Check your variegated Euonymus plants very carefully, inspecting both the stems and the underside of the leaves. Look for little tiny white specks that are firmly attached to the plant. Euonymus scale can be controlled if caught in the early stages. Once the infestations become severe they are difficult to over come and the plant is usually damaged to the point that it must be disposed of. Another insect that can quietly work away damaging your landscape plants without your knowledge are spider mites.
Spider mites are very tiny and extremely difficult to see.
Like the scale insects, spider mites are also sucking insects. They also reproduce by the millions. Most professional nurseryman alternate with two different chemicals when they spray for spider mites. They'll spray one chemical today, and ten days later spray a different one. That way they don't have to worry about the few spider mites that survived the first spraying building up a resistance to the chemical, and then breeding thousands of babies that are also resistant.