About a month ago, Piero and Jill, our neighbors across the street, presented us with a mystery: the seemingly ubiquitous appearance of tiny blobs of clay on their cars, their front porch, and on cars up and down our block of Holly Street, a couple blocks from North Berkeley BART.
Where had these blobs, probably in the thousands, come from? The theory I came up with: Maybe the dirt had been precipitated out from dust in the atmosphere. You know, dust that had been picked up in the Gobi Desert, say, and blown in the stratosphere clear to Berkeley, where it rained down on our street.
The idea isn't entirely loony: Dust from Asia and Africa is known to play a role in precipitation over California's mountains. But in that case, we're talking about minuscule particles that serve as nuclei for ice crystals that later fall as snow or rain. (Yes, sometimes there's so much dust in the air that it will precipitate as a muddy rain -- but that's different from what we were seeing on Holly Street.)
Kate, science teacher and certified California naturalist, appears to have come up with the most probable answer to the blob source: yellowjackets. Here's what the University of California's Integrated Pest Management site has to say about the nesting habits of these wasps:
Yellowjackets commonly build nests in rodent burrows, but they sometimes select other protected cavities, such as voids in walls and ceilings of houses, as nesting sites. Colonies, which are begun each spring by a single reproductive female, can reach populations of between 1,500 and 15,000 individuals, depending on the species.
The wasps build a nest of paper made from fibers scraped from wood mixed with saliva. It is built as multiple tiers of vertical cells, similar to nests of paper wasps, but enclosed by a paper envelope around the outside that usually contains a single entrance hole. If the rodent hole isn’t spacious enough, yellowjackets will increase the size by moistening the soil and digging.
The writeup doesn't say what the yellowjackets do with the material they excavate. But a Georgia gardening website does. In trying to answer readers' questions about the source of mysterious dirt balls, the site consulted an entomologist a University of Georgia entomologist, who said:
The yellowjacket is almost certainly the culprit here.
First is the time of year. Nests are expanded rapidly and grow almost exponentially during late June through September. To allow for this expansion the original nest hole must be greatly enlarged to accomodate the growing nest that will ultimately be at least soccer ball-sized and often larger.
After a good rain, excavating activity often approaches a frenzy level, and if you watch the traffic at the nest entrance 7 out of every 10 wasps will emerge with a chunk of clay in their jaws. They always airlift it generally out to within a few yards of the nest and drop the pellets like small bombs from several feet high, then immediately return to repeat the process. In this way they make room for their nest to grow, and it takes a lot of mouthfuls of mud to do so!
When Kate found this description, Piero said he'd been seeing a good number of yellowjackets around. Unknown, so far as I've heard, is whether the nest this industrious group has been working on has been located.
The photo above: a closeup of one of the clay blobs in question; below, to give an idea of scale, how they looked on the rear window of a Volkswagen Bug. More photos here. (Regarding the picture above: More alarming to me than the mystery blob is all the crud surrounding it; that, no doubt, is simply our normal urban fallout of dust, grit and particulate byproducts of burning hydrocarbons. We're breathing that stuff.)