Foreign Invaders Target Our Trees


Much has been written about protecting a specific tree from a specific pest. Seldom do we consider the broader picture to help us understand what is happening, whether we can affect these pest issues and even whether we should try.

Let’s first separate native tree pests from non-native or invasive pests. This is an important distinction, because it speaks to a significant difference in how we manage pests. Native tree pests originate on the same continent as the host, and have co-evolved with the host tree species. This infers a relationship between host and predator, which won’t generally kill the tree, especially when grown in situ from seed. Trees are weakened by such pests, but will usually survive to reproduce and continue the species.

Sudden oak death can be effectively treated for up to two years with trunk injections. PHOTOS: ROB GORDEN, ARBORJET

Native trees struggle with local pests when environmental factors such as drought, heat, flooding, salt, wind or ice stress the tree. Sometimes, also, changing environmental factors enhance the survival of the pest. The worst occurs when these environmental stresses combine with sudden pest expansion, as evidenced in the excessive droughts of the Midwest and Plains just a few years ago. In the mountain states, these perfect storm conditions favored a devastating outbreak of mountain pine beetle, destroying millions of acres of forest and exacerbating the region’s fire threat as well.

Planted ornamental and street trees, whether native or not, experience none of the luck of native, germinated trees. Too often, the concept “right tree, right place” is ignored when planting trees. As horticultural and arboricultural professionals, we are expected to consider this rule regardless of the demands of the client.

Pine beetles cause more damage than all other tree pests in the U.S., but can be effectively controlled with a combination of insect and disease treatments.

Understanding a tree’s native growing conditions will reduce plant stress and lessen the severity of native insect attacks on the tree. For instance, dogwood is an understory forest tree where roots are cool and leaves are shaded. Unfortunately, they are often planted in full sun to be seen and enjoyed. Is it any wonder that they are prone to attack by native borers?

Hemlocks prefer to grow on well-drained east- or north-facing slopes, and they grow slowly. They can live 900 years and grow to enormous sizes, but can’t survive years of native cool-season mite attacks when grown as a hedge. Redwoods grow in marine moisture conditions with frequent morning fog allowing consistent root moisture. The branches of redwoods cascade to ground level, with up to a foot of leaf and twig mulch, which keeps the roots cool. Because of their grandeur, they are often planted inland, where temperatures exceed 100 degrees, lower branches are removed for esthetics and limited mulch is applied.

Under these harsh conditions, ornamental trees are injured, killed or require extensive protection. Until recently, most work available to plant health care companies resulted from native pests and routine environmental stresses on trees.

Uninvited stowaways

Over the past 30 years, this has changed with the arrival or expansion of non-native and often invasive insect pests. Most arrived from Asia in packaging materials and untreated wooden pallets. Some of the early invaders were considered “native” due to their long presence here. These pests include Japanese beetle, gypsy moth and the hemlock wooly adelgid. Some of these introductions date to the turn of the 18th century.

Non-native pests pose such risk because the trees in North America did not co-evolve with them, and they lack natural defenses against attack. Most serious native insects will only kill weak trees. Invasive, non-native insects often kill healthy trees because these trees lack the defenses to fight them.

New pest challenges

While non-native insect or disease pests pose great problems, they also present opportunities to those protecting trees. Each new invasive pest brings its own unique management challenges to our country:

1. Scientific research – Identifying the pest, the host(s), the range of infestation and if any native predators could slow them down, are the first steps. This requires dedicated research, as well as funding and identifying stakeholders to pay for research without certainty of financial return. Experimenting with control options takes years for scientific verification, while the pest expands its range and damage.

2. Quarantines – The government agency tasked with evaluating new non-native threats in the U.S. is the USDA – APHIS (Animal and Plant Health Inspection Service). It often assumes a leadership role with invasive pests that pose significant risk to the U.S. The National Invasive Species Council (NISC) defines invasive species as, “An alien species whose introduction is likely to cause economic or environmental harm, or harm to human health,” adding that it is “Non-native to the ecosystem under consideration.” Some quarantines have been successful, and eradication of several pests has been accomplished.

A lightning strike exposes new and old pitch tubes on a redwood.

3. Government response – It is expected that state and federal authorities will help in times of natural disasters with funding, resources and technical expertise. This is not a fair expectation when non-native invasive pests arrive from other countries. Unless APHIS can eradicate the pest, responsibility falls to local jurisdictions, which are often ill-prepared with either technical expertise or adequate funding to correctly address the outbreak. When local government makes long-impact decisions (such as tree removal) based on short-term budgetary constraints, the outcome is seldom desirable for the community, and the real costs are seldom understood.

4. Extension service – The university extension system was developed to provide technical support for local constituents. However, the scale of invasive pests, the frequency of introduction and the resulting devastation they bring exceeds the capacity and reach of our extension system. Too frequently, extension agents receive dated news on solutions for invasive pests. Old information continues to populate the Internet and media, and decisions stemming from this data results in the loss of trees, which might be saved.

