It is nearly impossible to predict what sort of numbers of cicadas you will see in any one area, but the potential is there for damage to trees, especially young trees.  Young trees are tender, the weight of netting cold cause more injury than cicadas would.  Use it if it can be safely suspended to avoid breaking shoots and branches.  Pesticides will not work well against cicadas, as they feed very little and don't get much of a dose of a poison.  The damage to trees comes from their egg laying habits- they jab wounds into branches and then shove eggs into the wounds.  They can do this quite a few times before getting enough of the pesticide on them to kill them.