Damage to plants from adult cicada feeding is typically minimal, even for large, periodic emergences. Of greater concern is the damage that females cause through egg laying. Adult cicadas lay their eggs in woody tissue, under the bark. From the outside, these oviposition scars look like ragged cuts.

Second part of question is a bit trickier

Preventative treatments may be tempting, and soil treatments of insecticides have shown promise in reducing the number of egg laying attempts by female cicadas. However, preventative treatments must be made before insects are present. This means that they may not end up being beneficial if large numbers of cicadas do not appear. Some foliar insecticides may also repel adult cicadas.

For small plantings, netting is very effective at reducing or eliminating egg laying. In locations with young plantings a high likelihood of emergence, preventative treatment may be a good choice. For specific recommendations, contact your county cooperative Extension agent. Find your local Extension resources here at http://www.extension.org/.