By Shawn Bernick
Numerous emerging pest problems pose a significant threat to our clients’ individual trees, and can cause wide-scale mortality and substantial economic damage to larger populations of trees in both urban and natural areas. The global trade of shipping crates and packing materials, and the inherent difficulty of material inspection, creates an environment ripe for the introduction of harmful insects and diseases. Exotic pests have a greater likelihood of becoming epidemic problems as native trees have no natural defenses with which to protect themselves. Unfortunately, it is only a matter of time before the next Emerald Ash Borer or Asian Longhorned beetle is brought into the United States. Not all destructive pests of urban trees are the result of an exotic introduction. Often, native insects become prominent health issues of native trees when the trees are taken out of their natural environments and are planted in urban settings.
Discussion of the more prominent emerging pests in different locations throughout the United States is as follows:
Emerald Ash Borer (Agrilus plennipenis)
First documented in Southeast Michigan in 2002, this wood-boring insect has killed more than 50 million ash trees to date, and is of immediate or pending concern in communities throughout the Eastern and Midwest United States. Emerald Ash Borer (EAB) threatens all native ash trees and will continue to spread throughout the country. EAB is currently found in 15 states, and continues to be spread to new cities within these states and could potentially lead to the demise of native ash trees throughout the United States.
Asian Longhorned Beetle (Anoplophora glabripennis)
Another non-native pest that was introduced from Asia into the United States via wood packing material is the Asian Longhorned beetle (ALB). This beetle is a large cerambycid beetle that was originally discovered in Brooklyn, N.Y. in 1996. It was also found in Chicago in 1998, New Jersey in 2002 and in Worcester, Mass. in 2008. The Massachusetts infestation has recently spread into Boston and threatens the maple syrup industry in much of the Northeast. Asian Longhorned beetle attacks a wide range of common shade trees, including all U.S. maple species, elm, birch, horsechestnut, sycamore and willow to name only a few of the most commonly attacked. Given the predominance of [ital>Acer<ITAL] w:st="on" United States, ALB is definitely an emerging pest to keep an eye on. Currently, USDA APHIS is managing ALB infestations using sanitation, quarantine and soil and trunk injection applications of insecticide treatments.
Ambrosia beetles (Xylosandrus spp. and Xyleborus spp.)
Continuing the theme of emerging non-native pests that have been introduced into the United States on solid wood packing material, our next group of emerging pests are the ambrosia beetles. Twelve non-native ambrosia beetle species have taken residence in the United States since the early 1990s. These beetles commonly attack a variety of stressed or dying tree species. However, the red bay ambrosia beetle, Xyleborus glabratus, first identified in Georgia in 2002, can also attack healthy red bay trees. Not only does this beetle attack healthy trees, it also vectors the vascular wilt pathogen (Raffaelea lauricola) that causes laurel wilt. This lethal combination has caused widespread mortality of red bays in South Carolina, Georgia and Florida.
Brown Marmorated Stink Bug (Halyomorpha halys)
The Brown Marmorated Stink Bug was in the spotlight this past year as large-scale outbreaks occurred in the Mid-Atlantic United States. This pest has a very wide host range that includes many landscape species such as butterfly bush, catalapa and maple and it has been noted on numerous others. It also attacks and damages numerous fruit trees and has the potential to cause significant economic damage to fruit orchards. This pest also rivals the box elder bug and the Asian lady beetle as a nuisance pest as it invades homes in search of an overwintering spot. When agitated, this pest can also give off an odor, hence its common name.
Pine bark beetles (Dendroctonus spp. and Ips spp.)
Pine bark beetles are an important component of a healthy forest ecosystem. As native insects, they play a key role in maintaining a robust forest by quickly eliminating weak trees, allowing stronger trees to thrive. Most bark beetles attack stressed or weakened pine trees. However, some Dendroctonus species such as Mountain pine beetle (Dendroctonus ponderosae) and southern pine bark beetle (Dendroctonus frontalis) will attack healthier trees, especially in high-pest-pressure situations. A normal, healthy pine tree’s natural defenses prevent bark beetles from invading by forcing them out with sap. Pines that are growing in urban environments are stressed and weakened by a combination of drought, competition with turfgrass, and soil issues. This stress leaves urban pines highly susceptible to attack by pine bark beetles.
Bronze birch borer (Agrilus anxius) and two-lined chestnut borer (Agrilus bilineatus)
Although emerald ash borer is in the spotlight, there are native Agrilus species such as the bronze birch borer and the two-lined chestnut borer that continue to pose a significant threat to birch and oak trees. This has been especially true in recent years for urban areas that have experienced multiple years of below-average rainfall or drought conditions. In their native environments, these pests attack stressed and weakened trees, and are not considered serious pests of forest ecosystems. In the urban environment, low moisture conditions, heat, and root injury create situations in which birch and oak trees are more susceptible to these destructive borers. In addition, non-native birch trees are widely planted in landscapes for ornamental value. These non-native birch species have no natural defenses against North American insects, and are easy targets for the bronze birch borer.
