world except Australia, South America, and Antarctica (there are no bees in the Antarctic circle). Most species are active in early spring, and there are a relatively small selection of species active in autumn. They are typically small, yet some species are roughly the size of a honey bee. The related species Perdita are the worlds smallest bees with the highest diversity in the Southwestern US and Northwestern Mexico. Some species of Perdita are smaller than mosquitos! The Andrenidae, the taxonomic family that contains Andrena, Perdita, and many other genera is the largest taxonomic bee family with 45 genera worldwide.
Two different sized Andrena females on Ceanothus integerrimus
The majority of Andrena species are univoltine, where there is only one generation per year. Some species are bivoltine with two generations per year. Most species go through their dormant period (winter in North America, though there are winter-active species overseas) in diapause as adults, which likely gives them the advantage of being able to emerge quickly once conditions are favorable. If environmental conditions are not ideal for the flowering plants they depend upon, some species have been known to remain in diapause for at least two years before emerging.
Andrena pertristis (?) on common dandelion
Most Andrena are solitary, every female excavates and provisions her own nests. Some species are communal and dozens of females will share a single nest entrance, but they still lay eggs in their own cells inside the communal nest. It's comparable to a New York apartment building, where neighbors share a front entrance but still have to buy all their own groceries. Some Andrena nest in large congregations where nest entrances may be as close as an inch apart. Nest entrances may be hidden under leaves or fallen branches, and nests may be excavated in various soil types (i.e. sand or clay).
Male Andrena perched on Fritillaria affinis, near Pilot Rock
Unlike honey bees, Andrena excavate tunnels straight down into soil. They are very common in lawns and sometimes nest in soil covered by vegetation, unlike most other ground dwelling bees. Cells branch off the main shaft on short lateral tunnels. Nests of Andrena are typically between 6-12" (15-30cm) deep, but desert dwelling species nests can be incredibly deep. Andrena, like many solitary ground nesting bees, line their cells with a wax like substance produced from the Dufour's gland on the underside of the abdomen, and spread it around the cell wall with the trowel-shaped pygidial plate located at the tip of the abdomen. The waxy substance protects the larvae from bacterial infections and retains the moisture in the cell, reducing the risk of desiccation. Andrena also use the Dufour's gland secretions to mark the entrance of the nest to help them find the nest via the odor, especially important in communal nests where keeping intruders out is of great concern. Additionally, female Andrena scent-mark flowers with Dufour's gland secretions where resources were good so they may return on the next foraging trip.
Andrena on February blooming Scilla mischtschenkoana in my garden
Early spring weather is sometimes unpredictable. Rainy and windy weather can make it difficult for early bees to forage for resources and provision cells. With all solitary bees, cell provisioning entails collecting pollen and nectar, mixing them to form a pollen ball, depositing an egg, and sealing the cell before starting on the next one. Andrena are typically slow to provision cells, sometimes only completing a single cell per day. On average, each foraging trip can take over an hour for some species to gather nectar and pollen for a cell, and an average Andrena cell takes roughly nine foraging trips to complete. Some of these foraging trips are used for the mother bee to feed on nectar for herself. Nectar and to a lesser extent pollen is the sole source of carbohydrates and nutrients for adult bees. Pollen is the sole source of proteins and amino acids bees need, especially important for developing larvae.
Lomatium californicum on Upper Table Rock with swarming male Andrena

