According to Auburn University, the most common pest problems to affect hydrangeas include Japanese beetles, red spider mites, aphids, leaf tiers, rose chafers and oyster scale. Each of these pests leaves distinct signs on a hydrangea, so diagnosing the pest problem is simple. Once you have narrowed down the pest based on the signs, apply a pest remedy to rid your hydrangea bushes of the problem.
Mix detergent in a bucket with a garden hose. Add approximately 3 tbsp. of detergent per gallon of water to make a solution that is 2 percent in strength.
Place the bucket of soapy water next to your hydrangea bush.
Hand-pick Japanese beetles and rose chafer beetles off of your hydrangea bush, and throw them into the bucket of soapy water.
Squish any rolled or sandwiched leaves to kill any caterpillars nesting in the leaves. These caterpillars, known as leaf tiers, nest in the stacked and rolled leaves, eventually killing them.
Examine your hydrangea for signs of aphids. Ants crawling on hydrangea leaves are often a sign of aphid infestations. If you see ants, turn over the youngest leaves of your plants. The presence of small, pear-shaped, yellow or green insects means that you have an aphid infestation.
Mix an insecticidal soap according to the directions on the package. You can purchase insecticidal soap at a garden center.
Place the insecticidal soap mix into the holding chamber of a garden hose spray applicator.
Attach the garden hose to the spray applicator.
Spray your hydrangea bush with the insecticidal soap every two to three days, until the aphids are gone.
Oyster Shell Scale
Look on the upper ends of the stems for signs of oyster shell scale. Scale insects are typically covered with waxy shells that protect the insects while they feed. Insects may appear to be white, yellow, brown or black and will measure around 1/10 inch in diameter.
Purchase horticultural oil from a garden center. Mix the oil according to package directions.
Pour the oil into the holding chamber of a pressurized spray applicator.
Prime the applicator by grasping the handle and pumping up and down.
Grasp the holding chamber with one hand and the applicator wand with the other. Open the spray valve on the applicator wand, and apply the mixture directly over the oyster shell scale. The oil will coat the bugs and smother them without harming the plant.