Magnolia (Magnolia spp.) trees or shrubs adorn outdoor spaces across U.S. Department of Agriculture plant hardiness zones 4 though 12 with stunning, often-fragrant spring or summer blossoms. Hundreds of magnolia species and cultivars offer a vast range of habits and sizes and an equally impressive array of flower colors and forms. Even the most spectacular magnolias, however, are susceptible to infestations of sap-draining mealybugs and the unsightly black mold that accompanies their presence.
Obscure and Gill's mealybugs attacking magnolias form colonies on their branches, leaves and shoots. Although the former are pink and the latter gray, telling them apart is difficult because of the mealy white wax protecting them. Obscure mealybugs, however, have noticeable filaments edging their flat, oval bodies. Gill's mealybugs, on the other hand, often wear a shroud of whisker-thin transparent filaments resembling clumps of fiberglass. Large mealybug populations disfigure magnolias with what looks like patches of cotton wool. Although obscure mealybugs lay eggs in protective sacs and Gill's mealybugs give birth to live young, both species produce multiple generations of larvae, or crawlers, each year.
Mealybugs drain magnolia sap with sharp, tissue-piercing mouthparts. Prolonged feeding by large mealybug colonies deprives the plants of nutrients and moisture, stunting their growth and causing early leaf drop. These pests also excrete undigested sap as a gummy, transparent waste called honeydew. Ants flock to mealybug-infested magnolias to feed on the honeydew and defend the pests from their natural predators. Honeydew also attracts airborne sooty mold fungi that cover the goo with layers of powdery black mold. Heavy sooty mold may weaken a magnolia by interfering with photosynthesis.
Blasting your magnolia with a strong spray of hose water dislodges some of the mealybugs and rinses honeydew from the plant. Repeat spraying may be enough to keep the pests' numbers at harmless levels. Lady bugs, lacewings and syrphid flies all prey on mealybugs. To encourage them, rid your magnolias of ants with slow-acting boric acid bait. Dab small amounts of apple jelly and peanut butter on your magnolia's honeydew-coated leaves. Mix 1/2 cup of whichever one attracts the most ants with 1 1/4 teaspoons of boric acid powder and place the bait in a clean jar. After puncturing several holes in the jar's lid, seal it with tape and place it near your magnolia where children and pets won't find it. The stream of honeydew-collecting ants should begin tapering off in about a week.
If hosing down your magnolia and eliminating the ants aren't enough rid it of mealybugs, consider using an insecticidal soap solution. Mix 5 tablespoons of non-detergent, liquid soap in 1 gallon of water and spray the magnolia thoroughly on a cloudy day to kill any mealybugs the solution reaches. Rinse the plant with water after a few hours to prevent soap-related sun damage. Apply two more sprays over the next week to destroy pests the first application missed. When the mealybugs and their honeydew are gone, the sooty mold on your plant slowly starves and dries up.
- Magnolia Society International: Getting Started with Magnolias
- University of California Statewide Integrated Pest Management Program: Magnolia
- University of California Statewide Integrated Pest Management Program: Mealybug Species
- University of California Division of Agriculture and Natural Resources: Ferrisia Gilli: A New Mealybug Pest of Pistachios and Other Deciduous Crops
- University of California Statewide Integrated Pest Managment Program: Grape -- Mealybug
- University of California Statewide Integrated Pest Managment Program: Mealybugs
- University of California Statewide Integrated Pest Management Program: Sooty Mold
- Missouri Botanical Garden: Mealybugs -- Outdoors
- Montana State University Extension IPM Center: Ants in the Yard and Garden
- Colorado State University Extension: Insect Control -- Soaps and Detergents
- Photo Credit Jupiterimages/liquidlibrary/Getty Images