If the reservation is active (guests are there) you can also use the "change" option on the reservation and amend the price in the pricefield of the change form. (Airbnb service costs are also recalculated)
Yes, this sounds weird. I have a few flies coming into the house during summer as I usually leave the back doors open when I am home, but an infestation of them in late December in Wales? Have you managed to find out what the root of the problem is because, otherwise, you may have to keep issuing compensation to future guests.