Hydrangeas (Hydrangea spp.) grow in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the variety. Unfortunately, pests, such as mealybugs, live in those same climates. They infest plants indoors as well as outdoors, so managing an outbreak on your hydrangeas early is key to saving the rest of the plants in your garden and house.
A type of scale insect, mealybugs are sometimes difficult to find and control. They hide their tiny -- less than 1/4 inch -- waxy bodies on the undersides of leaves so you can't see them without turning leaves over. They pierce the leaves with their mouth parts and drink the sap, literally sucking the life out of your hydrangeas. It's difficult to see just one mealybug, but they tend to congregate in groups, making them easier to see.
Finding the Problem
If you don't have time to check under every leaf, every day, let your hydrangeas tell you when there's a problem. When leaves suddenly turn yellow and drop off, the plant stops growing or blooms quit forming when they should be flourishing, start your underside-leaf hunt for mealybugs. Also look for black mold developing on the leaves and blooms -- mealybugs excrete a sticky substance called honeydew, which encourages the growth of sooty mold. Groups of mealybugs resemble cotton wool attached to the bottom of the leaves.
When you first discover mealybugs on your hydrangeas, it might be early enough to control them without chemicals. Dipping cotton balls or cotton swabs in alcohol and wiping the backs of leaves can remove the mealybugs. You can also use a piece of cloth dipped in soapy water. Spraying the leaves with jets of water from a garden hose can dislodge the bugs, too.
When the infestation is large, it's time to break out the chemicals. Keep your kids and pets inside when you treat the plants to make sure they don't touch or eat it, just to be safe, and wear gloves when applying pesticides. Systemic chemicals work best for mealybugs because their waxy coating prevents pesticides from soaking through their skin. Systemic pesticides stay in the leaves and enter the pests' systems when they eat. Check the pesticide label to ensure it is effective against mealybugs and works with hydrangeas.
According to the North Carolina State University Cooperative Extension, pesticides containing acephate are good choice. Mix 1 1/3 teaspoon in 1 gallon of water, or according to the label instructions. Pour the mixture in a garden sprayer and drench the infested plant, soaking the root zone well. Check back every few days to see if more mealybugs appear -- acephate won't kill eggs.