Extension Service: Sidewalk stains mean your oak is bugged

I've never lived so intimately with oak trees and didn't realize how messy they were. Oak foliage and other parts contain chemicals called tannins. One can tell certain events are happening within the oak canopy by the brown staining which occurs when the tannins in the oak debris react with your alkaline concrete surfaces.

When part of the plant lands on your nice clean driveway or sidewalk, a brown stain appears after a few days. I have an ongoing study to track the source of what causes the staining, because I travel this obstacle course of debris and stuff, barefoot, to get to the morning paper. Most people know that acorns cause stains and mess in November and the flowers (catkins) and leaves drop in March. Then what is happening all of the other times to cause the unsightly staining?

So far, the research on my two oaks, which bombard my sidewalk and driveway, have revealed that it is insect activity.

Right now little red weevils, Attelabus species, are vandalizing our oak leaves. Look closely and I bet that you will find some of their leaf-rolling activity. The female meticulously clips the tip of the leaf and rolls it so that it resembles a miniature enchilada. This leaf-roll is pretty amazing when you consider that this little insect is working with a pretty thick leaf.

At some point she inserts an egg and the larva develops inside this leaf-roll. Often these little rolls drop off and leave stains. Earlier in the year, I noticed that the little granules in the brown stain on the driveway was frass. Yes, many caterpillar species feed in the canopy and one can tell when they are up there by the increase in staining activity on your clean sidewalk. So far the most common caterpillars have been the pink-stripped oakworm and the oblique heterocampa (yes, that is a moth larva).

This analysis helps me rationalize the "Why is this happening — again?" syndrome that I have every time I crank up the leaf blower and clean the driveway before the stains set and are harder to remove. At least we don't have gypsy moths, with those caterpillars you are talking piles of frass!

Maybe only a handful of people care as to what causes the staining. For those few, I hope that you have been elightened about your oak tree ecology and will appreciate the wildlife that it supports.

Who cares about a few rusty spots anyway?They will bleach out in the sun after a few months, or you can speed the process along with a little diluted Clorox solution.

Doug Caldwell is commercial landscape horticulturist and landscape entomologist for the Collier County-University of Florida Cooperative Extension Service. E-mail dlcaldwell@ifas.ufl.edu Call 353-4244, ext. 203. For updates on the Southwest Florida Horticulture Learning Center and more landscape pest management details, visit collier.ifas.ufl.edu.

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