Scale
Cactus scale diaspis echinocacti,
an armored scale on Spineless Prickly Pear
Scale, what are they?
Scale are common pests (insects) that colonize on your plants and feed on the sap and nutrients in plant tissue. There are around eight-thousands of different scale species. We have about 1000 species in North America. There are two main types of scales: armored scales and soft scales.
Soft scales have softer, irregular-shaped bodies 1/8"-1/4" and secrete honeydew. If you see black sooty mold growing on your plant, you may be dealing with soft-bodied scales. Most soft scales can move around the plant more easily than armored scales. They also don't stay around during winter like the armored scale.
Armored scales are much smaller than soft scales, usually 1/16"-1/8". These guys are usually round or oval-shaped; some can blend in with the plant they're feeding on. They sport a hard, waxy protective covering that protects their bodies. Armored scales can overwinter on our plants, making it very important to inspect them regularly.
Control
We first need to identify what type of scale we are dealing with. After reading about the two types above, if you still can't tell, Flip an adult scale over. You've got an armored scale if there's a soft body beneath a hard shell.
Note: Avoid synthetic insecticides and take care with contact insecticides as much as possible to avoid killing our good bugs, some of which are scale's natural enemies!
Soft scale treatment:
On soft scales, insecticidal soaps will do the trick. Treatments specifically for armored scales will also work, but you don't need more than insecticidal soap or a homemade Dawn and water mixture.
Armored scale treatment:
Spray contact insecticides:
Most contact insecticides cannot penetrate the waxy covering on the armored scale. However, horticultural oils may kill some of them by suffocation. Horticultural oil is one of the safest options for us and for beneficial insects. Care must be taken when using horticultural oils in the heat, whether by diluting the mixture or testing a spot on the plant before treating the whole thing. Ideal temperatures are between 45 and 85 degrees. Always spray insecticides in the early evening to avoid burning plants in the sun.
Most contact insecticides will take multiple applications depending on how bad the infestation is.
Rubbing Alcohol on a cotton swab, a paintbrush, or in a spray bottle can also be effective on soft and armored scales. When spraying, use a 1:5 alcohol-to-water ratio with a squirt of Dawn. Some even say a 1:1 ratio is okay on non-sensitive plants. You may have to remove armored scales after they die.
Systemic insecticides (applied to soil):
Products containing the active ingredient dinotefuran will work for soft and armored scales. It may be beneficial to combine this treatment with another contact treatment for improved control.
Ortho Tree & Shrub Insect Control Granules contains Dinotefuran.
Systemic foliar insecticide:
Products containing Acephate are effective in treating armored scales. Bonide Systemic Insect Control Concentrate contains Acephate.
Conclusion:
Scale insects suck and can be challenging to treat if you have armored scales. Inspect your plants regularly to avoid having to treat a heavy infestation.
Soft scale on Lemon tree leaf
By Amada44 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=60065956
Soft Scale on Bougainvillea leaf
By Jacopo Werther - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20156166
Black Scale on Draceana
By Secretlondon - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=112498056