Wild Side: Andrena nubecula

The Island holds an abundance of goldenrod, the perfect food for these bees.

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Female Andrena nubecula, with a full load of pollen. — Matt Pelikan

Yellow is the signature color of Martha’s Vineyard in late September: goldenrod is blooming everywhere! These members of the aster family — somewhere around 20 species and forms are known from the Vineyard — are the most obvious plant of our late summer landscape, growing in abundance in many habitat types and producing voluminous, bright yellow blossoms.

When something is abundant in nature, it’s safe to assume that, given time, other things will evolve to exploit that abundance. So it is with the goldenrods, which attract both generalist and specialist insects that feed on goldenrod flowers or leaves, as well as predators that prey on those visitors. You might say that flowering goldenrod represents a type of ecosystem, dispersed across the landscape and existing for only part of the year but supporting a distinctive network of ecological relationships.

One might as well start with the bees. By now, regular readers of this column know that while some bees are promiscuous in their feeding habits, visiting flowers of virtually any type at all, many other bees are more or less specialized, focusing their foraging on one family, genus, or even species of flower.

Both classes of bee depend heavily on goldenrod when it’s in season. Generalists like the common Eastern bumblebee, Bombus impatiens, visit goldenrod because it’s widely available and, presumably, deemed by these bees to be reasonably tasty and nutritious. But while Bombus impatiens will happily check out any other late-season wildflower it comes across, many other bees exhibit a stronger connection to goldenrod, timing their life cycles around peak bloom season and clearly preferring the clumps of yellow flowers to other resources that may be available.

One example would be Andrena nubecula, sometimes called the cloudy-winged miner bee. This is a small bee, with even the largest females less than a centimeter in length. And its deliberate movements — an individual typically works a goldenrod flowerhead thoroughly, sometimes for five minutes or more, before moving on to the next cluster of gold — make it easy to overlook. But once you have a sense of how to find this bee, it turns out to be rather common here.

While it’s known to visit various other members of the aster family, I can’t recall seeing A. nubecula on anything other than goldenrod. A black bee, it has obvious bands of white hair on the edge of each segment of its abdomen, except for the first, inboard segment — a pattern that, while perhaps not unique within the genus Andrena, is unusual enough to serve as a useful field mark.

There are also narrow but well-defined bands of white hair running along the inside edge of each eye, clearly visible in a head-on view or photograph. These “facial foveae” are a classic feature of the entire genus Andrena, but on few species do they stand out so sharply. The outer portion of the wings of A. nubecula are richly infused with dark pigment (hence “cloudy-winged” in the usual common name), another feature that makes this species easier to identify than most of its relatives.

As with most bee species, female cloudy-winged miner bees sport special, elongated hairs for transporting pollen. Some of these hairs are on the hind legs, a common though by no means universal arrangement among bee species that collect pollen. But like many other Andrena species, A. nubecula also has pollen transport equipment on the rear of its thorax: a bowl-shaped concavity on either flank, fringed with bristles. As it meanders around a goldenrod flower head, sipping nectar from individual flowers, pollen sticks to the bee’s body. Periodically, she grooms herself, wiping the pollen grains backwards toward the specialized hairs and the “pollen bowls” on her thorax.

These pollen-carrying hairs are elaborately branched; under high magnification, they look a bit like bottle brushes. As you might imagine, these feathery branches create additional surface area to hold pollen. I don’t know if it’s ever been formally studied, but my guess is that on goldenrod specialists like A. nubecula, the size, shape, and texture of these hairs are optimized specifically for holding the large, sticky pollen grains of these flowers. In any event, by the time she is fully laden and ready to return to her nest, a female nubecula carries an unbelievable amount of pollen, packed around her waist and hind legs like a floatation vest.

Some of that pollen, of course, may rub off on the flowers the bee visits next, fertilizing the receiving flowers and making future generations of goldenrod possible. But the vast majority of that golden burden is destined to be food for the female bee’s offspring, which will hatch underground, feed on golden pollen until they mature, and then emerge next year to repeat the cycle. An abundance of goldenrod will be waiting for the new generation.

 

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