Scale insects are sap-feeding pests that derive their name from the scale or shell-like waxy covering over the insect's body. The protective covering makes scale insects difficult to control through 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.
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. 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.
Here are some of the common scale insects found on ornamental plants in Iowa along with the approximate time the crawler stage is active and the phenological indicators you can watch for to better time your spray applications.
- 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.
More information on common scale insects, the timing of the crawler stage and available controls is available in publication IC-415, Scale Insects on Ornamental Plants.