Wild Side: Home invasion

Cuckoo bees take over the burrows of mining bees.

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Two cuckoo bees at the entrance of a mining bee burrow. — Matt Pelikan

Last Saturday, May 25, I paid a visit to a colony of mining bees I’ve been keeping an eye on. The site isn’t much to look at: An open area perhaps 50 feet across, featuring a mix of bare sand and sparse vegetation. But scattered across it are scores, maybe hundreds, of bee nest burrows, probably the work of one of the many species in the genus Andrena.

Each burrow, about the diameter of a pencil, is surrounded by a low mound of dirt, produced during the process of excavation. While the site is easily overlooked if you don’t pay much attention to insects, in fact it’s the locus of a great deal of activity: bees coming and going, entering and leaving the burrows.

Most people, when they think of bees, think of the highly social, hive-making bees such as the non-native European honeybee. But in fact, most of our native species are solitary nesters. Each female digs her own burrow, provisioning cells at the bottom of it with laboriously gathered pollen and then laying an egg in each cell. The pollen will feed the larval bees once the eggs hatch, and support them until they reach maturity.

On the occasion of this recent visit, I was surprised to see that in addition to mining bees I expected to see as they skimmed low over the ground, searching for their own personal burrows, the site was mobbed by much smaller bees, darting busily around and occasionally landing on the ground next to burrows. Reddish with small yellow markings on their abdomens, these smaller bees were easily recognized as members of my favorite bee genus, Nomada.

Known as “nomad bees” for the genus name and their habit of ceaselessly patrolling over open ground, Nomada are also collectively known as “cuckoo bees.” The sobriquet derives from the fact that, like the Eurasian cuckoo, nomad bees are nest parasites: Rather than making their own nests, female cuckoo bees (and cuckoo birds) lay their eggs in the nests of other species. (“Cuckoo bee” also applies to several other distantly related genera that have independently evolved the same habit, showing how unreliable common names for insects can be.) Since they don’t collect their own pollen, cuckoo bees of all sorts are recognizable because they lack the dense body hairs and “pollen baskets” that most bees use to collect pollen.

Larval cuckoo bees hatch quickly, getting a jump on the host offspring, and grow to maturity living on the pollen that their involuntary host stocked for her own offspring. Cuckoo bees don’t necessarily do any direct harm to the offspring of their host, though they may eat the egg or larva. But even if they just compete for the limited resources stockpiled in the nest burrow, they often end up causing the death of their foster siblings.

What I found myself watching, that is to say, was a swarm of nomad bees attempting to parasitize the nests of a colony of mining bees. Landing next to the mouth of a burrow, a nomad bee would plunge down into the tunnel. Sometimes she’d stay down for longer than I cared to wait, and I presumed she was laying one or more eggs in the host’s nest. Other times, the cuckoo bee would back out seconds after entering the nest, apparently deterred in her effort to lay eggs.

These attempts clearly failed because the host bees were actively defending their nests against parasitism. The female host bees were often visible facing outward just inside their burrows, and any Nomada trying to enter those burrows found themselves up against a much larger and presumably irritated mining bee.

Unfortunately for the mining bees, though, endlessly defending your nest is probably not a viable option. There’s still the need to provision your nest with pollen for offspring to eat. So periodically, the host bees emerged cautiously from their burrows, apparently checking first to make sure no nomad bees were poised by the entrance, and flew off to hunt for pollen. (There wasn’t much in bloom in the immediate area, and as far as I could tell, the mining bees were using honeysuckle flowers as their main resource.)

I’ve known of the relationship between cuckoo bees and their hosts for many years. And on occasion, I’ve seen a nomad bee sniffing around the entrance of a burrow. But this was the first time I’ve seen such a swarm of cuckoo bees working a large cluster of host burrows. It was fascinating to think of the complex dynamics of the situation: Cuckoo bees competing against each other for a chance to enter a burrow, mining bees trying to defend against invading cuckoo bees, and mining bees somehow calculating the value of leaving to hunt pollen versus staying home to play defense.

The action gradually moved around the mining bee colony, and was still in full swing when I had to leave. I’ll check next year to see how the mining bees fared, and how many nomad bees succeeded with their parasitic strategy.