Andrena are short tongued bees, and most forage for nectar on flowers with easily accessible nectar. Unlike honey bees and bumble bees which use their proboscis more like a straw, short tongued bees lap up nectar not unlike a cat. This doesn't mean all Andrena can't access nectar in larger or deeper flowers, some species have longer mouth parts or special adaptations that enable them to reach hidden nectar. Many species are pollen specialists and only collect pollen from a narrow range of plant species. For example, some Andrena species only collect pollen from Asteraceae (sunflower family), Apiaceae (parsley family), or willows (Salix spp.). The specialists will still collect nectar from a wider range of plants, including many nonnative plants. Most species aren't terribly picky, and some are important pollinators of cultivated plants in gardens and agricultural settings, though mining bees aren't commercially managed like alfalfa leafcutter bees or orchard mason bees.
Male Andrena with extended proboscis
Like many of the solitary bees, male Andrena emerge from diapause before the females. Males do not actively collect pollen, but they do feed on nectar. While male bees are not usually often valued for their pollination services, male Andrena do serve important roles in the pollination of some plants like bee-mimic orchids (Ophrys spp.) in Eurasia and Northern Africa where the orchids live. The bee orchids are only pollinated by male bees who are duped into thinking the orchid is a female ready to mate, and they recieve nothing for their efforts. For most other plants, males may be superior pollinators on a per bee basis. Female Andrena collect much larger amounts of pollen than males, but most of that pollen goes to the nest instead of the next flower since females frequently and thoroughly groom the pollen into their scopae (specialized pollen carrying hairs). Males don't groom frequently, and on a per bee basis likely deposit more pollen than females. Males don't visit nearly as many flowers as females, and are generally smaller in size. The importance of any pollinator to a specific plant species depends on how effective each flower visit is in regards to collecting or depositing pollen, and the rate of visitation. On a net basis, females are more important pollinators for the majority of plants visited.
Plum blossoms with an Andrena male
Females collect pollen by grabbing onto the anthers with their legs and bending the abdomen to push the anthers onto their bodies. On some flowers they contact the anthers by default as a result of reaching for nectar. Female Andrena are usually active for much of the day when weather is favorable, and visit many more flowers than males. Between flower visits, female Andrena groom all of the pollen onto their hind legs. Scopae, the specialized plumose hairs used for holding pollen, are located on the hind leg tibia and extending all the way up to the trochanter and propodeum (where the hind leg meets the thorax). Andrena with a full load of pollen can appear to have a belt of pollen spanning between the hind legs.
Female Andrena with pollen laden scopae
Flower visits by Andrena aren't always mutually beneficial. Some Andrena are nectar thieves of certain flower types, accessing the nectar without contacting the anthers or stigma and thus being of no benefit to the plant. Nectar is a metabolically expensive incentive plants make that is meant to attract pollinators to facilitate plant reproduction. Some plants keep the nectar hidden within parts of the flower inaccessible to some pollinators. This excludes the least effective pollinators for a particular plant species and encourages the best pollinators for the particular flower type (or at least that's how it's supposed to work). Many bees cheat by making incisions in the sides of the flowers to bypass the reproductive organs altogether and access the nectar directly.
Male nectar thief stealing nectar from a slit in the side of a manzanita flower.
In my observations, male Andrena appear to steal nectar more often than females. With shallow flowers that don't exclude pollinators, like many of the Asteraceae and most fruit trees, this doesn't appear to be the case. Flowers on plants in the Ericaceae (i.e. Arctostaphylos; Arbutus), Berberidaceae (Epimedium), or Fumariaceae (CorydalisAquilegia), to name a few examples, often have either restricted openings in urn-shaped flowers or nectar filled spurs are often victims of nectar thievery. In the case of Arctostaphylos, and many other genera in the Ericaceae, the flowers are only pollinated by bees capable of sonication where the bees vibrate their wing muscles to release the pollen.
Manzanita flowers 
