There are erect, hair-like growths on the upper leaf surface of my maple tree. Should I be concerned?

FAQ
Question: 

There are erect, hair-like growths on the upper leaf surface of my maple tree. Should I be concerned?

Answer: 

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

Maple spindle galls are yellowish green and about 1/5 inch long. They 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 spindle galls are caused by extremely small mites that are only 1/125 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, such as the maple spindle gall, are unsightly, they do not cause serious harm to healthy, well-established trees.  Galls can not be “cured” once they have formed.  Preventative insecticide treatments are seldom warranted.