Question
I have a young Sienna Maple tree.5" dia.
This summer it had little red bumps on the leaves. What are these bumps and should I be concerned?
Thank you
Fred

Answer
These are leaf galls caused by a small insect that lay its eggs on the leaf and secretes a chemical that causes the cells of the leaf to form a gall over the eggs. When the eggs hatch there is left a spot on the leaf. They cause no real health problems to the tree. The number of insect attacks is controlled by the weather and the inset population. There is no need to treat the tree. Efforts are best spent on good horticultural practices, such as watering and fertilizing, as needed, rather than spraying.  

Galls are irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites. Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf and twig galls are most noticeable. The inhabitant gains its nutrients from the inner gall tissue. Galls also provide some protection from natural enemies and insecticide sprays. Important details of the life cycles of many gall-makers are not known so specific recommendations to time control measures most effectively are not available.