I discovered this bee fly hovering along the side board of my raised garden bed a couple of days ago. It’s a Greater Bee Fly, Bombylius major, as told from the plump, furry golden body and the wings with dark fore-halves. I found the observation interesting for two reasons. First was simply the timing. The BugGuide page for B. major notes that the species can be found “from March to May (most common in April)”. Well, here we are at the end of May; it seems a little late for these guys to still be about. In fact, in thinking back on it I’m pretty sure that all of my previous encounters had been early spring. This is probably the latest I’ve seen the species by a few weeks.
The second was what the fly was doing. While the adults use that long proboscis to sip at nectar, the larvae are parasitic on the larvae of mining bees. I knew this, so I found it puzzling to watch the fly doing what appeared to my unaided eye to be laying eggs. It would hover in front of the board for just a moment and then land for a few seconds, pressing its abdomen against the board repeatedly while it hung there; can you see that yellow bit at the end of its abdomen, against the board? As I leaned over, peering at it while it did its thing, that little bit of light colour caught my eye so I could observe how the abdomen was being moved. Then it would pop up again, move over an inch or two, land and press its abdomen to the board again.
If it wasn’t laying eggs, I don’t know what it was doing. But why would it have been laying eggs on the boards of my raised bed? I don’t know if the flies lay their eggs directly on the host, or if they lay them in the ground or on a surface and let their maggots find the hosts; perhaps if the latter, it was doing that?
I find behaviour one of the most fascinating aspects of nature (alongside web-of-life level ecological connections), but it’s also one of the hardest to get answers for, especially since you can’t just ask the organism, “why are you doing that?” Anybody want to offer a hypothesis on this guy (gal)?