For Release August 14, 2005

Trees Now Showing Summer Stress

by Chuck Otte, Geary County Extension Agent

You don't need a calendar to tell you that it is getting late into summer. Just start taking a look at the trees as you travel around. The leaves speak volumes to the many different stresses that are hitting them as we move through the dog days of summer and into the late summer period.

    When evaluating damage on tree leaves this late in the season, it is important to remember that most trees, at least the ones that are mature or several years old, have already stored up most of the food reserves in their roots that they need to get them through the winter and growing next spring. Damage at this time of year may be aesthetically un-pleasing, but not necessarily damaging to the health of the tree.

    Oak leaves, especially the native bur and chinquapin oaks, have a lot of leaves turning yellow, gray and brown. The trees just look stressed. If you turn one of these leaves over, you will often see many small insects that have a lacy look to their wings and there may also be lots of little black dots on the leaf under surface. The insects are called lace bugs and the black specks are their droppings. Sometimes called plant bugs, they feed by sticking their mouth part into the leaf and sucking out the juices. Each one of these feeding spots then dies and all these little spots sort of merge together turning the leaf off color and looking half dead.

    By the time the leaves are looking this bad, it's not going to do much good to spray the insects. Killing the insects isn't going to make the leaves improve their appearance and often by the time you see the damage, they are through feeding. With small young trees go ahead and keep watering them as they may still put on some new leaves before fall sets in. If this feeding damage bothers you, then you need to start checking your oak trees next year in late June and start treating when you first see the insects, not when you see the damage.

    Honey locust trees are also looking rather distressed right now. The browning foliage is often seen on the tips of the branches and often associated with amass of webbing that mats the leaves together. The culprit here is the Mimosa webworm. The larvae of this small moth pulls the leaves together in a mat and then feeds in a somewhat protected little package of leaves, twigs and webbing. By now it is virtually impossible to control them and again, while looking bad, the damage is virtually non significant to mature healthy trees. Treatment with a biological control such as Dipel or BT would need to be initiated in early to mid July to stop this pest.

    Finally, we are seeing, and will continue to see, more of the walnut caterpillar and the fall webworm. Both of these pests feed on the leaves of many tree species. They will develop large webby masses that will linger on well after the leaves have fallen and the caterpillars have spun their cocoons. At different times people have used torches and burned these webby messes or clipped off the branch ends. In many cases, this action causes more damage and misshapen trees than the caterpillar feeding damage would ever do. You can try to break up the webbing with a pole and spray insecticide up in them. You can hit it was a high pressure sprayer, either with an insecticide or just straight water. Or you can do nothing and ignore them.

    Many of the trees are looking stressed right now. But most of the real stress is coming from the homeowners who are fretting about what they are seeing. Relax and just enjoy those waning days of summer!


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