Scale insects are divided into three groups: (1) armored scales, (2) soft scales, and (3)
mealybugs.  The armored and soft scales are one of the most destructive groups of insects that attack ornamental crops.  Mealybugs are not generally considered a problem on most woody ornamentals. 

        (1) Armored scales secrete a waxy covering over their bodies.  This covering is not an integral part of the insect's body.  Scales live and feed underneath this covering.  They vary in size from 1/16 inch to 1/8 inch in diameter and can be almost any color, depending on the species.  Armored scales may be circular, oval, oblong, thread-like, or even pear-shaped.  The female's armor is larger than that of the male, while the shape and color may be similar or distinctly different, depending upon the particular species.  There are over 350 species of armored scales in the US.

        (2) Soft scales also secrete a waxy covering, but it is an integral part of their body.  Soft scales vary widely in color, size, and shape.  They range from 1/8 inch to 1/2 inch in diameter and may be nearly flat to almost spherical in shape.  There are over 85 species of soft scales in the US.

        Eggs of scale insects resemble fine grains of sand and are found under the bodies of the adults.  The larvae are mobile with six legs (crawler).  The adult scale is heavily armored and it is necessary to look under this armor in order to see the living scale body.  Individual scale insects undergo a single cycle from egg to adult but populations consist of many generations of individuals and these separate generations may overlap.  Adult scales attach themselves tightly to the plant and often remain on the plant long after death.  Adult males emerge, mate and die, so it is generally the adult females that are observed on the plant.  The adult females also die, soon after producing eggs, so it is most productive to target scale insects during the more vulnerable crawler stage.

        Keep plants clean by periodically checking your plants and using a brush or a spray of soapy water on dirt and bugs. 

        Careful, periodic inspection of plants is critical to the prevention of major infestations of scale insects.

        Destroy the plant if the infestation is very severe.

Defeating with Sprays
        White oil (or any vegetable oil will do), soapy water, lime sulphur and baking soda.  
To these basic sprays you may wish to add herbal deterrents of your own, for example: garlic, mustard, pennyroyal, eau-de-cologne mint, rue, wormwood or pyrethrum.

Good spray coverage during plant dormancy or when crawlers are active is the key to successful control of scale.  Bark as well as foliage should be treated.

        Neem oil and organic insecticidal soaps are also very effective but may damage delicate flowers buds and kill important pollinating insects.

These sprays should not be used at bud burst or anytime during flowering (post fruit set is fine).  If crawler emergence coincides with bud burst, it is best to apply oil and soap mixtures with a sponge to avoid flower damage.  Scale can also be scraped off with a soapy toothbrush.

Treated scale may be hosed off after several weeks.  This will also help remove any black sooty mould present.  Avoid hosing off during crawler stage as juveniles may be easily dispersed to neighbouring plants.  Hosing off is a strategy best applied after spray treatment to remove insect carcasses.

        Natural enemies of scale include parasitical wasps, Chilocorus lady beetles, spiders and lacewigs and predatory mites.  Small birds also feast on scale.

Scales sources:

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