Fayette County

City to cut unhealthy pin oaks near airport

Twenty-five years ago, when rows of young pin oaks were planted along Man o' War Boulevard near the entrance to Blue Grass Airport, the idea was that columns of stately trees would someday welcome visitors to Lexington.

But no one took into account a small insect called the horned oak gall wasp.

In a couple of weeks, the trees — most stunted and covered with tumor-like galls — will feel the bite of a chain saw.

"We've removed several (infested) trees in the past, but once it gets to this level there's really nothing you can do but cut the trees down and start over," said Tim Queary, the city's urban forester.

Officials are hurrying to cut the trees and grind their stumps. They want the section of highway to be ready for the three varieties of new trees that will be planted in the spring, in time for them to get established before next fall's 2010 Alltech FEI World Equestrian Games.

Experts say the Man o' War pin oaks are an excellent example of the pitfalls of monoculture — the planting of only one type of tree.

Although a long line of matched trees is pleasing to the eye, it sets up a situation where an insect or a disease can quickly get out of control, leaving a line of stumps.

Dan Potter, a University of Kentucky entomologist, has spent four years studying the horned oak gall wasp.

The insect is especially fond of pin oak trees, he said. And Lexington has accommodated the wasp by planting rows of pin oaks along streets or on horse farms.

"It gets going and it's like an all-you-can-eat buffet," Potter said.

Another visible example of the horned oak gall wasp's work is near the Maxwell H. Gluck Equine Research Center on Nicholasville Road. An infestation started there about five years ago; now so many galls are on the trees that they threaten to break branches.

Lexington is facing the same problems with other trees.

Whole neighborhoods have been planted with ash trees, which now are threatened by an insect called the emerald ash borer.

Bradford pears were once the tree of choice for new developments, but their limbs break easily and they are susceptible to fire blight.

Pin oaks also face other problems, including a disease called bacterial leaf scorch. It is caused by a bacterium called Xylella fastidiosa, which makes the trees die of thirst, even in the wettest years, by effectively blocking their arteries.

Lexington's Tree Board has been pushing, so far without success, for an ordinance that would prohibit developers from creating future monocultures.

The horned oak gall wasp doesn't necessarily kill trees, but can stunt their growth and make them unattractive.

It does this through a complex, multi-stage process, Potter said.

First, a female wasp will lay her eggs in a bud of emerging leaves in the spring. That causes a small gall, almost like a blister, on the leaf. In June, a small wasp emerges from the leaf. It flies to a twig, where it lays eggs and deposits a plant hormone that causes a gall — Potter compared it to a human cancer — that grows around the wasp larva. The larva grows inside the gall for nearly three years before it emerges and the process begins again.

Potter has been looking, without success, for a way to control the wasp. Insecticides sprayed on the galls or injected into the tree don't work well because the gall apparently protects the larva inside, he said.

The best course for people who spot an early gall infestation is to cut off as many as they can reach, he said.

Queary, the city's forester, said new trees to replace the pin oaks will cost about $8,000. The three species will be planted in groups of four or five, he said.

The replacements include 33 swamp white oaks, 44 bald cypresses and 45 prairie crab apples.

Queary said they were chosen because they are native species, provide variety and don't have obvious insect or disease problems.

"Hopefully in our lifetimes we won't have to do this again," he said.