Updated: 3rd February 2021
I sometimes get asked for advice about bee nests beneath garden decking. It's usually a bumble bee nest, and the person asking for advice is wanting to carry out some repairs to the decking, or in the case of the query below, wanting to wash and stain the deck.
So, if this is your concern, you may find the following useful.
Here is a summary of the query:
Firstly, thank you for caring about the bees, and yes, I would be concerned that a power washer would harm the nest and the bees, and the oil treatment wouldn't do them much good either.
It would be better and very kind of you if you could wait until the nest is no longer active. I can't be certain when that will be. It all depends how established the nest is already, as to how long it will be before the new queens have left the nest (they are the last to emerge and ensure future colonies of bumble bees), but it could take a week or two, or longer.
You'll know when the nest is no longer active, because the nest will cease to be buzzing with bees going back and forth. At that point, it should be okay to clean the decking. If you can get access, you could take a look to check and see. If not, don't worry, just keep an eye on things with the nest.
In the meantime, if you are concerned about being disturbed by bees or being stung by bees flying around the decking, you could take simple actions to encourage them to fly in a different direction from seated areas (for example), by strategically placing plant pots or other objects on the decking (without concealing access and entrance to the nest for the bumble bees).
However, I must say, we have both a manuka bush and jasmine very close to our seating area on the patio. Both are busy with bees foraging on the flower (along with the odd wasp) and they never bother us.
I hope this helps, and thank you again for caring.