This is both a plant and insect question. I manage a property that has a small stand of planted English and Garry oaks. This summer, there has been a very large population of bees and yellowjackets attracted to these trees. There isn’t a nest or hive on the 5 acre property that I have found, so these bees seem to be coming in from a distance. Indicators that I am observing are:
Many sap (sucker) holes on these trees, one in particular.
A heavy black dusty coating on most of the leaves.
The sound of droplets, not humming or eating.
Based on your description, my first thought is that the coating on the leaves sounds like sooty mold. First comes the sticky honeydew left by leaf-sucking insects such as aphids, and then the sooty mold develops on the sticky substance. It would make sense that bees and wasps might be attracted by the honeydew, but might also be preying on the smaller insects that cause it.
This article, which comes from a beekeeping perspective, may also be of interest. Here is an excerpt:
“Although it may be off-putting to think of eating insect excrement, honeydew honey is prized in parts of Europe and New Zealand and often fetches higher prices than floral honey. It is rich in mineral content, amino acids, and may have stronger antibacterial properties. When a psyllid insect or aphid ingests the plants sap, it digests the small amount of protein present and expels the rest of the water, sugars, tannins and other indigestible material as honeydew. If you have ever parked your car under a tree and found it covered in a sticky substance the next morning, chances are the tree is infested with sap-sucking psyllids or aphids. For many other insects including ants, wasps, and bees, this is a valuable food source.”
Your poetic description of “the sound of droplets, not humming or eating” might be the sound of transpiration from the leaves, or just the honeydew (part tree sap, part insect excrement) seeping from them.
I work on a restoration site and this fall I have been noticing weird fuzzy growths on many of the Nootka roses (Rosa nutkana) growing there. Do you know what is causing this? And is it related to similar strange growths on all of the Scotch broom? In the case of the broom, it actually kills them completely—they turn brown or black, and their roots are pretty much non-existent, which makes them very easy to uproot (which is what we are trying to do). I just don’t want to lose the roses or other desirable plants on the site.
What you are describing on the roses sounds like mossy rose gall (Diplolepis rosae). Washington State University Extension’s HortSense page says that these galls which are caused by cynipid wasps will not harm the host plants. You could picky them off the roses, but that seems impractical in a restoration site, and besides they are fairly benign and attractive curiosities. The particular species of cynipid wasps which cause it are unlikely to affect plants which are outside the rose family.
Your other question about dying Cytisus scoparius (Scotch broom) is especially interesting. I think what you are seeing on those browned and blackened plants is also a gall, caused by the Scotch broom gall mite (Aceria genistae). This insect is apparently on the cutting edge of controlling invasive broom. According to this informational page from University of California Statewide Integrated Pest Management Program, the mite was first seen on broom plants in Tacoma, WA and Portland, OR in 2005. It has since spread through the Pacific Northwest. An abstract of an article entitled “The Scotch Broom Gall Mite: Accidental Introduction
to Classical Biological Control Agent?” (J. Andreas et al.) appeared in the 2011 XIII International Symposium on Biological Control of Weeds. Studies are underway to see if the mite affects non-target plants such as lupine. For now, you can rejoice in the fact that the mites are curbing the growth and reproduction of the broom, and making your work a little bit easier!
A database to find native plants by zip code which is sorted by the type of plant (herbaceous or woody) and by the number of moths and butterflies supported. Created by the National Wildlife Foundation.