The best product I have used for killing and preventing scale, especially boisduval scale, is Azatin, containing an ingredient from the Neem tree. It's a bit on the pricey side, but a little goes a very long way. Just add a few drops to a standard-size hand sprayer and mix with water. If the scale is a major infestation, add a teaspoon. Adding a little sticker/speader, mineral oil or even a drop or two of dishwashing liquid, will ensure excellent coverage. Just do a search in Google to locate vendors.
Cheers and Good Growing, Paula in Boca Raton, Fl _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com