I walked through terrible mosquito clouds on the VDLP one September. A local told me they usually come out in August, but they were later that year (or maybe it was that they were staying around later). They weren't the kind of mosquitoes that we have in the U.S. in that their bites didn't leave you with large, itchy welts. They were smaller and blacker, but got on EVERYTHING. Even though I had a wide-brimmed hat on, in the worst spots (forests where there was little breeze) I had to walk swinging my trekking poles in front of me, or else they'd be in my eyes, ears, nostrils, etc. When I took out my camera and took the lens cap off, they'd immediately be all over the lens. It was awful.
This was the stretch from Salamanca on. The worst areas/forests were around Puebla de Sanabria to about A Canda.
Note: I didn't have bug spray, so maybe I would have fared much better if I had some. Also, locals told me the bugs were called mosquitoes. I don't know if there are different kinds, as Johnnie Walker's description of them sounds like the kind we have in the U.S., and these definitely looked quite different and didn't have bites that left the typical itchy welts. In fact, you just felt little pinches but I didn't end up with hardly any kind of marks on my body. Still, they were annoying.
Melanie