Are the leaves of your cottonwood trees starting to turn brown and fall off?
You probably have an infestation of leafminers. They are usually the larvae of flies, moths, or beetles that feed or "mine" between the upper and lower epidermal leaf surfaces. The larvae tunnel through the leaf creating a narrow, whitish colored serpentine (winding) mine. They can be very destructive to the leaves of tree, as evidenced by the appearance of many of the larger cottonwoods around town. Sometimes the leaves will have "circles" in them like those in the photo to the right. If you scratch this open you can see the little white worms that are getting ready to hatch. The leaves are almost all brown, and are beginning to drop like it is fall.
Systemic insecticides usually provide the greatest control of leafminers. A systemic insecticide is absorbed into the leaf tissue, killing the insects inside. Non-systemic insecticides stay on the leaf surface and do not affect insects inside of them. That is why thorough spray coverage of the upper and lower leaf surfaces is especially important--- and almost impossible on these large trees. Of course it should also be mentioned that you should always apply the pesticide according to directions outlined on the container label. Unfortunately a systemic is not going to do that much good for you now. Because it has to leach down into the roots and then travel up the tree - it is best applied in the early Fall. That way it has time to do it's job before the eggs would hatch in the spring.
We will have a reminder notice in our enews in the Fall. Till next time, stay cool and in the shade if possible..............it's HOT out there. Susan