AirBnB does NOT reimburse you for an insect infestation. That was their answer when I asked them before I started hosting.
IF YOU REPORT THIS TO AIR THEY MAY SHUT DOWN YOUR LISTING until you pay mucho bucks for an exterminator to certify that you no longer have them.
If the folks have been there that long and didn’t notice it before, THEY brought them in from somewhere or they would have seen them earlier. Do any of them work in hotels, been traveling lately, or purchased used clothing, bedding or luggage?
Don’t hire an exterminator that sprays chemicals, because they don’t kill them. The ONLY thing that kills them is to heat the entire space and its contents to 180 degF for 12 hours or more. Good Bedbug exterminators use heat which also kills the eggs. Chemicals will just make them disburse further into the rest of the house.
But before you do anything else, first make certain that they are actually bedbugs. Get a bedbug trap which uses pheromones to attract them and compare the pictures with some from a reputable insect site. I once had some tiny wood weevils crawl into a bedbug trap (they are there to be checked after each guest leaves to make sure they didn’t bring any in) and guests of the peel-back-the-throw-rug variety left without telling me and went to a motel (which HAS had many bedbug problems). Air was convinced when I sent them comparative pix, but that was back when they had a staff that actually answered the Superhost line on one ring.
Here are the traps:
SenSci ActivVolcano Bed Bug Monitor and Lure - 2 Pack - Volcano Bed Bug Monitor and Activ Bed Bug Lure