In the Garden:
Pacific Northwest
December, 2010
Regional Report

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Dormant season spraying helps ensure a healthy crop of apples.

Apple Tree Scale

My apple trees tend to develop populations of scale insects every few years. They are sometimes hard to detect, so I inspect my trees in winter when the branches are bare.

When a scale insect finds a likely place to live it attaches itself securely and spends its entire life in that same spot. Females produce clusters of eggs that are protected beneath their bodies until they are mature enough to crawl away in search of their own special spots on the branch. Because they spend their vulnerable youth protected by the mom%%%s hard outer shell, the offspring are safe from predators and from pesticides.

Scouting for Scales
The best time to inspect your trees for scale insects is during the winter months. If you find mysterious bumps on the branches, try scraping them off with your fingernail. If you discover a fluid or sticky substance on the bark, you've most likely found a scale insect.

Scale insects attach themselves to the twigs and branches of my apple trees, looking like lumps or bumps on the bark. If I find just a few, I squish them with gloved fingers. If there are many, I sometimes scrape them off with the side of my trowel. If the bark is young and tender, I resort to spraying the tree with horticultural oil. It effectively suffocates the scales, along with any overwintering insect eggs, without harming the tree bark.

Timing is Everything
When the weather begins to warm in the spring, just as the leaf buds begin to swell, an application of horticultural oil will suffocate the adult scale insects and you'll likely catch the offspring as they begin to crawl in search of their own new places on the branch.

You'll find several different formulations of horticultural oil in the garden center. Those sold as "dormant oil" are classified as "heavy oil" and are designed for use on bare trees and shrubs before they leaf out in the spring. You'll also find "summer" oils which are more refined and therefore lighter. These can be used on trees and shrubs that have leafed out. It's important to choose the right formulation for the time of year you intend to spray.

Lighter oil sprays, which have a higher degree of purity, are further classified as superior, supreme or summer oils. In many situations, these lighter oils can be applied both in the dormant stage of a plant's life or after bud break and leaf emergence.

To avoid problems with leaf burn, I find it easiest to stick with the dormant oils and apply just as the leaf buds begin to swell. That's also about the time the scale crawlers are searching for their new homes, so you'll get both the adults and their offspring with a single application. And you'll also suffocate any overwintering insect eggs which will help reduce insect problems later in the season.


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