How to Treat Scale on Shrubs

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Scale insects feed off of plant sap with an appetite that can kill off branches and even entire shrubs. They appear as black balls of immobile wax, so they are not immediately identifiable as insects. Their excretion is a honeydew which is consumed by ants. The ants return the favor by protecting the immobile scales from predators and parasites. Therefore, one strategy to eradicate scale is to eradicate ants. Another is to use their waxy adhesion to branches against them, with the slipperiness of soap.

Things You'll Need

  • Toothbrush
  • Mild detergent
  • Duct tape
  • Purchase a mild detergent, perhaps something ecologically friendly like Simple Green non-toxic all-purpose cleaner. Simple Green needs to be water-diluted 30-to-1 for scale application.

  • Apply the diluted detergent manually to the scale insects with a toothbrush. They should become slippery and detach from the branch. If they don't, increase the concentration of detergent.

  • Deter scale insects from returning with monthly application of horticultural soap or oil. This can be sprayed effectively; this is the way commercial groves treat scales.

  • Look for new scales periodically. Expect to have to brush on more detergent every half year or so, to intercept scales after the migration stage (by flight) of their life cycle.

Tips & Warnings

  • There are many techniques for getting rid of ants, the scale insects' guardians--for example, poisoning the anthill with boric acid dissolved in boiling water. A barrier can be set up to block the ants' access to the shrub, with a collar of duct tape or a band of Tanglefoot wrapped around the bottom of the shrub's trunk.
  • Details for the best timing of the year for scale treatment can be found on the UC Davis webpage.
  • Don't confuse Simple Green detergent with Simple Green Lime Scale Remover, which treats limescale from hard water, not scale insects.
  • Commonly used as a barrier for indoor ants, diatomaceous earth may not be a good idea on outdoor ants. It could desiccate scale-eating insects on which the ants prey.

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