Guest Author - Susan Taylor
I’ve just come inside from the greenhouse and am thoroughly disgusted with myself. I didn’t follow my own advice and quarantine some new plants and now I have an infestation of scale. For those of you who have never had it, scale is one of the most important of orchid pests and one of the most difficult to treat. I don’t think that it is possible to actually get rid of it once you have it, but it is controllable.
The first sign that you might have scale will usually be small round yellow spots on new leaves. Or a sudden total yellowing of older leaves. If you have a small collection and consistently check your plants on a weekly basis, you should be able to spot these problems before they get to be too bad.
Look at the yellow spots both on top and bottom of the affected leaf. If you have scale you will see a small pinhead sized or smaller brownish dot. This is the female scale and if left in place she’ll form a hard protective shell with eggs for the next generation under it. Take a Q-Tip with either rubbing alcohol or diluted dishwashing soap on it and gently rub until the insect comes loose. The liquid will kill the insect, but if there are eggs under the scale it will not kill them. Carefully check the whole plant if you find even one scale on it. Chances are that there are many more that you will not see. They especially like to get under either green or brown sheaths, in the crevasses of leaves, wherever you have tied up growths, in new growth and around the roots. If you can, soak the whole plant, pot and all to kill all mobile nymphs and males. Remove all female scales you can and then quarantine the plant. It will take a minimum of two months to completely eradicate them from a plant using the means above every week.
If you see an older leaf that suddenly goes yellow, check under the leaf right where it attaches to the pseudobulb or stem. If you have scales, you will see masses of white cottony male scales. They can be killed with either alcohol or soapy water. If you have males, females are there too—you just can’t see them. See above for information on what to do.
DO NOT ignore the signs of scale. It’s not going to go away and it’s not going to get better. For more information on how to treat scale including insecticides and growth inhibitors, The South Dakota State University has a wonderful article Scale Insects on Orchids by Paul Johnson. Although this article is directed toward northern orchid enthuiasts, it is applicable to southern growers also.