Manzanitas, the common name for Arctostaphylos, are one of the earliest blooming shrubs in the Rogue Valley, and they attract a wide range of pollinators, including many hungry Andrena. Bumble bee queens and anthophorine bees (Eucera sp.) use their mandibles to cut slits at the base of manzanita flowers. Smaller bees like Andrena then follow in their place. Most Andrena are not equipped to access the nectar from the flower opening unless the flower is damaged or malformed, and it is unlikely they contribute towards the pollination of Arctostaphylos in any significant way.
Lasthenia californica ssp. californica and male Andrena on Upper Table Rock
Despite being occasional nectar thieves, Andrena are very important early pollinators for many plant species. In some areas, they are the most abundant bee when the weather is cool. Although they are active in early spring in low elevations, they are active in early summer in higher altitudes such as subalpine and alpine mountainous environments. In either high or low elevations, Andrena can't warm their bodies the way that bumble bees do by vibrating their flight muscles and burning calories for warmth. All bees are ectothermic (cold blooded), while some bees, like bumble bees, are facultatively endothermic (having the ability to warm themselves). To cope with cool environmental conditions, Andrena sunbathe on sunny vegetation until their body temperature is adequate for flight.
Fritillaria affinis with possible Andrena pollinator?
Most Andrena feed on nectar and pollen from a very wide range of plant types. While there are many specialist species, most are highly adaptable generalists. Species whose ranges span multiple ecoregions (such as A. prunorum which is found throughout most of the west in the United States and Canada) are not picky eaters, and may feed on completely different plant species in different parts of their range. There are species that collect pollen from many wind pollinated plants, like grasses and oak catkins. Planting flowers for Andrena in the garden is not challenging, so long as you include many species with open or small flowers. Some of the plants in my garden that I have observed Andrena visiting include bulbs (i.e. Crocus; Scilla), strawberries, Apiaceae (Lomarium; others), California lilac (Ceanothus), Berberidaceae (Epimedium; Mahonia), and many others. It's worth experimenting and observing to see what they like, but if you include a large diversity of flower shapes and overlapping bloom periods, there are good odds you'll see some Andrena.
Andrena female gathering pollen on oak catkins, I was very excited to capture this photo!
If you don't detect any Andrena in your area, they may be detected by the presence of some of their parasites. The fuzzy bee-like beefly (Bombylius) is a nest parasite of many ground nesting bees. They have a long needle-like proboscis and visit many of the same flowers as Andrena. Beeflies hover over the ground looking for bee nests, and when one is detected, they skillfully toss some eggs into the nest with a flick of their abdomen, all while remaining in flight. Another group of nest parasites are the cuckoo bees in the genus Nomada. Nomad cuckoo bees are very wasp-like in appearance, often in bright colors like red. While these waspish bees don't collect pollen, they feed on many of the same nectar plants as Andrena and may play a small role in pollination. There are others, and the presence of any of these parasites is a sign of a healthy host population.
Nomad cuckoo bee (Nomada)
I hope you enjoyed reading this introduction to the mining bees, Andrena. This wasn't meant as an identification guide, and there is much more that I could have written to further illuminate the biology and behavior of this large genus. If you would like to learn more, please take a look at some of the resources below.

Danforth, Bryan. The Solitary Bees: Biology, Evolution, Conservation. 1st ed., Princeton University Press, 2019.

Fowler, J. (2020). Pollen Specialist Bees of the Western United States.

Michener, Charles. The Bees of the World. 2nd ed., Johns Hopkins University Press, 2007.

Miliczky, Eugene. “Observations on the Nesting Biology of Andrena (Plastandrena) Prunorum Cockerell in Washington State (Hymenoptera: Andrenidae).” Journal of the Kansas Entomological Society, vol. 81, no. 2, 2008, pp. 110–21.

O’Toole, Christopher. Bees: A Natural History. First Edition, Firefly Books, 2013.

Stephen, William Procuronoff, et al. The Biology and External Morphology of Bees. Amsterdam University Press, 1969.

Tang, Ju, et al. “Pollinator Effectiveness and Importance between Female and Male Mining Bee (Andrena).” Biology Letters, vol. 15, no. 10, 2019, p. 20190479.

Wilson, Joseph, and Olivia Messinger Carril. The Bees in Your Backyard: A Guide to North America’s Bees. Princeton University Press, 2015.