Scale Insects

Mid-May is the start of the time when we can control many of the scale insects found on ornamental plants in Iowa. These sap-feeding pests derive their name from the scale or shell-like waxy covering on their bodies. Scales may attack most species of shade and fruit trees and ornamental shrubs and may be found on plant stems, twigs, foliage or fruit.

Sap loss due to scale insect infestations may cause yellowing or wilting of leaves, stunting or unthrifty appearance of the plants, and eventually, death of all or part of the plant when infestations are heavy. A more noticeable symptom for some scales is the sticky, shiny honeydew excreted by the scales or the black sooty mold that grows on honeydew.

Scale insects can be difficult to control because they are protected by the waxy or cottony covering for most of their life. Therefore, great emphasis is placed on understanding the life cycle and applying controls at the time the eggs are hatching and the pest is vulnerable to insecticides. The new scale nymphs are called "crawlers" because they crawl about on the plant before settling at a new feeding site.

Scale control on heavily infested shrubs or vines should begin with pruning out and discarding heavily infested stems. Then spray the remainder of the plant at the time of the crawlers to complete the control process.

The time of the crawler stage is fairly predictable but varies with the weather in spring and early summer. This year, for example, scale crawlers are going to be delayed from "normal" the same as other springtime activities. One way to account for year to year weather variation is to use phenology (known influences of weather variations on plant and animal rhythms) for timing the sprays.

Thanks to years of careful observations by several researchers in the Midwest we know that specific insect activity such as egg hatch coincides with the blooming of certain plants in the same area. By watching the phenological indicator plants (even though they themselves are not infested with the scale in question) you can accurately predict when a particular scale's eggs hatch will hatch in your area. Information given below is adopted from the book Coincide: The Orton System of Pest Management, by Donald A. Orton. Plantsmen's Publications, Flossmoor, IL, 1989.

In the following list of some of the common scale insects found on ornamental plants in Iowa is the approximate time the crawler stage is active and the phenological indicators you can watch for better timing of spray applications.


Insect Time Active Indicators
Pine Needle Scale mid to late May vanhouttei spirea in bloom
Oystershell Scale mid May to early June vanhouttei spirea has finished blooming.
Euonymus Scale late May Japanese tree lilac and catalpa in early bloom.
San Jose Scale mid-June Japanese tree lilac and catalpa in full bloom.
Lecanium Scale mid to late June Hydrangea 'Grandiflora' in full bloom, and Yucca filamentosa blooming
Cottony Maple Scale Early July Hydrangea 'Grandiflora' in full bloom, and Yucca filamentosa blooming

Insecticides for control of scale insect crawlers include the following: insecticidal soap, horticultural oil (summer rate), acephate (Isotox or Orthene), carbaryl (Sevin), malathion, diazinon, and chlorpyrifos (Dursban). Check label to confirm product is labeled for the plant to be treated. Always read and follow label instructions for directions on mixing, usage and application safety.

This article originally appeared in the May 17, 1996 issue, p. 81.


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