These days our parks are quiet, often covered with a blanket of snow, the trees dormant and flowers having long disappeared…is the staff on vacation? Not at all! The winter months are full of activity. Friends Project Manager Bob Mulcahy, Collections Care Manager Sarah Hutt, and consulting arborist and soil scientist Normand Helie are hard at work planning for the year to come, and overseeing winter tree work.
Sarah and Bob evaluate every piece of sculpture throughout the year, planning out the annual cycle of care and cleaning. Together they prepare the budget, receive proposals from conservators, ready contracts for the proposed work, and notify Boston Parks and Recreation Department, the Boston Arts Commission and Boston Landmarks Commission about upcoming annual maintenance. Once the contracts are signed and paperwork filed, Bob and Sarah will work with the conservator contractors to schedule when the Women’s Memorial, among many, will get the TLC they need.
The Friends has been funding the expert care of trees since 1970 as part of our mission to preserve and enhance the Boston Common, Public Garden, and Commonwealth Avenue Mall. This work would not be possible without the financial support of our Members. We are always delighted when Members express a genuine interest in learning more about the details of the work we do. We noticed an increase in questions about tree care coming in over the past few months, and in particular inquiries about our work related to the elm bark beetle and Dutch elm disease, which seem to have been sparked by our Members Reception presentation (Digging In: Beyond the Roots of Urban Tree Care). We asked our contractor Christine Helie to explain some of the work she does for us. She is an entomologist and field scientist who works with her husband Normand at The Growing Tree. Chris is directly involved in developing an Integrated Pest Management program to preserve the mature and young elm trees in our parks. Here is what she had to say:
Among the trees in the Public Garden, on the Boston Common and on Commonwealth Avenue, is a unique collection of elm trees. This valuable assortment of European, American and Asian elms are susceptible to Dutch elm disease (DED). This disease is caused by a fungus that compromises the conductive tissue of the tree and eventually kills it. The primary vector of the fungus is the European elm bark beetle. Through its breeding and feeding behavior, this bark beetle transfers DED from diseased trees to healthy trees.
In 2012, with the support of Greg Mosman, Tree Warden of the Boston Parks & Recreation Department and on behalf of the Friends and its tree care program, a monitoring and management system for this insect was designed as part of a new elm tree preservation program for the mature and young elm trees in the three parks. The manner in which insects are monitored and managed can vary depending on the habitat in which they exist. For our purposes, a three sided box of plywood, painted green was built to house an 18”x25” sticky trap with a pheromone lure attached in the center.
Pheromones are chemicals produced by an organism that elicit a response from another organism. They are used by insects or animals to communicate with individuals of the same species. Depending on the type of activity, different pheromones will be used to relay a message.
For example, ants use a trail pheromone to mark a path leading to food that other ants in the colony can find and follow. However, when encountering a dangerous situation they use an alarm pheromone to warn their nest mates. The pheromones used in our beetle traps signal to both male and female elm bark beetles that this is a great spot for breeding and laying eggs.
There are over 24 traps in use throughout the parks. From the beginning, our goal was to
make them easy to access but discreet. Rather than placing unsightly posts throughout the parks, we decided to install our traps on trees at least 150 feet away from any elm trees.
Because the bark beetle is attracted to elm trees weakened by stress, one of the components in the pheromone mimics volatiles released by a stressed elm tree. As a result, the trees that we chose to place our traps on became substitute elms, luring the elm bark beetles away from the elm trees.
Pheromones are effective at very low concentrations and insect specific. This fact becomes evident when you compare some of the trees with traps to an actual elm tree. The vase shaped elm tree with upright branching is quite different from the pyramidal shaped Norway spruce with drooping branchlets.
The elm has a broad leaf with a serrated edge, whereas the spruce has needle-like foliage. The bark of an American elm tree has deep crevices that form diamond-shaped furrows, while the bark of a Norway spruce tree has thick round scales.
Regardless of these features though, the Norway spruce in the Public Garden has consistently captured high numbers of the European elm bark beetle on its trap.
Below are images showing the physical differences between Elm trees (top photos) and Norway Spruce trees(bottom).
Bark beetles appear to use different methods when locating a proper host tree. By crawling on the bark, they can sense the texture and determine whether the tree is susceptible to attack. Dispersing beetles are also guided by odors from weakened trees. From what we have observed in our program, it seems apparent that when the beetles land on a potential host, one of our stand-in elms, the odors detected override the physical clues they pick up from the tree. As a result the beetles continue to search for the source of the pheromones until they are caught on the trap or die trying to find the “elm.”
These traps have also allowed us to monitor the location, concentration, and pattern of movements of this disease host, helping to indicate the optimal times to treat, prune and, in some cases, remove a diseased tree.
Since their implementation, the elm bark beetle traps have become important tools in our fight against Dutch elm disease. The twenty four traps in use throughout the parks and surrounding areas are installed on thirteen different tree species. While these trees may be Oaks, Locusts, Maples, Lindens, or even a Norway spruce, they actually serve as substitute elms and are important allies in the preservation of our real elm tree population.
Landolt, Peter J. “Sex Attractant and Aggregation Pheromones of Male Phytophagous Insects.” American Entomologist Spring 1997 12-22. Print.
This past Saturday, based on recommendations by the consulting arborists of the Friends of the Public Garden, the Boston Parks and Recreation Department removed 4 worrisome trees. Another was pruned due to a major crack in a main limb. The 4 trees were first targeted for removal late last fall due to declining health. In all, three species of Maple, a Norway Maple (Acer platanoides) a Sycamore Maple (Acer pseudoplatanus) and a Silver Maple (Acer saccharinum) and one Scots Elm (Ulmus glabra) were removed. A fifth tree was slated for removal but extensive pruning was done to further preserve the tree, a Callery Pear (Pryus calleryana). Special thanks go out to the efforts of Greg Mosman, Tree Warden for the City of Boston, for the help in coordinating these removals.