Andrena: A Closer Look
By Travis Owen
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 themselves 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.
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 world’s 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.
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.
Most Andrena are solitary, meaning every female excavates and provisions her own nest. 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).
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 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.
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 receive 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.
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.
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.
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.
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