From ticks to tapeworms, parasites lead the list of reviled animals. And the insect world takes parasitism to particularly repellent extremes: The young of entire families of wasps and flies, for example, comprising tens of thousands of species, mature inside their hosts, literally eating them from the inside out.
But nature has no regard for human ethics or aesthetics. The only point is survival, and parasitism, which involves appropriating resources rather than producing your own, offers obvious advantages. Why work yourself when you can steal the fruits of someone else’s labor?
Bees currently (and rightly) enjoy widespread human approval for their role in pollinating plants, lending productivity to agriculture and helping native plants flourish. So it might be dismaying to find that parasitism is a common strategy among these popular insects. One rough estimate is that about 15 percent of all bee species are parasitic, a ratio that is not far off among the bees known on the Vineyard.
The name “cuckoo bee” is often loosely applied to these parasitic species, after the Old World cuckoos of the bird world, which lay their eggs in the nest of other birds and stick the host with the labor of raising the hatchlings. Cuckoo bees generally parasitize other bees, laying their eggs in the nests that have been built and provisioned by the victimized species. While the colonies of social bees, such as bumblebees, are sometimes the target, most parasitic species exploit so-called solitary bees, which nest individually, often in burrows dug in the ground. Often the parasite/host relationship is a very specific one, with a cuckoo bee parasitizing only one or a small number of other species.
In evolutionary terms, cuckoo bees descended from species that built their own nests, and the parasitic lifestyle has evolved many times, in multiple bee families. Somewhere along the line, some bees simply began laying their eggs in the nests of other species, rather than building their own, giving rise to parasitic lines of descent. With no need to gather pollen for their offspring, parasitic bees gradually lost the physical traits — hairiness, the presence of hairy “baskets” on the hind legs for transporting pollen — that help other bees provision their nests. As a result, most parasitic bees look rather wasplike, often spiny but lacking hair.
Parasitic bees do visit flowers, eating pollen as a source of energy. And in the process of such visits, they do pick up small amounts of pollen on their legs and bodies (I’ve taken photos that show this). However, these bees clearly carry much less pollen around than do their hairier, nonparasitic relatives. Moreover, because they only take pollen to satisfy their own needs, not the needs of all their offspring-to-be, cuckoo bees spend much less time visiting flowers. As a result, they are probably much less helpful as pollinators than nonparasitic bees.
They spend a good deal of their time patrolling promising habitat, looking for the burrows of suitable hosts. In my yard, parasitic bees in the genus Nomada patrol with a seemingly mathematical precision, crossing open areas in a series of tight S curves, and pausing to investigate promising holes in the ground. I’ve yet to see a parasitic bee actually entering a host’s burrow, but they locate those burrows reliably, and often spend several minutes at a time walking around a burrow’s mouth, perhaps determining if it belongs to a suitable species, or if the owner is at home.
With a suitable burrow located, a female cuckoo bee waits until the burrow’s proprietor is out foraging. Descending into the vacant burrow, the parasitic bee often eats any eggs or larvae present, lays her own eggs near the pollen supply in the burrow, and then seals the tunnel on the way out. The interloping larvae hatch and then mature, eating the pollen that was gathered by the nest’s original owner.
If bees in general are beneficial insects, does that mean that cuckoo bees, which victimize other bees and are less efficient as pollinators, are bad? My answer would be, in part, that we should avoid thinking of wildlife as “good” or “bad.” Insects simply are, and to my mind, every species has a right to life. (Well, except the deer tick.)
But in a backward kind of way, parasitic bees may benefit their hosts. Think of it this way: The best way a host species can defend against nest parasitism is by spending as little time as possible away from her nest. Host bees that select sites close to rich supplies of pollen can spend relatively little time foraging, and as a result, may experience less likelihood of being parasitized.
Assuming that some innate tendency, and not just luck, guides a bee’s nest-site selection, cuckoo bees would prey more heavily on individuals with poor judgment about nest sites. In other words, parasitism may be a selective force that lets the most fit members of the host species reproduce more often than less fit members.
In any event, cuckoo bees are beautiful insects with complex and interesting behavior. My advice is to set value judgments aside and appreciate these animals as unique, highly adapted pieces of an infinitely complicated natural world.