Small wasps bring down big black oaks

A gall wasp can be anywhere from 1 to 8 mm in length.
Photo courtesy of Wikipedia Commons

A gall wasp can be anywhere from 1 to 8 mm in length.

Last week, homeowner Christine Todd watched as tree workers removed two large black oak trees, each about 60 feet tall and estimated to be 60 to 70 years old, from her small back yard on Pennacook Avenue in Oak Bluffs. The oaks died as a result of a small wasp that laid its eggs at the ends of the branches. Galls, small growths, developed around the eggs that prevented the leaves from getting the nutrition they need. When the leaves died or failed to develop over successive growing seasons the trees died.

Arborist Ken Cottrill of Lickity Split Logs in Chilmark pointed out the problem to The Times on trees in West Tisbury last week. Many had lost lower branches and had dead, leafless, branches covered with moss. He said he has seen trees in a similar condition all over the Island.

Tom Clark, curator and collections manager at the Polly Hill Arboretum in West Tisbury, explained the problem in a phone conversation last week. “An infestation of the cynipid gall wasp over the last several years has been responsible for damaging many black oaks on the Cape and on the Vineyard,” he said.

The Arboretum has been getting reports from many of the Island arborists since the problem was first brought to their attention three or four years ago by John McCarter of McCarter Tree Service in Chilmark.

“It was a bit puzzling to us. At that time either there wasn’t a lot of it around or people weren’t noticing it,” Mr. Clark said. “It really came to the fore last year, people began seeing terminal branches dying back and some of the current season’s growth being stunted. It really caused quite a stir particularly in the tree care community.”

Mr. Clark said they first thought that stressed trees might be more at risk, trees subject to salt or other damage, but after surveying a range of trees on the Island. they realized that all black oaks are at risk. “It is important to note that it is only black oaks that are at risk and not the white oaks or the post oaks.” He said mild infestations of some scrub oaks has been noted. “Of the six species of oak native to the island, black oak is one of the most widespread,” he said.

“We even took down a couple of trees here at the arboretum. They weren’t dead but they weren’t ever going to regain a full form or be structurally sound.”

“The life cycle of the wasp is not entirely understood,” he said. “There has not been a lot of research on this insect. We do know that a gall, an abnormal growth, develops around the egg the wasp lays in the terminals of the branches. This time of the year we can see those swollen areas and the fine holes that the adult wasps have emerged from.”

Mr. Clark said that the wasp problem may be similar to the infestation of winter canker moth caterpillars back in 2005, 2006, and 2007 that destroyed trees by eating the leaves. “It was a terrible problem for several years killing many trees and then it went away,” he said. “The insect is still here but that is often what happens. The population will get out of control for a while and then some biological factor will change the situation.” He said that it may have been a bacteria or a fungus that was introduced into the canker worm population that limited its growth and knocked it back into balance.

“That’s what we are hoping will happen to this infestation. There was a similar infestation on Long Island in New York back in the nineties, that eventually went back into remission, back into balance,” he said.

A report from the U.S. Department of Agriculture Forest Service on the six-year Long Island infestation bolsters Mr. Clark’s view. It said the fact that, “unaffected oaks were found next to declining oaks suggests that microsite characteristics may have contributed to the decline.” The report also states that while trees of all conditions were subject to attack, trees that were healthier before had a better chance of surviving and that “no specific control tactics are known.”

Some arborists have had limited success with trunk injections of a systemic insecticide “We have injected some trees here at the Arboretum mostly as an experiment,” Mr. Clark said. “It has helped but it hasn’t eliminated the problem.”

There are over 800 species of gall wasps in North America, according to Wikipedia, and 70 percent of them live in various types of oak trees. They range in size from 1 to 8 millimeters. It is a gall wasp that forms the golf ball sized gall, sometimes called an apple gall, on the leaves of some Vineyard oaks. The apple gall does not harm its host.

The trees behind Ms. Todd’s house once shaded her small yard. But this spring one tree produced no leaves and the other only had about a hundred leaves, according to Ms. Todd, who is executive director of the Oak Bluffs Association. “I landscaped the yard with plants that like shade. There wasn’t enough shade this summer. My plants were dying.”

The lack of shade gave flying predators a view of the small goldfish pond she had built. “I watched an osprey swoop down and grab a large fish from the pond,” she said, “right in front of me.

“I noticed the trees didn’t have the leaf growth and the fullness I was used to about two years ago. And then I noticed the branches were covered with fungus last year, a sign they were dying, I learned.

“This spring it was apparent the trees were in very bad shape. They became a hazard to my house and to the neighbors’ houses. I had to have the trees taken down. It cost me $4,000, money I don’t have to spend on tree care.”

Two arborists from Tabor Tree and Land Company working 40 feet off the ground took down the two dying black oak trees from the top down last week. One, Elliot Bilzerian of Oak Bluffs, worked with a chainsaw on one tree from the safety of a bucket at the top of a cherry picker while the other, Tyler Chronister of Vineyard Haven, chainsaw in hand, brought down the other tree a section at a time while suspended by a series of ropes and harnesses.

“Those trees have seen a lot since we moved here 13 years ago,” Ms. Todd said. “Graduations, birthday parties, anniversaries, Christmas decorations. I am really sad to see them go.”