If you live in the west, then you are familiar with all the varieties of oak trees that grow here. California is particularly known for its redwoods and sequoias, and rightly so, but those grow in very specific areas, while oaks grow across the state in four or five different plant communities that are widespread. It took me a long time to understand California's plant communities, but here is a blurb from Wikipedia to help explain this one:
"California oak woodland is a plant community found throughout the California chaparral and woodlands ecoregion of California in the United States and northwestern Baja California in Mexico. Oak woodland is widespread at lower elevations in coastal California; in interior valleys of the Coast Ranges, Transverse Ranges and Peninsular Ranges; and in a ring around the California Central Valley grasslands. The dominant trees are oaks, interspersed with other broadleaf and coniferous trees, with an understory of grasses, herbs, geophytes, and California native plants. Oak savannas occur where the oaks are more widely spaced due a combination of lack of available moisture, and low-intensity frequent fires. The oak woodlands of Southern California and coastal Northern California are dominated by coast live oak (Quercus agrifolia), but also include valley oak (Q. lobata), California black oak (Q. kelloggii), canyon live oak (Q. chrysolepis), and other California oaks. The foothill oak woodlands around the Central Valley are dominated by blue oak (Q. douglasii) and gray pine (Pinus sabiniana)."
Didja get that? :)
Here in our yard, we have a Valley Oak, which is no surprise, as we live very near open space dominated by Northern Oak Woodland. It was here when we moved in 12 years ago, and has grown quite a bit in that time, despite the fact that I've done pretty much everything wrong in taking care of it (planting edibles underneath it, and then adding regular irrigation, pruning it at the wrong time of year, etc). It's now larger than any other tree in our yard despite my ignorant attentions.
In the fall of our first year on this property, I noticed the above pink growths on the oak leaves. I took the leaf in to a local nursery and asked what it was. They very helpfully explained all about gall wasps, tiny microscopic insects that inject a chemical into the oak leaf or branch, and then lay eggs into that area. When the eggs hatch, the area swells to accommodate the larvae. I have been into nature my whole life, and a California resident since I was 20, and a hiker pretty much for forever, and I never knew this until an oak was in my yard. It surprises me still when folks don't know about galls, and I'm always so glad to share about them, because they are fascinating. Good old Alfred Kinsey was originally an entomologist and studied these creatures extensively before he moved on to sex.
Every year since, I've noticed new kinds of galls on this same tree. It's fun, every fall, to see what's there. Gall wasps are good guys, and these little nests don't hurt the tree, so there is no reason not to welcome them.
These pictures (not my best effort, sorry, all very high up and I was teetering on a ladder) were all taken this evening, after my mother and I were standing outside talking, under the oak tree. Suddenly we both noticed the huge amount of honeybees flying around the tree. I've NEVER noticed bees in the oak tree before. Oak trees are largely wind pollinated, like many trees, and I've honestly never even really noticed the blossoms on this tree, though I do of course notice the acorns (which have just started to get big - it is clearly NOT a mast year, as there are only a few).
So we looked closer and definitely saw the bees landing and feeding on these little knobs. To me, they looked for the world like galls, but why would a bee feed on a gall? It just doesn't make sense. So I promised mom I would immediately do some research, and sure enough, with some digging, I found the answer.
There's not a whole lot of information on the interwebs about these particular gall wasps. But one thing is clear: In this particular kind of gall, the larvae turn the plant starches that its feeding on into sugars, which then exude from the gall and form honeydew on the outer surface of the gall. This is to attract ants, wasps, and yes, honeybees, so that they will guard and protect this food source and stop other parasitic wasps from invading the nests! Isn't that incredible???
We don't usually think of honeybees as honeydew eaters. When I see the word honeydew I think of aphids and ants, and that symbiotic relationship (aphids produce honeydew to attract ants so the ants will protect them from predators). But apparently if it is available, and food is scarce (which in the late summer and early fall is always true here in hot dry CA), bees will take advantage. It's unclear how much nutrition it provides for them, and it's unclear how it affects their tummies; in fact some large-scale beekeepers do everything they can to prevent the bees from eating it, because it can turn into a hive-sized diarrhea issue! But of course I'm not going to do anything extreme. I'm just going to watch and wait and see what happens. Like I said, I've never seen this before, so maybe these wasps only came to my tree this year for the first time!