This is a tiny (2-3mm) lady beetle (Coccinellidae) in the genus Psyllobora (Fungus-eating Lady Beetles). It is found region wide in the PNW, although it is more common west of the Cascades. This is one of the smallest of the lady beetles, alluded to by the fact that Psyllobora means ‘flea of the North’ in Greek. The species epithet vigintimaculata means 20-spotted, although that isn’t evident on this species in the PNW, because many of the spots are joined (confluent). In fact the joining of the lateral spot to the median spot is diagnostic for this species in our region. P. borealis is the only other Psyllobora in our region, but its lateral spot is much smaller and always separate.
20-spotted Lady Beetles feed only on powdery mildew (PM; family Erysiphaceae) at all life stages. As such farmers and gardeners of all stripes and sizes consider them to be a highly beneficial insect, because unchecked PM infestations can not only drastically reduce production, but the consumer consumption of this mildew can have serious consequences, if the consumer is part of the 10% of the population that is allergic to mold.
I found this specimen sweeping my net through a mixture of Cape Jewelweed, Stinging Nettle, and Reed Canary Grass, so I didn’t observe the PM it was feeding on. And I almost dumped it out, because I mistook its tiny form for a seed. But something in the pattern made me look closer, and I realized it was some kind of beetle. I really didn’t think I’d be able to identify it, at least not without help, but after a little refrigeration it was cooperative for these photos, which turned out decent, although hardly great. Then I paged through “Pacific Northwest Insects” by Merrill Peterson (a truly outstanding field guide which every bug hunter in our region should own; I have 2 copies so one is always close at hand), starting with the leaf beetles and then going to the lady beetles, where I found its cousin P. borealis, with a note about P. vigintimaculata having joined spots. A bit more research and Voilà!, I had a positive identification.
Size- 2-3 mm
Habitat- Wherever powdery mildew is present; usually high humidity micro habitats
Range- Region wide, more common west of the Cascades
Eats- Powdery mildew obligate as larvae and adult
Flight Season- Adults can be found year around; may not be evident during the coldest and wettest months