It makes them dry out and they die. I use Orthene or its equivalent. At this time I have found the white crawlers are widely seen.

Try the same thing I use on white scale (see earlier post). Mix white oil, you might know it as Pest Oil (80% petroleum base) with water at HALF strength plus a dash of dishwashing liquid.
Spray it on both sides of the fronds.
The miscible oil smothers the critters and the DW liquid helps it to stick to the fronds better.
You may have to repeat in 4 or 5 days but be sure before hand that the scale is not dead. A squeeze will tell you if they have dried up - if they have they are dead.
Even though this is a fairly harmless spray there is no point in using it more than you have to.
Remember, HALF strength mix to avoid any possibility of frond burn.
Best of luck, John