Beewatch: more fun than a prime time soap opera
by Terry Morse
Insect weather has hit the Oregon coast, as I can tell from the numerous flies sunning on the south wall of my apartment complex. Sawflies, bee flies, and hover flies are also on the wing. Our early damselfly, the Swift Forktail (Ischnura erratica), is emerging in large numbers around wetlands.
Perhaps the most interesting insect flying now, though, is a ground-nesting digger bee that lacks a common name, Habropoda sp. These dark, 15 mm long bees have nearly naked abdomens, but the rest of the body is covered with short, thick gray fur. The pale coating of hairs down the middle of the face appears to the naked eye to be a white blaze mark, such as a horse might have.
Males emerge from their underground burrows earlier than females, then tirelessly cruise the area searching for emerging females to mate with. When they detect a female digging out of her burrow (possibly by smell) males begin to aggregate in the area, and one or more may start digging down toward the female. When she emerges, all the males in the area converge on her like hockey players after a loose puck, forming what I call a "mating ball": a female in the middle encased in a spherical horde of suitors, all struggling to be the one to mate with her. These bee balls, some of which may be 2 or 3 inches across, appear to roll around on the ground until, eventually, one male succeeds in mating with the female. The unsuccessful males leave and the ball breaks up, revealing the mated pair in the middle.
After mating, the females begin visiting local plants to collect nectar and pollen, both for their own nourishment and to provision their larvae for the nearly year-long immature period, which is spent underground. The females excavate burrows that may be 3 feet long. At the bottom of the burrow, they lay eggs in individual cells provisioned with pollen. Bee flies and other parasites may lay eggs in the burrows; their young will feed on the young digger bees, or the pollen their mother left for them.
Only the larvae overwinter. The mating season is brief, lasting from late March through late April. The adult bees do not survive long after mating and egg-laying, but their young will be safe at the bottom of their deep nests until the following spring. Unless they've been parasitized.
What: Digger Bees (Hymenoptera, Family Apidae, Subfamily Anthophorinae).
Where to find them: These digger bees seem to prefer open, sandy soil and sandy hillsides that receive plenty of sunlight but are reasonably sheltered from the wind
When to find them: Now! The mating season is brief. Don't procrastinate! Based on my observations at Mike Miller park today (29 March), the females are just beginning to emerge, so you should be able to see the entire range of adult behaviors within the next few weeks. If you wait very long, you may miss the excitement of the mating balls.
How to spot them: Look for numerous dark bees flying rapidly and apparently aimlessly throughout an area, within a few inches of the ground. Though there may be many in an area, they will not be very dense, so it would be easy to overlook them.
What to look for: Observe the males patrolling for females. If you see a small number of bees together on the ground, some of which are digging into it, watch to see whether they are males digging down to an emerging female. If you see a large cluster of bees apparently "rolling" over the ground, watch it. Eventually, it should break up and you'll see the successful male and his mate. Watch also for females excavating their burrows. They will generally be solitary, unlike the males digging for females. Notice how they sweep sand out of the burrow for a period, then leave the burrow for a few minutes before returning. Watch for females returning to the burrows with bright yellow pollen on their hind legs.
These are very good-natured bees, intent on their own business. You can sit or lie down in the midst of them for a close look, and they will completely ignore you. (Please try not to trample any burrow entrances while doing this). I don't know whether the females are capable of stinging, but if you're just watching, I wouldn't worry about it. Bring along a magnifying glass (not a hand lens) for a closer look, or a camera with closeup capabilities. You won't need binoculars to observe them.
Other insects to look for: As you watch the burrows, you may also see parasitic insects depositing their eggs in the Habropoda burrows. The larvae of the parasitoids will subsist on the eggs or larvae of the digger bees. The two types of parasitoids I have seen around the colonies are a 10 mm black wasp with red abdomen, which you will see entering the burrows, and furry bee flies (Diptera: Bombyliidae), which will hover above the burrow entrances and drop their eggs into the burrows like bombs.
Barthell, John F., and Howell V. Daly. 1995. Male size variation and mating site fidelity in a population of Habropoda depressa Fowler (Hymenoptera: Anthophoridae). Pan-Pacific Entomologist 71(3): 149-156.
Barthell, John F., Daniel M. Bromberger, Howell V. Daly, and Robin W. Thorp. 1998. Nesting biology of the solitary digger bee Habropoda depressa (Hymenoptera: Anthophoridae) in urban and island environments. Journal of the Kansas Entomological Society 71(2): 116-136.
Tentative identification of the bees from photographic slides courtesy of Doug Yanega (formerly) of the Illinois Natural History Survey. Thanks also to Bob Loeffel of Thiel Creek, who first brought them to my attention.