Scale Insects: Naughty, Nice or Neutral?

To help protect your outdoor trees and shrubs from scale insects, there are many organic control methods you can try.

| December 2004/January 2005

  • Scale Insects
    Tiny brownish bumps on the fruit, leaves and stem are scale insects feeding.
    Illustration courtesy Tom Quirk

  • Scale Insects

Scale are small creatures closely related to plant hoppers, whiteflies and aphids in the order Homoptera. The scales have tiny beaked mouths that suck juice from plants, finding food in a manner very similar to aphids. With the naked eye or even a hand lens, you would wonder if they were insects at all, for they do not have the outward appearance of a bug. From 3 to 4 millimeters in length, they appear like tiny turtle shells with no visible legs or wings, and they don’t show the more typical division between head and abdomen. In fact, the females are unusual because they have no wings at all, and often no legs.

To help protect your outdoor trees and shrubs from a scale infestation, paint Tanglefoot on cardboard, or something comparable, and wrap it around the tree trunk. This keeps away ants, which “farm” the scale and drive off the scale’s natural predators, parasitic wasps. Different scale species have their own companion parasitic wasps; if ants are not present to interfere, the wasps will usually kill the scale.

For houseplants, your best defense is to examine the plants carefully before you buy them, and then monitor them after you get them home. If you notice clusters of bumps on the leaves or stems, see if they rub off. If the bumps rub off and leave a sticky residue, you’ve just discovered a scale colony.

Many different species of scale exist, and all are susceptible to the same organic controls. As with aphids, scale produce a sticky honeydew (which is what attracts the ants), and if the honeydew builds up on leaves, it can turn dark with an unsightly black mold.

If mold has built up on the honeydew secretions, small amounts can be washed off by hand or with a gentle hose spray before you treat the plant. Mold on fruit sits only on the surface, so it can be washed off safely, too.

The best organic control of scale is a horticultural oil spray. The oil sprays are highly refined vegetable or petroleum products. Scale breathe through pores in their skin (again, similar to aphids); the oil plugs up the pores, suffocating the insects. If used in early spring on outdoor plants, it will suffocate scale embryos, too, and early application should prevent the mold from forming.

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