Cottony maple scale (Pulvinaria innumerabilis) is an insect that lives its life cycle on many landscape trees. It favors silver maple but will also infest other maple varieties, linden, honey locust as well as other species. Most often this insect is a curiosity and nuisance rather than a true pest. However, there are times when this insect population gets out of control and can cause serious damage to landscape trees.
The Life Cycle of Cottony Maple Scale
Immature female scales are flat, brown, oval insects that attach themselves firmly and almost imperceptibly to twigs and branches of host trees. They have no noticeable wings, legs or antennae which further helps them blend into the tree’s bark. Once mature, these insects produce cotton-like egg sacs. These sacs can contain over 1,500 eggs.
Beginning in mid-June and extending through August, the eggs hatch and crawlers emerge from the sacs. These larvae move to the undersides of leaves where they feed for the remainder of the summer. In the fall, as the insects mature, they move to the twigs and branches of the tree where they attach themselves and start the cycle again.
Damage from Cottony Maple Scale
Generally, cottony maple scale is merely an oddity and a nuisance. The cottony egg sacs may be unsightly to some but most often are barely noticeable. In cases of severe infestations, an entire branch may be covered in egg sacs, diminishing the aesthetic value of the tree.
At the crawler stage, the insects excrete honeydew which is a mixture of sugar and water. Honeydew is sticky and can coat cars and patio furniture. If left on surfaces, honeydew supports the growth of sooty mold which is a grayish black fungus. Honeydew can also attract bees and wasps to honeydew-covered patios and decks.
If a tree is stressed or otherwise weakened, a heavy infestation of cottony maple scale can further weaken the tree. In cases of severe infestation a tree can suffer yellowing of leaves, premature defoliation and branch dieback. On rare occasions a heavy infestation can kill a weakened tree.
Controlling Cottony Maple Scale
Cottony maple scale has many natural predators and parasites that usually keep this insect under control. If the infestation is small, treating with pesticides runs the risk of seriously damaging these beneficial insect populations for several years. Destroying these beneficial insects can actually make cottony maple scale infestations much worse.
Maintaining tree health is always important to stave off damage from infestations and infections of any kind. Properly water and fertilize trees to ensure vigor. If egg sacs are present, simply remove and destroy them to prevent them from hatching. If an entire branch is heavily infected, it may be beneficial to prune out and destroy that branch.
If a tree is too large or an infestation is too heavy, pesticides can be applied. Mature insects and egg sacs will not be affected by pesticides so it must be applied when the insect is in the crawler stage. Applying pesticides too early or too late will result in destroying beneficial insect population while leaving the scale population unharmed. Using insecticidal soap has less impact on beneficial insects and should be considered. Imidacloprid is also indicated for cottony maple scale.
Cottony maple scale normally will not seriously harm trees. It’s more of a nuisance for homeowners. Understanding this pest is the first step to managing it in the landscape.
Sources:
Shetlar, D. J. Cottony Maple Scale and Its Management. FactSheet HYG-2019-95, Ohio State University Extension.
Krischik, V. and J. Davidson. IPM of Midwest Landscapes. “Pests of Trees and Shrubs: Cottony Maple Scale.”
Paula Lovgren - Paula is a freelance writer, gardener, garden designer and mother to two children and three cats. She has previously worked as a marketing ...
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