I have a very large Staghorn ( about 100 fronds /5 ft diameter), and recently found scale. It has lost about 15 fronds but I have gone over every frond and scrapped off the scale with finger nail, But , a week later I see more young scale ; I am thinking of applying oil with a cloth rather than spray . ( About 5ml /liter of water) . Any alternatives?
This article Scale Insects on Staghorn Ferns might be useful. You've done the right thing so far by chipping them off with a fingernail, but these ferns are pretty sensitive to treatment for scale.The recommendation is to use horticultural oil or insecticidal soap as a spray, but it's important that you do not allow any sunlight to reach the plant after its been treated for some days, because this will increase the risk of damage from the treatment you've used.