We went to the beach today. It was the perfect beach day—hot, and not too much wind (not at the beach, at least). The waves were big and great for body surfing and boogie boarding, and as usual, we ran into friends who also happened to be there, and had a good catch-up.

But the very best part of the beach today was the bees.

Thousands of native bees on the dunes just above high tide line. Swarming in the air, just 30 cm off the soil surface. I couldn’t help but watch, and I was rewarded with a fantastic show.

The bees almost all had loaded pollen baskets, and at first I wondered if they were feeding on something on the sand, because they would dip down to the surface, take a few steps, then fly away. I watched dozens of bees do this, but still couldn’t tell what they might be collecting.

Then I saw it. A bee landed, then quick as a flash, dove head first into the loose sand. It took her only a moment, and she had vanished, leaving nothing but a slight divot in the sand to show where she’d gone.

Then I knew. They were burrowing into the sand, provisioning nests for their larvae. They had dug the burrows earlier, and the brief touchdowns on the sand were to locate the right burrow. I watched for a long time, and saw several bees dive into their burrows. I even saw one go part way, decide she had the wrong spot, and scramble back to the surface to try again.

Then I saw another insect in the crowd—a wasp. It, too, was hovering over the sand and dropping down now and again to the surface. I surmised that it was a parasite, looking for the hidden bee burrows. I guessed it would enter a burrow and lay an egg on the bee larva, and the wasp larva would eat the bee.

These were all guesses based on my observations. I really didn’t know if there were dune-nesting bees here, or if they were parasitised by wasps.

At home, I was able to confirm my observations. My bees were the native Leioproctus metallicus, and they are parasitised by a gasteruptid wasp that lays its own eggs in the bee burrows.

I was pleased to have pieced together this puzzle by watching bees on the beach. (The only part I couldn’t see was that the bees try to fool the wasps by digging many burrows, not all of which contain larvae.) Entomologist Tom Eisner once wrote, “There is a saying that ‘5 minutes in the library can save you weeks in the laboratory,’ which has considerable merit. I prefer the naturalist’s version, which says that ‘weeks in the field can save you minutes in the library.'”

I’m with Eisner on that one. Watching those bees and trying to piece together what was happening was pure magic.