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Cottony Scale Insects on Ornamentals

- Scales are related to aphids and do similar damage -

Summary: Scale insects tap into plants and feed on plant sap. They weaken and kill plants over time. Cottony scale insects produce a cottony egg mass from which the crawler stage hatches. The crawler stage is the easiest to control.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Scale insects are closely related to aphids. Some scale insects are so highly modified they often don't even look like insects. Most appear to be legless and attached to the plant. These insects feed by tapping into the plant stem or leaf and removing sap. Like aphids, they are often associated with honeydew which supports the growth of sooty mold.

Some scale insects produce a cottony sac that contains hundreds or thousands of eggs (right and below). These are the cottony scales. Eggs generally hatch in early summer and release the crawler stage, the only highly mobile stage in the insect's life cycle. This is also the stage that must be targeted for effective control.
cottony scale insects and egg sacs

cottony scale insects with egg sacs (the white cottony masses)
 

There are many scale insect pests of ornamentals and they have complex life cycles and host plant interactions. The best comprehensive source of information about these pests on ornamental plants is Insects That Feed on Trees and Shrubs by Johnson and Lyon (see below).

Plant damage caused by scale insects

Plant damage is related to the sap that scale insects take from the plant. Over time as more sap is extracted the plant weakens, leaves may drop and eventually whole branches may die.

Plants that are stressed by drought, root damage or disease are better hosts for many pests including scale insects. These same stressed plants may be killed by high scale insect populations. In addition, sooty mold growing on the leaf surface can interfere with normal processes and can further weaken the plant.

Scale insect control

The most important part of scale insect control is timing. Control measures must be timed to coincide with hatching of the crawler stage which usually occurs in early summer for cottony scales. If properly timed, and good spray coverage is achieved, soft insecticides like insecticidal soaps and oils are just as effective as conventional insecticides. Over the long run soaps and oils may be more effective since they preserve the natural enemy complex which may account for most of the longterm control of these plant pests.

Look for scale crawlers

Scout plants starting in late spring. Use a hand lens to look for scale crawlers on the underside of leaves or near cottony egg sacs. Scale crawlers will be about the size of spider mites but amber in color. Once crawlers are found control treatments can begin.
cottony camellia scale

cottony camellia scale - egg sacs and sooty mold

It may take several seasons to completely control a severe scale infestation. Keep at it. Scale infestations often take years to develop and it is unlikely that you'll eliminate them overnight.

Identification of insect and mite pests of landscape plants

There is one standard reference book for identification of landscape pests, Insects That Feed on Trees and Shrubs by Johnson & Lyon (right). This is generally the first book I turn to for a difficult landscape pest problem. It has detailed life history information and photographs for most of the major landscape plant pests, including scale insects. The pest information is arranged by host plant so it is fairly easy to narrow down the problem. The photographs throughout the book are especially good at showing damage symptoms.

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Identification guides, reference and gardening books, etc.

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Mission: To provide accurate, up-to-date and unbiased information for solving common insect and mite problems around your home, business and landscape using least-toxic methods.

Jack DeAngelis, Ph.D.

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