Question
What can be done for a locust tree that seems to be dying because of some little green bugs.  Thanks.

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.  

The adult is a small flattened beetle that is about 5-6 mm long. The head is black and the thorax and the outer margins of the elytra are orange. It has a prominent black dorsal stripe running down the middle of the back. The elytra are deeply pitted and have three longitudinal ridges each. The eggs are flat, white and oval. They are laid in small clusters of three to five on the underside of the leaves. The yellowish larvae are flattened as well.  

The larvae create mines (tunnels) between the top and bottom leaf layers, which is where they feed.  Eventually, the larval feeding causes the leaf to dry and turn a rusty-brown color.  The brown leaves will remain on the tree for some time, giving an off-color to the trees.  The larvae pupate in the leaf mines and emerge as adults in mid-summer.  There are two generations of beetles per year.

The adult beetle feeding has little affect on the tree health.  However, the mining by the larva can cause growth loss or death in trees under stress.  Typically, parasites and predators will reduce the leafminer populations and chemical control rarely is warranted.

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 like Orthene 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.

There is a newer product that should work on these insects called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the tree and the roots transport it to the leaves killing the insect. It is also used a preventive to keep the insects off if used early in the spring. Check with your local nursery fro this product.