Question
I have 3 young locust trees that were planted when my home was built last September. They were thriving well and suddenly many of my neighbors with similar trees have lost their trees or many of the leaves on their trees.  Mine has many yellowed leaves with brown little spots like freckles on them.  The next door neighbor lost both her trees in a matter of two weeks and they were replaced.  What could this be and how do I treat it?

Answer
Sounds like locust leaf miner.
The primary and most conspicuous damage is caused by the leafmining activities of the larvae. Larvae feed inside the leaves on the inner layers. In the beginning the mine is small, but it is gradually enlarged until most or all of the leaf is affected. A single leafminer may attack several leaves. The mine or blister is at first clear, but later turns brown as the leaf dries out. Leaves damaged by the locust leafminer may turn entirely brown and cling to the tree for some time, giving the tree a blighted look. This damage is most noticeable in the late summer along major interstate highways where it will seem that miles of trees have brown leaves.

Ornamental and shade trees can be protected with a foliar spray directed either at the adults in late spring or the larvae as they hatch out in early summer. An August spray at the newly emerging adults that skeletonize the leaves and create the "fired" appearance would not totally protect the foliage, because early adult pinholing of the foliage and larval mining would have already occurred.
Systemic insecticides may provide the greatest control of leafminers. A systemic insecticide is absorbed into the leaf tissue, killing the insects inside. Non-systemic insecticides stay on the leaf surface and do not affect insects inside of them. Thorough spray coverage of the upper and lower leaf surfaces is especially important. Apply the pesticide according to directions outlined on the container label.

I would try either Bayer Advanced Lawn Complete Insect Killerm or Bayer Advanced Tree & Shrub Insect Control



Here is a web link to locust leaf miner damage.
http://na.fs.fed.us/spfo/pubs/pest_al/llminer/llminer.htm