5. Finding the funds – Acquiring federal funding to fight invasive pests has been challenging, if not impossible, with states lamenting this reduced federal aid. It has also been difficult to help states and cities to formulate the right decisions impacting community livability, ecosystem protection, property value retention and preservation of urban canopy. Too often the chainsaw has won the battle, while the greater community has lost. Recently, most notably in Illinois, government agencies have ruled that tree protection is able to be capitalized, which means instead of an emergency appropriation, bond money can be used to spread the cost of invasive outbreaks over a longer repayment window. This makes it much easier to respond to these natural disasters.

For arborists and horticulturalists, these invasive pests represent challenges and opportunities. While cities grapple with the larger issues of street and park trees, you can educate yourself on the problems, solutions, treatments and protocols that will result in pleased clients and saved trees. For those who want to start protecting trees as a part of their services, begin by learning about the key pests and solutions for your market.

Pitch tubes are the most obvious symptoms of pine beetle infestations.

Here is a glimpse of today’s invasive foreign and domestic pest threats and opportunities:

1. Emerald ash borer (EAB) – Found in 22 states, this pest bores into and kills ash trees. Treatments are available offering two years of protection with extremely high success. Visit for guidance.

2. Polyphagous shot hole borer (PSHB) – This boring insect found in Southern California attacks more than 200 species of trees and reproduces in more than 20. It also brings a fungus into the tree. Two key hosts include sycamore and sweet gum. New treatments are now available. It has spread from Los Angeles to Pasadena, California, and another outbreak exists in San Diego.

3. Pinewood nematode – This pest attacks several non-native pines, especially Austrian, in the Midwest. When pine sawyer beetles bring this invasive nematode to trees, they show rapid decline. Treatment for healthy nearby trees is advised and effective.

4. Walnut twig beetle – This boring beetle is part of a growing trend of insect-vectored diseases, resulting in Thousand Cankers Disease. The tiny bark beetle introduces a fungus to the tree.

5. Pine beetles (southern, western, mountain, IPS) – All in the Scolytid family, these beetles are now effectively controlled with a combination of insect and disease treatments. These beetles cause more damage than all other tree pests in the U.S., and many introduce Blue Stain fungi to the tree.

6. Hemlock wooly adelgid – Devastating hemlock species from Georgia to Maine, it weakens then kills trees. New treatments offer multiple year protection.

7. Sudden oak death – A disease of coastal oak and tanoak in California, trunk injection treatments are very effective for up to two years.

8. Gold spotted oak borer – This relative of EAB and two-lined chestnut borer attacks oaks near San Diego.

9. European elm scale – This pest is very messy leaving honeydew and sooty mold on trees, cars and homes. There’s a concern for resistance with older insecticidal choices. New data suggests more effective controls and correct timing can provide good protection.

10. Magnolia scale – From Florida north and west to areas where magnolia flourish, it is easy to control with injection.

Your treatment options

It is not enough to understand that there are pests, which one can control, saving trees. You must also understand the changing protocols of pest management on trees.

Foliar sprays – The go-to solution for years, it was simple, easy to train on and offered a blanket treatment strategy. For ornamental trees (flowering, smaller), it is still used with regularity, but is less successful on invasive pests because the worst insect and disease problems bore inside the trunk and limbs. Public sentiment on spraying has changed, and broad sprays are less acceptable. Rain and wind pose big issues when treatments are made using this method.

Soil drench treatments – These were adopted widely after sprays became less desirable. Systemic products are taken into trees’ roots, but require more product due to the nature of the treatment. They also require longer lead times to get product from the soil into the tree. They are less effective on serious boring pests and raise concern over groundwater contamination from leaching. Most soil drench chemistries limit annual use on trees per acre. They require large tank sprayers and, while less impacted by wind, are still impacted by precipitation or drought.

The Polyphagous shot hole borer, found in Southern California, attacks more than 200 species of trees and reproduces in more than 20 other tree species.

Bark sprays – A more recent addition to the pest solution protocols, they are effective on some pests, with the greatest success on scale insects. There is some public exposure concern, shorter residuals and increased risk of leaching, with per acre limits. University trials suggest this method is less effective against boring insects.

Trunk injections – A variety of methods have been created to inject insect and disease treatments, as well as micronutrients, directly into the tree. Some are very successful, while others are less effective. Different methods can improve or reduce productivity, are easier or more difficult, can protect or injure the tree and are or are not suitable for both conifers and hardwoods. So how does one determine what to use?

Published research is the most effective way to evaluate treatment options.

Questions to be answered include: Can I deliver the treatment in a reasonable period of time, generally 15 to 30 minutes? Will the system protect the tree’s cambium during treatment? Which system encourages rapid injection site closure? Do chemistries remain sealed in the tree? Does the system protect both the applicator and the environment? How long will the product protect the tree? How effective are they? Most products protect during light infestations, but only a very few help through heavy infestations.