Aphids distort the new growth and coat the leaves with sticky honeydew. Spray with Sevin, rotenone or malathion. The insects can be dislodged with a high pressure water spray from the garden hose.
Four-lined plant bug causes round, brown, sunken spots on the leaves. The injury is often thought to be a disease. Sprays of malathion control the pest although they will be needed but rarely.
A leaf tier webs the leaves over the tip of the branches. Some formulations of diazinon have a label for hydrangea leaf tier. The insects may also be picked off by hand.
Rose chafers are light tan with red, spindly legs, though they can be darker. They occur in large numbers where soils are sandy. Chemicals are ineffective because more rose chafers quickly move into a treated area to replace those killed by pesticides. If you want to try chemical treatment, use methoxychlor or rotenone.
Oystershell scale infests the upper stems of hydrangea and often goes unnoticed. Sprays of dormant oil should control overwintering stages and are less harmful to predators that help control scale. Control crawlers with sprays of malathion or Sevin. The preferred method of control is a combination regular inspection to detect the infestation at an early stage, then use dormant oil.
Mites cause yellowish foliage.