[xoxoxoBruce]

I checked...

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.

The female's ovipositor slices into the wood and deposits the eggs. One to several dozen eggs can be laid in one branch, with up to 400 eggs being laid by each female in 40 to 50 sites.
Cicada eggs remain in the twigs for six to ten weeks before hatching. The newly hatched, ant-like nymphs fall to the ground where they burrow 6 to 18 inches underground to feed.

Here's a cut away of two egg clutches. Now I see how they do so much damage.