Coral Gables

Those stains on your car from black olive trees? Blame a bug, not the tree

Kenny Martinez, who works for the city of Coral Gables, injects pesticide into a black olive tree.
Kenny Martinez, who works for the city of Coral Gables, injects pesticide into a black olive tree. City of Coral Gables

It’s hard to hate trees. But many people would rather chop down the messy black olive than enjoy the marvelous shade it provides.

The black olive is blamed for the rust-colored stains it leaves beneath its canopy. Driveways, sidewalks, cars, boats, roofs and walls can look like they’ve been sitting under a leaking vat of strong tea.

But the Bucida buceras, commonly known as the oxhorn bucida or black olive, gets a bad rap. The staining is not caused by the tree’s leaves or flowers but by the droppings of a caterpillar which feeds on the foliage and bores into the stringbean-shaped pods caused by a mite.

Yes, those ugly brown blotches on your walkway or the hood of your Honda are from caterpillar poop.

Coral Gables is fighting back. Designated a Tree City for 34 years and proud of the green canopy provided by 38,000 trees, Coral Gables is systematically injecting the trunks of its 10,000 black olive trees with a pesticide that kills the caterpillars and mites.

“These critters excrete what they digest and their excrement is highly concentrated with tannins,” said Jorge Rivera, landscape coordinator for the city and a certified arborist. “The tree senses it is being attacked and increases the tannin production in its sap by 400 percent. The stain can be difficult to remove.”

Coral Gables treated 2,200 black olives this spring, up from its initial round of 25 when it began the experimental program five years ago. Flowering is most intense from April to June. Soaking rains tend to embed the stains.

“People complain and complain and want the trees taken down, but black olives make up a quarter of our urban forest,” Rivera said. “Instead of eradicating these mature trees we decided to be proactive and control the nuisance effect.”

Rivera has a beautiful black olive that towers over his family’s cars at his home.

“They all happen to be white cars so every morning we have to hose them off because if you let it sit it becomes permanent,” he said. “To clean, I apply a weak solution of bleach. It’s a majestic tree but cleaning up after it is a chore.”

The pesticide, abamectin, is injected with an applicator through holes drilled into the trunk. The treatment reduces the mites’ formation of galls, or curly pods, that the caterpillar tunnels into and eats. The cost to the city is $25 per tree.

“It’s a sustainable practice with no risk of contamination to surrounding plants and no harm to pets or humans,” Rivera said. “We’ve found it reduces staining by 80 to 90 percent. We need to respond to the residents and make them happy to live in Coral Gables.”

Stains from tannins in black olive trees.

Caterpillars and mites were identified as the dirty culprits in a study by Doug “Dr. Dougbug” Caldwell, a landscape entomologist and horticulture educator at the University of Florida’s Institute of Food and Agricultural Sciences Extension Service in Naples.

The black olive, originally from the Greater Antilles and Leeward Islands, does not produce olives and isn’t even in the olive family. It is a host for the Eriophyes buceras Cromroy mite and the Characoma nilotica caterpillar, also known as the “bungee caterpillar” that you may have bumped into when passing under the tree.

“This name came about due to its habit of rappelling around on silken webbing and getting in people’s faces. These ‘in-your-face’ encounters are a big annoyance factor, and another reason to avoid planting these trees along sidewalks and in parking lots,” Caldwell wrote. “The oxhorn bucida caterpillar’s digestive processes apparently concentrates the tannins and/or unbinds the staining agents. The serious, turn-your-head staining occurs when the caterpillar frass collects on the sidewalks, and subsequent rain, dew, or irrigation water soaks it and the tannins are released. On a scale of 1 (very subtle) to 10 (very dark), fruit and leaf stains may rate a 2 or 3, but the caterpillar frass boosts the staining right into the 8 or 9 range.”

Caldwell said spraying an insecticide containing Bacillus thuringiensis when the tree is in peak flower is another way to reduce the caterpillar population.

Coral Gables started planting black olives about 40 years ago when it was “in fashion,” Rivera said.

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Kenny Martinez, who works for the city of Coral Gables, injects pesticide into a black olive tree. City of Coral Gables

“The problem arose because people decided to use them as a street tree,” he said. “If they’re planted in an open area, there’s no problem. The shade is incredible and they can grow 65 feet high.

“Every single type of tree has one issue or another. Oaks, mahoganies, tamarinds — people think they don’t stain but they do. Royal Poinciana flowers stain. Mahogany pods can dent your car. All deciduous trees have tannins as part of the arsenal they use to defend themselves. We take trees for granted and forget that they are living organisms — and more like humans than we might think.”

Coral Springs and Naples are among other cities trying the treatment.

“Mites are known to become resistant to pesticides,” Rivera said. “We’re in the process of finding another one we can use in rotation or replacement.”