Why have I been seeing healthy trees with patches of dead leaves?

This is called flagging! It can be caused by a variety of things, according to the Missouri Department of Conservation website, from weather-damage to insects to fungus and other diseases. At this time of year in this area, the flagging was probably caused by the periodical cicadas.

We checked www.cicadamania.com and In Ohio’s Backyard: Periodical Cicadas by Gene Kritsky for more information. They explained that cicadas don’t eat that leaves, and, while they use their mouthparts to suck sap from the trees, that isn’t what’s causing the flagging. The female cicadas use a pointed appendage called an ovipositor to deposit their eggs in the new growth on the ends of tree branches, preferably deciduous trees along the edge of a forest or otherwise in full sunlight. Sometimes this causes the branches to break and droop down, causing flagging. Small or young trees are at the most risk of permanent damage, but most trees will bounce back once the dead branches drop off. It’s in the best interest of the cicada not to cause permanent harm to the tree, because their young will feed on the juices from its roots for seventeen years as they develop. (The young cicadas do not remain in the tree branches. The eggs hatch after six to eight weeks and tiny nymphs fall to the ground, eventually tunneling a foot or more into the earth.) If you see small lengthwise slits on the branches, these are oviposition scars, a good sign that the flagging was caused by cicadas. Again, it’s likely that the tree will soon be back to normal. Protect it from further stress by making sure it has adequate water and pruning it only very lightly until it’s dormant again in the winter.

For other tree troubles, The Tree Doctor: A Guide to Tree Care and Maintenance by Daniel and Erin Prendergast is available for borrowing at the library.