Question
We live in the Park Ridge area (a suburb of Chicago) in Illinois, and in the last few days have noticed that most of the trees in the area are showing brown clusters of dead leaves, from a small amount to sometimes covering half the tree, evenly spaced, almost like a checkerboard.  Most trees in the area are affected, perhaps oak trees more than most.  What is the reason for this and how harmful is it to the trees affected by it?  There were a lot of cicadas recently but supposedly they don't cause harm to the trees.  So what is going on??

Answer
If you are talking about there are small dead twigs in the foliage that are scattered then I would say if you had a large population of cicadas this is the cause. Periodical cicadas damage trees above and below ground. The most obvious damage is that caused by egg laying in small twigs. This damage causes twigs to split, wither, and die, causing a symptom called "flagging." Flagging is especially serious on young plants (four years or younger) because more of the branches are of the preferred size for oviposition, 1/4 to 1/2 inch in diameter. Some of the more favored trees for oviposition include maple, oak, hickory, beech, ash, dogwood, hawthorn, magnolia, willow, apple, peach, cherry and pear. It is the female's egg laying that creates the most visible damage.  Each female cuts slits in the twigs of trees and lays eggs in them.  The eggs hatch and the young nymphs fall to the ground to find a root to feed on for the next 17 years.  The wounded branches are weakened and often break off or wilt.  While the damage is noticeable, it is not life threatening except for very young or very small trees.

The trees will out grow the damage and after the dead twigs fall off there will be no noticeable effect to the health of the trees.