Scale insects on shade and fruit trees as well as shrubs can usually be controlled in late winter with a dormant oil spray according to David Robson, University of Illinois Extension horticulture educator, Springfield Center.

Dormant oil is lightweight petroleum oil usually sold as dormant oil, superior oil or Volck oil spray. It is applied to deciduous woody plants while they are dormant to kill exposed overwintering insects. Cottony maple, lecanium, euonymus and San Jose scales, as well as European red mite, are the most common pests controlled because they are difficult to control at other times of the year.

Scales are protected from insecticides for most of the year by either a hardened body wall or a secreted waxy covering. Outside of dormant oil treatments, scale insects are only susceptible to pesticides as crawlers shortly after egg hatch.

The crawler stage lasts only for a few days before the insect settles down into a leaf or branch to feed. It then molts to the next nymph stage and develops its protective covering.

Mites are difficult to control during the summer because they reproduce quickly and are not controlled by most insecticides. Even the use of miticides may kill mite predators that were keeping the harmful mites in check.

When dormant oil is applied, it covers the overwintering mite eggs, the overwintering scale insects and any exposed insect eggs, Robson adds. This coating of oil shuts off these animals' air supply and suffocates them

This pesticide is used while the plant is dormant because the oil can also be toxic to the plant. Applications before leaves start to emerge reduce this problem.

For the same reason, dormant oils should be applied only if the temperature for the 24 hours after treatment stays above 40 F, so that the oil quickly evaporates off the bark.

Evergreens and hard maples, such as sugar maple and Norway maple, should not be sprayed with a dormant oil spray due to possibilities of damage to these plants. However, it is possible to spray Euonymus vines (Big Leaf Wintercreeper) and lilacs to control scales on them.

Don't worry if the leaves fall off the plants on the Euonymus. They would have dropped when new growth starts in a few weeks.

Dormant oil only needs to be applied once every three or four years. Make sure to spray the areas beneath the plant as well.