Mystery Clay Blobs in Berkeley Neighborhood


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.)



Filed under Berkeley, Science

3 Responses to Mystery Clay Blobs in Berkeley Neighborhood

  1. Gari

    We observed the particles on California St. as well. The breadth of spread of the dirt particles extended at least to California St. behind you and down Jaynes. The coverage on our patio cover, roof and window sills, and on the sidewalks around us, was uniform in distribution. The fallout occured during a time of significant sidewalk construction on Holly, California, Jaynes and Saccramento. I still feel it is dust kicked up into the air by construction, aglomerating and falling in a fairly even pattern across our neighborhoods.

  2. Could be. I’ve worked and lived around construction zones many times, as well as having grown up in an area with dirt roads and lots of farm activity, and the dust that I’ve seen settled in those areas looks like just that — dust.

  3. Yellow jackets nested under a rose bush beside my house this summer. I wasn’t surprised to discover this – for weeks I had noticed a lot of them buzzing about in my backyard – but the way it went down was unfortunate: I disturbed the nest while watering and the nasty buggers came after me in a swarm. I ran crazily into the house, swatting them away as I went and tearing off my shirt, which one had somehow gotten under. But they nailed me in five spots, ranging from knee to ear. That was some painful shit and, worse, a day or two later, exceedingly itchy!
    Since these f—ers were obvious threats to life and limb, I undertook a campaign to destroy the nest. At first I put a cap (a kitchen bowl) over the nest entrance – this was suggested on a few websites as a chemical-free way to take care of the problem. It didn’t work. The ground under the rose was layered with old leaves and other organic matter and the actual opening to the nest was far below. They just tunneled through the fluffy stuff and made a new entrance/exit at one spot on the edge of the bowl. So this is the spot I began targeting with a wasp/yellow-jacket spray sold at the local supermarket. The spray is best suited for the hanging-style nests that you might find under eaves; you wait until nighttime when they’re all home in the nest watching Fox News or whatever it is they do in the evening, and then drench the nest from 25 feet away. Still, I figured I’d give it a try with my ground nest.
    The morning after the first after-dark chemical assault, there were slow to get going, but by noon were buzzing in and out. After sortie No. 2, they were quiet for a full day, but then came back. It ultimately took three $5 canisters of the spray to get the job done. After a week of no activity, I removed the bowl and found an impressive mound of clay pellets around the nest hole. These weren’t mottled like the ones you picture, but were smooth. Like pebbles. So that’s the point of my story: My yellow jackets left their clay blobs right at the next entrance/exit, and they were smooth, not mottled.
    As a PS: After I removed the bowl and marveled at the mound, I pushed down on it with my foot, trying to work it into the hole. My thought was that if I left the hole, some of their ilk might come back next spring. I was surprised to find the ground give way a few inches. I kept feeding material into the hole and pushing down and the ground kept giving away, spreading out from the entrance/exit. Clearly this was quite a subterranean complex they had developed. Ultimately I had to shovel on soil from elsewhere to stabilize and level the ground.

Leave a Reply

Your email address will not be published. Required fields are marked *