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Impending Peril of Urban Forests

By: Chris Keil, MLA

Trees provide vital habitat for animals by offering cover and food. Complex food webs comprised of numerous species can revolve around a patch of forest or in some cases even a single tree. We have all witnessed squirrels gathering acorns from an oak tree, but it is less apparent that many fungi, insects, birds, plants, and bryophytes also depend on mature trees. The insects, nourished by the tree’s tissue, play a role in pollinating other plant species and in providing food to other animals higher up the food chain. While native insects feed on host trees they rarely threaten entire tree populations or even individual trees. However, certain nonindegenous insect species, free of their typical predators, can decimate an entire tree species and radically alter the forest landscape. In New York, there are several of these insect species that have garnered attention in recent years. The damage they have caused and continued threat that they pose is very real.

Emerald Ash Borer (EAB) (Agrilus planipennis)

Emerald Ash Borer Image courtesy of

Since 2002 this exotic beetle has killed an estimated 50 to 100 million ash trees.
Image courtesy of Maine.gov

An exotic beetle from eastern Asia first identified in the U.S. in 2002 after hitching a ride in shipping material. Since the beetle’s accidental introduction it has spread to 15 Midwestern and Eastern states and two Canadian provinces. The larvae of this beetle feed on the phloem (food carrying tissue) of ash trees inhibiting the trees’ ability to transport nutrients and water, usually killing the tree within two to four years. In May 2012 the Society for Municipal Arborists estimated that EAB has killed 50 to 100 million ash trees so far and threatens to kill most of the 7.5 billion ash trees in North America. In New York alone, one in ten trees in hardwood forests is an ash. Ash trees (Fraxinus sp.), are important in multiple forest communities in New York including wetlands. They are commonly planted in urban environments because of their beautiful fall colors, dense shade, and tolerance to difficult conditions. Ash is also a valuable lumber species and excellent firewood, which only escalates the spread of the EAB. Dead ash is often transported across the region as firewood which augments the spread of EAB. Scientists have been monitoring the EAB through an extensive program that has installed purple traps, sometimes known as purple prisms, in 47 states. These traps reflect light at the same wavelength as ash leaves and are covered with a botanical oil that traps the insects. In addition, New York and other states have taken efforts to prevent the spread of EAB by establishing quarantine zones.

Asian Longhorned Beetle (ALB) (Anoplophora glabripennis

Asian Longhorned Beetle  Image courtesy of

Despite successful eradication in New York quarantine zones still exist in Brooklyn & Queens. Image courtesy of AsianLonghornedbeetle.org

First discovered in Manhattan in 1999 and in Staten Island in 2007, this nonindigenous beetle is also thought to have come to the U.S. through shipping materials from Asia. ALB feeds on many different species of tree but favors maples, elms, and willows – all important species in eastern hardwood forests. ALB can spend several life-stages and multiple years within an infected tree, feeding on the host tree’s cambial tissue, sapwood, and heartwood. As an eradication effort, thousands of severely infected trees were removed and chemical treatments were used on minor infestations and at-risk trees throughout the city and nearby Suffolk County, Long Island. In 2011, New York declared eradication was successful but this does not mean the threat is gone. New York city, state, and federal agencies have remained focused on identifying future threats and quickly mobilizing to mitigate them. Current quarantine zones still exist in Brooklyn and Queens.

Hemlock Wooly Adelgid (HWA) (Adelges tsugae)

HWA Image courtesy of the National Parks Service

Spot HWA by the appearance of tiny “cotton balls”. Image courtesy of NPS.

A tiny true bug, also of Asian origin, infects native hemlock species (Tsuga sp.). Eastern Hemlocks are important evergreen conifer tree usually appearing in mixed forest communities alongside hardwood species, though rarely in urban environments. Hemlocks are long-lived trees that can survive in deep shade beneath the understory but eventually grow to towering heights adding structural heterogeneity to forests. Hemlocks are also associated with greater macroinvertebrate diversity and quality brook trout habitat. HWA was first discovered in the Hudson Valley of New York in 1980s and is now present in 25 New York counties. The adelgid feeds on phloem of young hemlock branches causing the tree to drop needles and stop producing new growth. It can take up to ten years for a tree to die from HWA infestation but weakened trees may die from secondary causes.

These three insect invaders have garnered public attention lately for threatening New York’s native trees, but many more biological threats exist. Most people are familiar with two notorious fungal pests, Dutch Elm Disease (DED) and Chestnut Blight, which have ravaged American elm and chestnut trees. Verticillium wilt is another fungus that affects more than 300 plant species, many of which are important in agriculture and horticulture. Bacterial pests, such as Bacterial Leaf Scorch, affected Red Oak trees quite dramatically in parts of the Northeast this summer. Combine these pathogens with other environmental threats such as climate change, severe storms, diminished municipal budgets for tree care, and development pressures and you can see that urban forests and forest ecosystems are truly imperiled. In a globalized world, with increasing international trade and shipping we must assume the spread of harmful tree pests will continue jeopardizing the health and function of our native forest habitats. However, there are steps that can be taken to minimize risk, combat pests, and promote tree health despite these pressures.

Landscape architects, arborists, and knowledgeable landscape contractors should all be able to assess tree health and give clients recommendations on specific treatment and maintenance for infected trees. Accurate diagnosis is critical once a problem has been identified. Risk prevention is always the best option and designers should be aware of the insects and diseases that are likely to affect newly installed trees. For specific valuable specimen trees, treatment options such as insecticidal foliar sprays and soil soaks should be considered, but are often costly, long-term solutions which may have unintended consequences on other flora and fauna. In some cases of minor infections, pruning can delay the affects of an infection. Generally, promoting healthy soil and optimal growing conditions is always wise and a pre-condition for other treatment options. There have been some hopeful developments in the horticultural industry, such as the development of pest-resistant tree species. For example the Ulmus americana ‘Valley Forge” has shown excellent tolerance to DED. Resistant cultivars may eventually restore ecologically and culturally important mature trees to our forest landscape. In the meantime, it is worth fighting these pests with the tools available and it is important for states like New York to develop comprehensive forest management plans that include urban ecosystems to ensure that the damage is not too severe, sudden, and costly.

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