Scale insects (numerous species)
During the past two growing seasons, scale populations have escalated in incidence throughout many parts of the United States. Soft scale species such as magnolia, cottony maple, European elm and calico scale appear to be rising in localized areas throughout the Midwest and Eastern parts of the United States. Likewise, numerous armored scale species including pine needle, obscure, oystershell and white peach, as well as prunicola, euonymus scale are being reported more frequently. The cause of these outbreaks is unknown, but localized environmental conditions may be part of the reason we are seeing increased levels of scale populations.
Diseases that affect the vascular tissue of a tree are among the most deadly ailments with which trees are faced. Vascular wilt diseases can be caused by fungi, bacteria, mycoplasma-like organisms and nematodes. Vascular wilt pathogens directly block the flow of water and sugars or induce the host tree itself as part of its host defense mechanism in an attempt to stop the progression of the invading organism. Dutch Elm Disease (non-native) and oak wilt (thought to be native) are famous examples of vascular wilt pathogens that can quickly infect and kill majestic shade trees in a matter of a few weeks. While these are well known, there are a few noteworthy vascular wilt diseases that have been increasing in incidence and severity in specific regions during the past few years.
Laurel wilt (Raffaelea lauricola)
As mentioned earlier, laurel wilt is a deadly vascular wilt disease of red bay (Persea borbonia) and other trees in the Lauraceae that is vectored by the red bay ambrosia beetle. Laurel wilt is caused by a fungus (Raffaelea lauricola). As the ambrosia beetle bores into the sapwood of stems and branches, the pathogen is transmitted and moves systemically where it plugs up the water-conducting cells in the tree and causes wilting. After becoming infected with the laurel wilt fungus, a redbay will wilt in a matter of weeks to a few months. The dying tree is then colonized by numerous ambrosia beetles that create galleries in the wood, where they reproduce. Laurel wilt has caused widespread mortality of red bay trees in parks, forests, and residential landscapes on the coastal plains of South Carolina, Georgia and Florida. This complex also threatens camphor, pondspice, swampbay, sassafras and is of significant concern to avocado growers in southern Florida.
Pine wilt disease (Bursaphelenchus xylophilus)
Pine wilt is a fatal disease of pine (Pinus sp.) caused by the nematode Bursaphelenchus xylophilus. As a native organism to North America, it is a primary pathogen of non-native, 2-3 needle pines. This includes Scots pine (P. sylvestris), Austrian pine (P. nigra) and many others. The disease is vectored by pine sawyer beetles (Monochamus sp.) as they feed on the bark and foliage of susceptible pine trees. Trees begin to die in midsummer and can die within weeks of the initial infection, leaving brown, dead needles still attached to the branches. This disease is a major concern for arborists, as well as Christmas tree growers in the Midwest.
Bacterial leaf scorch (Xylella fastidiosa)
Bacterial leaf scorch is an important disease of shade trees. It is caused by the fastidious xylem-inhabiting bacteria Xylella fastidiosa. This disease is vectored by a variety of xylem-feeding insects including leafhoppers and treehoppers. A variety of tree species are susceptible to bacterial leaf scorch. Notable examples include red oak, pin oak, bur oak, red maple, American elm and sycamore.
It has been reported as far north on the Eastern Seaboard as New York and is prevalent in the Southeast, Texas, and extends northward to Illinois. After being discovered in the early 1990s in New Jersey, the disease has been documented to infect more than 40 percent of the red and pin oaks located in Camden, Gloucester, and Burlington counties. This disease is also common in Southern California on liquidambar and landscape olive. Bacterial leaf scorch is a frustrating problem for arborists because there is no known cure and mature infected trees die prematurely and their appearance deteriorates over their lifespan.
Bur Oak Blight (Tubakia species)
Not a vascular wilt, but still worthy of mentioning, is a disease specific to bur oaks called Bur Oak Blight (BOB). BOB has been documented since the late 1990s/early 2000s in the upper Midwest, and has been documented in southern Minnesota, Wisconsin, eastern Nebraska and Iowa. Although scientists are learning more about the specific pathogen or pathogens responsible for this disease, it appears to be associated with a number of Tubakia species which normally only cause leaf spots. The disease causes leaves to curl and turn brown late in the growing season (July/August) and, in some cases, entire canopies can have brown, curled leaves. Although it is not clear if other abiotic agents increase the severity of this disease, it is clear that individual trees infected with BOB typically increase in severity from year to year. Trees subjected to drought and infected with BOB will be more susceptible to two-lined chestnut borer, which may result in crown dieback and decline. Thankfully, this disease appears to spread slowly, and, at this point, is confined to the aforementioned locations.
As an arborist, it is important that you are up to date on the latest news and information related to common pest problems in your area. This can be challenging given the number of introductions of invasive pests into the United States in recent years. In addition, regional outbreaks of certain pests have increased in incidence and severity over the past few years, requiring you to brush up on past and current information. Staying updated on information about emerging pest problems is important not only for keeping your clients’ trees healthy and protected from these pests, but for ensuring that you are viewed as a knowledgeable and valued source of information for your clients.
Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn. For more information, visit www.treecarescience.com