I noticed a lot of sawdust coming out of the wood stumps propped up and drilled to make habitat for mason bees. A stakeout with camera should solve the mystery.
After several minutes, a tiny iridescent black wasp zipped into a hole in the stump. Definitely not a mason bee! Getting a photo was like catching a launching rocket – these guys go in full speed and exit like they’re shot from the holes in the wood.
Luckily, I had time and at least with a digital camera I wouldn’t waste any expensive film. I noticed signs of emergence: the insects push sawdust from the holes as they excavate, then (sometimes) crawl out before zooming away. Others would sometimes hover in front of the hole before entering, and with bright sunlight I could stop the lens down to minimum and perhaps have enough depth of field for a clean enough image for an ID.
Eventually, I succeeded. Out of about 40 images, I managed to get five good shots. Once transferred to the computer, it was time to go wasp hunting.
A bit of poking around insect-oriented web sites, and there they were: Pemphredon, also known as aphid wasps.
Yes, another species that dines on aphids! These wasps nest in hollowed out pith in stems, but apparently a hole drilled in an old stump is a perfectly good nesting place for them.
The mother wasp collects a lot of aphids (one site says about 24 aphids per wasp larva), piles them in the hole, and lays an egg on them. Then she seals the chamber, fetches another two dozen aphids and repeats the process. The larval wasps feast on the aphids, eventually to emerge as wasps ready to eliminate even more aphids. There are a lot of wasps in this stump, and more nests in another stump nearby – all apparently packed full of incapacitated aphids in various stages of being devoured.
It’s always fun to discover another beneficial insect, especially when you didn’t even know these guys were out there. I couldn’t find much on the adults, so I don’t know how good a pollinator they might be. I do know that an old stump drilled with holes does double duty, harboring mason bees (other holes are full of telltale leaves) and aphid wasps. Both pollination and pest control in one convenient location!
It’s possible that I goofed on my ID for these guys, although they match the description more or less exactly. If so, perhaps some trained hymenopterist (is that even a word?) will come to my rescue with a definitive ID.