2022 update: There were lots of mosquitos first week of August 2022. We were in the M loop and we were being eaten alive. Bug spray with DEET helped a little, "natural" bug spray was useless. Citronella candles were almost useless. We didn't have a Thermocell, so no way for us to know if that worked or not. Talking with other campers, the south loops will suffer from mosquitos longer than the north loops because of the proximity to the lake. So, keep that in mind when booking. This year, 2021, we camped for nearly a week during the first calendar week of August and no mosquitos and no flying bugs. We didn't have any use for bug spray. We have been told that June and July are the mosquito months and to avoid the park then or come prepared.

Not sure? I was told by a local about them and then sure enough they were there, but they didn't really seem to be biting.

Not sure about August but in May there were, but not bad.

There was apparently a big issue in 2016 but the park has since taken measures to prevent a major infestation.(https://stateparks.oregon.gov/index.cfm?do=v.news-story

I have been several times in the area and never seen mosquitos while there.