Just reporting an observation...
Yesterday evening, shortly before dusk, there was a steady humming from one of my belts of cypress (Cupressus sempervirens). It was similar to the buzzing from a large number of bees (eg. when you get a large number in a citrus grove), but a slightly deeper tone. As I approached the trees, I could see dozens of flying insects, bigger than bees. They were just flying around the branches, somewhat manically, I thought. They largely ignored me. It was frustrating trying to see what they were because they did not seem to be landing.
Then, by following one or two, I happened to find a female, who was sitting on a stem nearby (a Pelargonium stem as it happened). A few males were scrambling to mate with her. I presume similar goings-on were happening in the branches of the cypresses. I could see the insects were beetles, and from their shape and colour they looked like European chafers (Rhizotrogus majalis); about 15mm long - much smaller than the cock chafers many gardeners will be familiar with. The
evening swarming behaviour is also correct for this species.
These critters
feed mainly on the roots of grasses. Since I cultivate no grasses, I do not consider them a pest. Obviously, other folks may have a different opinion!
If I manage to get a photo, I'll add it here....
update 20may12: success (a tad out of focus, mind)