By Whitney Cranshaw, Colorado State University Cooperative Extension, Entomology
They may be little but they can be mighty.
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.
Cottony maple scale
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.
The most significant of these pests is the oystershell scale, so named because it resembles a minute oyster attached to the bark of trees. Aspen, ash, lilac, cotoneaster, willow and poplars are among the many plants in the region highly susceptible to this insect. Oystershell scales kill, often because of the increased susceptibility to disease caused when the scales sap the plant's vigor.
European 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
Pine 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.
Pine needle scale
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.
Eggs under scale insect
Horticultural 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.
Lechanium scale crawler
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. Below are some general guidelines to help you know when crawlers emerge in the Denver Metro area:
Pine needle scale Late April to early May (with a second generation in mid-to-late July)
Oystershell scale Mid-to-late May
Pine tortoise scale Early-to-mid June
European elm scale Mid-to-late June
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.
Photographs of lechanium scale, pine needle scale, oyster scale, scale eggs, crawler and cottony maple scale by Judy Sedbrook
Photograph of European Elm scale courtesy of Curt Swift.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010