Not necessarily as they must have come from somewhere else. I would have sprayed horticultural oils during the dormant season (winter) and then either horticultural oils or insecticidal soaps during the growing season to suffocate them. I would then plan on repeating them one or two more years and see if they are not needed afterwards.

These are common pests in the northeast. They "appear" in hydrangeas around mid May or later and then move on to camellias later on as they move to warmer areas.

https://extension.umd.edu/hgic/topics/cottony-camellia-scale-shrubs

you could have used a systemic rather than spray. might as well start digging. these are not considered fast-growing shrubs and the pests will not avoid them when they resprout, assuming they do.

Luis, due to health issues, I don't use any sprays in my garden. I have not seen this problem on hydrangeas at all. Seeing as Cottony Camellia Scale is very fond of holly and there is an enormous holly tree directly across the street, and I found a holly leaf with the pest clearly on it under one of my camellias, I deduce that this bug came from the holly. Jeff, as said, I don't use chemicals in my garden. This includes systemics, which aren't allowed for sale in Canada anyway. So I have as you suggested, removed all camellias from my garden. They are gone.