Minna was inking the last panel in stream 1,5 weeks ago. I asked whether Rey is looking at Tuuribird and she answered on the lines of “No, he’s actually looking at... something else”. Obviously she might just lie
In any case I’m not buying the mosquito theory. He could slap a mozzie but wouldn’t look that surprised. A childhood memory from the lake (from the very area if the island is Selkäsaari like I think) offers the option of a horsefly. Their bite hurts, enough to be a surprise too. But as mentioned above, nothing in this comic happens without a reason, so that seems a bit mundane as an explanation. Although Minna could just put it in to have some slapstick for us.
Now I had another idea, although perhaps a bit far-fetched. What if Onni can “enlist” some local bugs to work as sentries (similar to what the sentinel mages do, although perhaps just a few and for shorter periods) and he made one of those sting Reynir, who is then surprised by am unprovoked attack from an unexpected bug, or can somehow see it’s not a normal bug. At first I thought of a tiny drone, but that’s implausible in the setting