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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&#x