I have seen a number of questions about white scale on ferns but none have suggested a cure which works in sub-tropical Queensland and that is to spray both sides of fronds with a mix of white oil at half normal strength plus a small spash of dishwashing detergent in water.

I always use a white oil (sometimes called Pest Oil) made commercially from petroleum oil (concentration is 80% petroleum oil)because the mixture is consistent however there are numerous recipes for homemade white oil, just google 'white oil ingredients'

You will probably have to spray again after 4-5 days to make sure all the bugs are dead but be careful after that because the dead scale does not drop off the fronds and you may be tricked into think they need another spray - washing with a strongish spray can work as can brushing or wiping.

The dishwashing detergent assist as a wetting agent and the miscible oil sticks to frond and scale better.

I hope this helps but remember HALF normal strength to avoid frond burn.

Regards, John