We all have our waymarks that guide us through the seasons. For me, one sure sign that winter is fading is the appearance of Bombylius bee flies in sunny woodland patches. I’m not the only one: early western entomologist Frank Cole described them as the “hairy and handsome harbingers of spring.”
The Bombylius I watch for is Bombylius major, the Greater Bee Fly. They look like little plush toys, with a long, thin, proboscis. They are common across North America and also occur in Eurasia. Their early spring activity is timed to coincide with that of solitary bees, such as those in the genus Andrena. As adults, Bombylius feed on nectar by hovering over early spring flowers. But their larvae are parasites in the nests of solitary bees, feeding on the food stores and larvae of the hosts.
Thus, one sees adults Bombylius prospecting for the burrows of solitary bees before the holes are sealed up. I’ve watched them searching for burrows, investigating what I presume must be likely-looking (from a bee fly’s point of view) holes, the forceful breeze from their wings tossing and scattering grains of soil as if a tiny tornado was attacking a square-inch patch of ground. It would seem most straightforward if Ms. Bombylius just went directly into the nest burrow, but I suppose that the hosts have all sorts of defenses against such an intrusion. Instead, the female Bombylius hovers over the hole and flicks her eggs inside. Females of many bee fly species pack sand grains into a special abdominal chamber so that they stick to her eggs. Presumably this gives them heft or prevents dessication.
Other bee flies in the Bombyliidae family occur throughout the spring and summer. Many are parasites on bees like Bombylius major, others target grasshoppers, tiger beetles, moths, or other insects. Some bee flies are very convincing bee mimics and all, to me, are fascinating!