If you look closely, you can see that this bee is not alone. There is a cluster of passengers on the back of the thorax, below and between the wings. They are probably mites, although they could also be eggs. Either way, they illustrate a couple of really interesting things about the life cycle of bees.
The first thing to understand is that most parasites of bees feed on the brood (that is, the larvae and their food), rather than feeding on adults. Adult bees are small and mobile: they’re not a particularly rich meal, and they live exposed lives. There are parasites of adult bees, of course, but it’s a risky career: many a parasite has joined its host as the meal of a larger predator. (There are also parasites that expect and exploit that, but that’s another set of posts.)
Exploiting the brood presents a much easier life. Bee larvae are fat, well protected, and frequently live on a small pile of protein-rich food (pollen). Most of them live communally: if you find one, there are at least a few more nearby. It’s no surprise that many parasites treat adult bees as nothing more than a free ride to get to the next larva.
The next thing to understand is the odd genetic system of bees (and their relatives, wasps and ants.) It’s called haplodiploidy. It’s a complicated system, but it boils down to this: when a bee lays an egg, it’s either fertilized or unfertilized. Fertilized eggs develop into females. Unfertilized eggs develop into males. That turns out to have fascinating side effects (bee sisters are more closely related to each other than to their mothers, and more closely related to their mothers than their brothers,) but the bit that’s relevant to parasitism is that if a bee can choose whether or not to fertilize an egg when it is laid (and all of them can,) then she can choose the sex of each egg that she lays.
Why does that matter? It matters because in the world of bees, males are mostly useless. They can’t sting (stingers are modified ovipositors, and egg-laying males are few and far between.) They don’t collect pollen or help raise the brood. With few exceptions (actually, I say that out of habit: off hand, I can’t think of any exceptions at all,) they’re basically just flying packets of sperm. And if there’s one central rule in the evolution of sex, it’s that sperm is cheap.
The last thing to understand is that Osmia nest in tubes. In the wild, they’re fond of beetle holes, but it turns out that they’re perfectly willing to build a nest in a paper straw. Inside the tube, they create a row of neat wax-lined cells, each one containing a lump of pollen, a dab of nectar, and an egg. (You may see where I’m going with this.)
Put all that together, take another look our Osmia here, and you’ve basically told his life story. (He is a male: you can tell by the long antenna.) When his mother started building her nest, she began by laying female eggs – deep in the nest, safely distant from the entrance. At some point, as she worked her way closer to the edge, she switched to laying males, including this guy.
At some point – late in the cycle, when he had already completed metamorphosis and was waiting for good weather to emerge – some parasites breached the nest. Arriving too late to feast on brood, they took the second best choice: they found an adult and climbed on, waiting for a ride to a newer, richer nest.
The joke is on them. There’s a reason his egg was laid near the entrance: in cases like this, his job is to convey his passengers to oblivion. He is a male, and once a male Osmia leaves its natal nest, he never returns – to that nest or to any other.
If these are mites, they may be able to jump to a female during copulation, but it’s not very likely. Many males never mate at all, and even if the mites get lucky and make the transfer, the female may just groom them off before they have a chance to latch on.
In all likelihood, these parasites are already dead. They just don’t know it yet.