If spider mites were a problem in your yard last year, apply horticultural oil to the trees and shrubs that had the infestations in late winter or early spring before bud break. These tiny pests probably won't be active yet, but the eggs or the red or orange adult females will still be hidden in the bark of the plants. Adult spider mites are about the size of a period, or smaller. It will be difficult to see the eggs or adults, so just spray the plant if it had a problem last year. Avoid blanket sprays to all ornamentals, however, because spider mites are controlled by many beneficials.
Scale insects are easier to spot before trees and ornamentals leaf out.
These insects construct a protective cover over themselves and attach it permanently to the trunk, branch, twig, or needle of the plant. Some are light colored, some are brown or grey and can be difficult to see. Scales affect evergreens, fruit trees, shade trees, and many ornamentals. High populations can damage trees, so an application of horticultural oil is in order if you find them. Applications of other insecticides will be ineffective unless the young happen to be crawling about without the protective shells.