The hawthorn mealybug feeds on the sap of twigs and small branches. Heavy infestations weaken plants and cause branch death. Black sooty mold grows on the large amounts of honeydew produced by the bug, and greatly detracts from hawthorn’s appearance.
White, waxy clumps scattered among the bark scales of the trunk and large branches are another sign of hawthorn mealybug infestation. Much of the insect’s life is spent on the bark and only the young or nymph stage feeds on the leaves.
Because this insect spends so much time under loose bark and in other protected sites where insecticide is difficult to apply, it is difficult to control. Predators include three species of lady beetle and the green lacewing larvae.
The best time to attempt to control hawthorn mealybug is in early May when crawlers are moving from overwintering sites in bark crevices to feeding sites in twig crotches. Insecticides containing imidacloprid applied to the soil provide excellent control. Dormant season oil sprays in the winter may provide some degree of control.
For more information, see the following Colorado State University Extension fact sheet(s).