Scale Insect Pests

They may be little but they can be mighty.Massi’s Gardens & Landscaping

They are scales, one of the most insidious insect pests affecting landscape plants in Colorado. They are tiny, typically inconspicuously colored and largely immobile. They spend their lives quietly removing sap from trees and shrubs.

Several important scale insects, however, can cause serious damage, causing dieback of branches and occasionally killing the plant.

Scales are close relatives to the more familiar aphids. The main difference is the presence of the plate-like scale covering, a waxy material secreted through pores on the back. This covering gives the insects substantial protection from the environment, as well as from many of the control sprays we might want to use. Because of this, scale insects often are one of the more difficult pests to manage.

Dozens of scales can occur. The knowledgeable gardener will become familiar with a few important ones.

Oystershell scale

Massi’s Gardens & LandscapingEuropean elm scale chronically occurs as a pest of the American elm. Anyone who's ever parked an automobile beneath an elm tree and returned to find small droplets of sticky honeydew on the car, knows how European elm scales work.

 

European Elm Scale

Massi’s Gardens & LandscapingEuropean elm scale chronically occurs as a pest of the American elm. Anyone who's ever parked an automobile beneath an elm tree and returned to find small droplets of sticky honeydew on the car, knows how European elm scales work.

 

Scales of pine and spruce

Massi’s Gardens & LandscapingPine and spruce can be affected by two common scales. Pine needle scale is an elongate, white insect that attaches itself to the needles of the spruce and several pines, notably mugho. This insect, occurring in great numbers the past few seasons, has resulted in premature needle drop on pine and spruce.

A more recently introduced pest in the state is the pine tortoise scale, a "soft" globular-shaped scale that occurs on the twigs of Scots pine and a few other pine species. Pine tortoise scale also produces large amounts of sticky honeydew, which attracts nuisance visits by yellowjackets and honeybees.

Control of scales

If scales are damaging your plants, you might need to take several approaches to control. For oystershell scale on small trees and shrubs, a simple and highly effective practice is to gently scrape the scales off the trunks and branches with a soft, plastic scrub pad. Once dislodged from the protective covering, the eggs soon die.

Massi’s Gardens & LandscapingHorticultural oils also are useful for controlling many scale insects. These are specialty oils refined to allow their use on plants and are sold in many garden centers under various trade names. "Dormant oil," "supreme oil," "superior oil" and "spray oil" are among the descriptions. Mixed with water to a 1 to 3 percent dilution, the oils cover the insects and smother them. Most oils are used during the dormant season, but some of the newer oils allow use after leaves have emerged. (Uses and precautions are on the label of each product. Read carefully before use.)

A well-timed "crawler spray" often is the most effective way to control scales. Newly hatched scale insects are called crawlers because during this brief period they are unarmored and mobile. In this state, they are susceptible to sprays of most insecticides. Common crawler treatments include Sevin, Orthene, malathion, insecticidal soaps and horticultural oils. The trick is properly timing the treatment because the crawler period is brief and soon is followed by the secretion of the protective scale covering.

Each season can differ, however, and crawler emergences varies across the state. The best way to determine emergence is by sampling infested plants. Do this by shaking branches that contain living scales and eggs over a piece of paper; examine for the presence of minute crawlers. Alternately, on some plants, you can capture crawlers by placing a piece of double-sided sticky tape on the branch.

 

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