Scale insects do a lot of damage to many kind of woody ornamental plants every year. Most kinds are tiny, hard to distinguish without a magnifying glass and are likely to be overlooked until the infested plant is nearly dead. Many have a protective wax shell which makes them hard to kill. They feed by pushing threadlike mouth parts through the bark and sucking the sap. Usually they settle down in one place when they are a few days old and remain there the rest of their lives.
Two of the worst are Oystershell scale and San Jose scale which feed on my feed on many kinds of plants, including privet, lilac, spirea and pyracantha.
Late winter is a good time to check trees and shrubs in the garden, particularly those doing poorly for some unknown reason, to find out if they are infested. Use a magnifying glass and inspect the bark for abnormalities. On trees they are usually on the younger branches and twigs at the top and it is only when a small branch is broken off by the wind and examined closely that they can be detected.
One of the easiest and most effective methods of eliminating scale insects is dormant oil spraying. By spraying when the plant is dormant, the oil can be used at greater strength without danger of injury to the plant.
One of the best oils for dormant spraying is Pratt's Scalecide, according to specialists. Directions on the label should be followed closely.
This oil should not be used on thin bark trees such as sugar maple, Japanese maple, beech, black walnut, butternut, hickory, redbud, juniper, palms and maidenhair ferns because they are almost certain to be damaged by it.
Scalecide is one of the least dangerous sprays the gardener can use. It is harmful if swallowed, but otherwise is safe for the applicator and for pets and wildlife.
Another advantage of the dormant oil spray is that insects have not been able to develop resistance to it.
The oil spray should be applied just before buds open on trees and shrubs in the spring. Do not spray when the temperature is likely to go below 40 degrees during the next 24 hours. A dry, sunny morning is preferred, when the temperature is between 40 and 65. The less wind the better the coverage will be.
The dormant oil spray is effective in destroying scale eggs and immature over-wintering adults, aphid eggs and over-wintering adults, mites and mealy bugs.
Eggs of mites, aphids and scale begin to stir as warmer weather approaches. As the eggs develop, their need for oxygen increases. There is also some correlation to the development of trees. As the trees begin to push out green buds, so in turn do the eggs develop. Because of need for more oxygen at this time, an application of dormant oil will prevent oxygen from being taken by the insects and will cause suffocation.
Lime sulfur is recommended for use on trees and shrubs which may be damaged by oil sprays. But don't let lime sulfur get on the paint of your house because it may stain it.
Do not spray just for good luck. It probably would be a waste of time and money. Spray only is you know you have an infestation and spray only plants knownto be infested.