Doug Caldwell/Special to the Daily News

This string bean-like gall is also a food source for the oxhorn bucida caterpillar, as evidenced by the frass accumulations.

Doug Caldwell/Special to the Daily News

This is typical staining underneath an oxhorn bucida due to caterpillar droppings (frass).

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I’m pushing for doing away with a common name for one of our common street trees, the "black olive" (Bucida buceras).

This name was confusing to me when I first moved here. One of the landscapers whom I drove around with to learn more about Southwest Florida horticulture exclaimed, "And there’s a black olive. It can be a problem with the fruit staining."

I immediately thought of the black or European olive tree (Olea europea) that is used in California. Our South Florida "black olive" isn’t even in the olive family — not even a close relative.

Another earlier, common name for this tree is Oxhorn bucida ("The Native Trees of Florida," by E. Westand L. Arnold, published in 1956; and "500 Plants of South Florida," by J. Morton. published in 1981). This name came about because of a curious structure, a 4- to 12-inch-long string bean-like gall that forms on some of the fruit. It is induced by the feeding of eriophyid mites. Sometimes the gall is straight and sometimes it curls and resembles, to some, an oxhorn. However, from my observations, these galls are not common on the "shady lady" cultivar of Bucida buceras that we see here.

This tree is associated with staining on driveways and sidewalks. Tannins are a component in Bucida leaves, bark and structures, much like oak trees. The tannins cause staining, which may be cleaned up on concrete with harsh (swimming pool-strength concentrations) of chlorine or bleach. Use great caution to protect eyes and skin with these products and avoid rinsing onto nearby plant foliage.

On my morning walks over the years, I have observed that the major staining is not associated with the plant parts, however, especially not the fruit (botanically called a nutlet or coriaceous drupe — I call it the more generic term, "fruit"). The leaves may cause some slight staining, but the really nasty discoloration that landscaper Mike Malloy calls looks-as-if-somebody’s-old-radiator-blew staining is due to caterpillar frass.

Yes, the black olive, er, Oxhorn bucida caterpillars (Characoma nilotica) apparently concentrate the tannins in their digestive systems, and when their frass drops and gets wet, there is some serious staining. These caterpillars are unique in that they feed on the flowers, leaves and even bore into the galls and feed.

Fortunately, the caterpillars are only active about six weeks out of the year, from mid-April through early June. There is apparently only one generation per year, unlike the life cycle of the oleander caterpillar, which has repeating generations all year.

The Oxhorn bucida caterpillar numbers also can build up and defoliate trees, as happened in 2003, but the trees re-foliate in another four weeks or so. Due to parasites and predators, the caterpillar infestations are usually light about seven years out of eight or so, which means the staining shouldn’t be a nuisance every year.

Another name for this caterpillar is the "bungee" caterpillar. This came about due to its habit of rappelling around on silken webbing and getting in people’s faces; this is a common sidewalk and parking lot tree. But beware, as its roots can cause some upheaval. Plant it about at least 10 feet away — 20 is better — from sidewalks and pavers.

So the good news is that the severe, radiator-strength staining should only be a problem for about six weeks, from late April through early June. If the stain and rappelling larvae are a problem, a spray of a selective insecticide containing B.t., Bacillus thuringiensis, when the tree is in peak flower and maybe again two weeks later, should reduce the caterpillar population.

Note, B.t. is selective for caterpillars; should there be some honeybees around the flowers — I’ve not noticed many, if any — they won’t be affected.

Doug Caldwell, Ph.D., is the commercial landscape horticulture extension agent and landscape entomologist with the University of Florida Collier County Extension Service. E-mail dougbug@ufl.edu; phone, 353-4244, ext. 203. For updates on Southwest Florida Horticulture visit collier.ifas.ufl.edu.