Yard and garden: FAQs on maple trees

Iowa State University Extension

The tiny red bumps or gall-like growths on maple leaves are maple bladder galls and are caused by tiny foliar-feeding insects called mites.

AMES, Iowa — Maple trees can provide a beautiful addition to a landscape as well as shade for the owners. As with all trees and shrubs, they can get pests. Here are some common questions and answers about maple tree pests from Iowa State University Extension and Outreach horticulture specialists. 

Question: There are small, white objects resembling kernels of popcorn on the branches of my silver maple tree. What are they?

Answer: The small, white, popcorn-like objects are likely cottony maple scale. Cottony maple scale is an insect. It is most commonly found on silver maple trees. However, it can also be found on other maples, oak, linden, hackberry, honey locust and other trees. 

In June, female scales begin to produce large, white, cottony egg sacs that may grow to the size of dimes (up to 1/2 inch in diameter). Large numbers of egg sacs look like popcorn strung along the branches and twigs. 

The eggs within the expanded egg sacs (up to 1,000 per sac) begin to hatch in early July. The new scale nymphs crawl to the undersides of leaves where they feed on sap from the leaf tissue. The scales grow to adulthood on the leaves and mate in August or September before the females return to twigs to spend the winter. There is one generation per year. 

Cottony maple scale insects excrete a clear, sticky substance called honeydew. The honeydew drops onto leaves on the lower portions of infested trees. It also drops onto plants or other objects (patio furniture, cars, driveways, etc.) beneath trees. Oftentimes, a sooty mold fungus colonizes the honeydew, resulting in a black sooty appearance on leaves, branches and other objects. 

Cottony maple scale usually causes little harm to healthy trees. Natural predators will control the infestation within one or two years. Treatment of cottony maple scale with insecticides may do more harm than good, as the insecticide will kill their insect predators.

Q: There are erect, cone-like or roundish growths on the upper leaf surface of my maple tree. Should I be concerned?

A: The growths are likely galls. Galls are abnormal growths of plant tissue induced to form by mites, insects or other small organisms. The cone-like gall on the maple leaves is probably the maple spindle gall and more roundish growths are maple bladder galls.

Both types of galls range in color from greenish to red and are about 1/5 of an inch long. Maple spindle galls are as thick as the lead in a pencil. The galls are somewhat thicker in the middle than at the ends, hence the common name of spindle gall. Maple bladder galls are similar in size and have an irregular round shape.

Both types of galls are caused by extremely small eriophyid mites that are only 1/125 of an inch long. The adult mites spend the winter under the bark and other protective places on the trees. In the early spring, the adults move to the developing, unfolding leaves and begin feeding. The leaf responds to the small irritation by rapidly producing extra cells that form the abnormal growth at the feeding site. The gall encloses the mite, which continues to feed and lay numerous eggs within the gall.

Reproduction is prolific and as the new mites mature, they leave the gall and move to other newly emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. Adult mites leave the foliage in the fall and move to the overwintering sites. 

While galls are unsightly, they do not cause serious harm to healthy, well-established trees. Galls cannot be “cured” once they have formed. Preventative insecticide treatments are seldom warranted.

Q: There is a large growth on the trunk of my maple tree. What is it?

A: The growth on the trunk of the maple tree is likely a burl. Burls are abnormal swellings or growths that develop on the trunks and branches of trees. Burls can be found on deciduous trees and evergreens. The exact cause is unknown. Possible causes include bacteria, fungi, insects, wounds or environmental stress. 

Burls do not kill trees. However, they may reduce the tree’s vigor. On a positive note, the unusual swirling grain pattern found in burls makes them prized by woodworkers. Burls can be carved into bowls, furniture and other objects. 

Q: There are small, fuzzy insects on the branches of my maple tree. Are they harming the tree?

A: The white, fuzzy insects on the branches of your maple tree are wooly alder aphids. (The insect is also known as the maple blight aphid.)

Wooly alder aphids feed on the sap of maple trees from bud-break until late June. Then winged adults, some with abdomens covered in white fluffy wax, are produced in the colonies. These winged migrants readily fly when disturbed and create the illusion of tiny masses of cotton floating through the air. The winged adults leave the maple tree and fly to alders where they establish new colonies on the secondary host. Wooly alder aphids require both maple and alder trees to complete their life cycle. 

While the presence of white, fuzzy colonies of wooly alder aphids on a maple tree may cause alarm, they don’t cause serious harm to infested maples. Damage is usually limited to the loss of some leaves. Large wooly alder aphid populations usually collapse from predation and parasitism. Control efforts are not necessary.