Question
I have five maple trees lining my corner lot.  Two of them have to be over 3 stories tall.  They both have something growing on the leaves that look like pimples.  Another maple seems to have an aphid problem because it is losing its leaves in July and they are sticky.  I would like to know if either problem is major or should I just let nature take its course?  We have had a dry, cool summer but have recently had rain, should that help?

Answer
The "pimples" on the leaves are galls from an insect. the small wasp like insect lays its eggs on the leaf surface and secretes a chemical that causes the leaf cells to grow over the eggs. They hatch and leave the spot on the leaf. these are not a real health hazard to the tree and the population of gall wasp vary year to year and are not a real concern.

The sticky sap is either a scale or aphids--both will suck the plant juices and secretes a substance called Honeydew. This is high in sugar content and sticky. For large trees there is a product called Bayer Advanced tree and Scrub Insect Control that is applied to the soil around the tree and the roots absorb the insecticide and transports it to the leaves and stems and when the insect feeds on the plant juices they are killed. here is a web link to this product.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html

If the sticky sap does not bother you AND the trees foliage looks healthy (except for the spots) You can let the aphid population go. But over time the population may build up to the point that the sucking will effect the tree. Damage to look for is curling leaves smaller leaves than normal, heavy sap flow and a black sooty mold buildup.