Managing Beetles to Preserve Elm Trees

American Elm removal Commonwealth Avenue Mall June 17, 2014
A Commonwealth Avenue Mall Centenarian is removed due to Dutch elm disease (2014)

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

Beetle counter, PG, May 23, 2014, EAJ
beetle trap

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.

Bidding Farewell to a Centenarian Elm on Commonwealth Avenue Mall


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A  giant American elm tree that lived on Commonwealth Avenue Mall between Dartmouth and Exeter Streets for more than 100 years had to be removed due to Dutch elm disease on Tuesday, June 17.  The Friends remain vigilant in treating the elms in our parks against this disease, in order to protect the elm tree population.  For almost roughly 150 years, trees have been a treasured sight on this historic promenade and in 2013, for the first time in more than 43 years, each planting location on the Mall had a tree.  Originally all the trees on the Mall were elms, and the population was ravaged by the disease. They have been replaced with a variety of trees of similar scale and profile, including disease-resistant elms.

Dutch elm disease has been affecting the U.S. elm population since the 1930’s. The disease can kill an entire elm tree anywhere from weeks to years. The disease is brought on by a fungus that is distributed by the Elm Bark Beetle or via a root graft infection.

To confirm Dutch elm disease, branches are cut out of the tree and bark is peeled back to see if there are signs of streaking
To confirm Dutch elm disease, branches are cut out of the tree and bark is peeled back to see if there are signs of streaking

Signs of Dutch Elm Disease
• Sudden wilting of leaves in branches
• Curling and yellowing leaves
• Branch die-back

When to spot infections
• Spring and Summer

Confirmation of Dutch Elm Disease
• When branches are cut out, look below peeled bark for signs of streaking
• Confirm through lab results

Learn more about the Friends tree care program and how you can help.




Parks Department to monitor Dutch Elm Bark Beetle

As part of the ongoing maintenance of elm trees in the Boston Common, Public Garden and the Commonwealth Avenue Mall, the City, in partnership with The Friends of the Public Garden, has been working on an elm bark beetle monitoring program.  This new approach is in line with current integrated pest management (IPM) strategies to reduce the amount of chemicals used to control harmful insect populations.  The traps set will give us important data on the elm beetle populations such as numbers of beetles and timing of their generational hatching, this data will then help us form a more effective and environmentally friendly management program.

The traps themselves will be 18.5″x 28″ plywood painted green with a pheromone trap in the center, attached to trees at a height of about 15′.  The current plan is to have 4 each in the Common and Public Garden and 3 in the Fenway Victory garden.  Each trap must be at least 150′ away from any elm trees which is why they cannot be placed on the Mall.  There will be signage included explaining the function of the traps, and the Park line as a contact number for any questions.  There are no pesticides involved in the traps.  The traps will go up no earlier than the first of March and will be removed no later than the first of October.