One of our landscapers brought a white oak leaf in yesterday which had been "skeletonized" meaning the foliage has been carefully devoured leaving all the veins behind. This appears to have been done by a small black caterpillar which is smooth and threadlike measuring about 1/4 inch long. There were some traces of webbing. Just as we choose the tastiest foods, the caterpillar has chosen to eat only the tender part of the leaf leaving behind the tough and fibrous veins. Pretty smart critter.
One of the primary functions of the leaf is to produce food for the plant through the process of photosynthesis. The loss of foliage can interrupt the food production process and cause damage to the overall health of the plant. In the case of this white oak, the problem has occurred late in the growing season, thus food has been produced through most of the growing season and the leaves will stop production naturally in about 6 weeks. For this reason, there will be a lesser impact on the health of this particular plant. This defoliation is not good for the plant, but the damage may be within our tolerance level because it is so late in the season.
In horticulture we often refer to Integrated Pest Management (IPM). This is the process of evaluating the problem relative to the affected plant and developing an overall control plan, if needed at all. IPM employs mechanical, cultural, biological, and, as a last resort, chemical control measures to contain a pest within the boundaries of an established level of tolerance. Tolerance levels are usually set by the health threat to the plant as well as the level of aesthetic damage the gardener is willing to accept. In most cases perfection is not needed and the "live and let live" principal can be applied to the pest. In the event that the tomato horn worm is demolishing your tomatoes, control measures are needed. In the case of the oak leaf skeletonizer, the need for control measures are probably unnecessary and impractical.
Additionally, what percentage of the foliage has been destroyed? You must perform a through evaluation prior to making a decision. Picture a canvas painted green and then evenly brush tan strokes over the entire surface of the canvas until you have covered 5% of the canvas with tan strokes. Now step back. The impact is dramatic. Look at all that brown!
The white oak leaf in this case is from a group of mature trees with heights of 50 to 60 feet. How could you spray these trees with a pesticide? If you are capable of spraying plants of this size, how much of the environment surrounding the trees will be affected by such a massive introduction of pesticides? Additionally, sprays would also kill the parasitic wasp which is a natural predator of the skeletonizing caterpillar. Do your homework before you take drastic measures.
My conclusion in this case of the oak leaf skeletonizer on these white oaks would be to monitor the situation without action. Let nature take its course.
Andy Lynn