Three Tree & Shrub Pests To Watch


Last December, a CNN article declared 2020 as the year of scary bugs. But 2021 has perhaps been worse with the return of the infamous murder hornets, 17-year-dormant Brood X cicadas, spotted lanternflies, and more.

Unfortunately, pests can sabotage the enjoyment of outdoor spaces. And 75% of people deemed outdoor spaces as indispensable this past year, according to a survey conducted on behalf of TruGreen by OnePoll. To ensure outdoor spaces remain a place of reprieve, here’s the latest information on some of the most problematic invasives that affect trees and shrubs.

Emerald Ash Borer

Originally from Asia, the emerald ash borer (EAB) was first discovered in the Detroit area around 2002. Strong flyers, they have since spread to 35 states (AL, AR, CO, CT, DE, GA, IL, IN, IA, KS, LA, MD, MA, MI, MN, MO, Nebraska, NH, NJ, NY, NC, OH, OK, PA, TN, TX, VA, WV, and WI) and four provinces in Canada.

pests Emerald Ash Borer
Emerald ash borer damage. (Credit: Piotr- ADOBE STOCK)

Though half an inch long, the metallic green EAB causes great damage to the circulatory systems of green, white, and black ash trees. Adults emerge in late May (or earlier in warm weather), with females laying eggs shortly thereafter. Upon hatching, larvae quickly bore into the tree, feeding on the cambium and disrupting the tree’s vascular system. Feeding damage inhibits the tree’s ability to transport photosynthates, water, and nutrients between the roots and leaves, resulting in canopy thinning, branch dieback, and epicormic sprouting.

Often, EAB presence and damage may not be obvious to clients after the first year. However, EAB use sensory organs to detect the presence of other insects and plant volatiles given off from feeding. Therefore, EAB will migrate where others are feeding year after year.

After pupation the next spring, adults dig their way out, leaving a distinct exit hole in the shape of a “D”. The unique exit holes, coupled with flagging of branches or defoliation, are indications of infestation. Increased bird activity is another potential indicator, since birds will dig to reach larvae.

Early detection is critical to salvaging the tree; if defoliation is 40-60%, there may be no chance of remedying the damage. Before arriving at a property, know if possible infestations have been detected in the area; if so, chances of infestation are probable. Binoculars and a keen eye will be necessary to confirm infestation and determine the appropriate treatment plan—be it a root zone injection, direct trunk injection, or removal. After assessing the level of damage, and determining if the infestation is recent, a systemic insecticide is the preferred treatment option. Current guidance also suggests treating all ash trees within a 15-mile radius for protection.

The choice of insecticide is dependent upon formulation, timing, application technique, and equipment used. For best results when conducting soil drenching or injections, use dinotefuran or imidacloprid, or a combination of both in liquid form. For dinotefuran specifically, it’s best to apply in early spring; imidacloprid can be applied in both spring and fall.

For direct trunk injections, emamectin benzoate is the recommended insecticide. This method requires drilling into the infected tree and using specialized equipment to administer the proper dosage per tree size. Using diameter at breast height tape (DBH), measure the tree trunk at chest height—approximately 4.5’ off the ground. Once DBH is determined, cross-reference it with the product label for the dosage rate and administer accordingly. It’s important to make sure the application is as uniform as possible around the tree to ensure material is well dispersed. Trunk injections can last up to three years, but a soil injection should be reapplied every one to two years, as long as there is pest activity.

When removing infected trees, check state and federal regulations. Many states have rules regarding removal and disposal of infested trees. Many have also banned distribution of firewood. The USDA Animal and Plant Health Inspection Service (APHIS) provides federal guidelines for tree removal and proper disposal for many invasive pests.

It’s important to report any infestation to the local agriculture extension office. Reporting is more critical than ever following the lift of quarantine regulations in early 2021.

The USDA has begun developing a biological control using parasitic wasps to reduce EAB populations. These wasps are natural predators of EAB in Asia and have shown some promise for isolated control in the U.S. without negatively impacting ecosystems. It’s currently too early to know how effective this strategy will be over time.

Hemlock Woolly Adelgid

Originating in Asia, hemlock woolly adelgid (HWA) was first found in Virginia in the 1950s; in the 1980s the infestation rapidly spread, wreaking devastation on the natural hemlock forests in the Southern Appalachian Mountains. Woolly adelgid has since spread into at least 17 states, from the Smoky Mountains through Maine.

pests Hemlock Woolly Adelgid
Hemlock woolly adelgid. (Credit: Nordreicka – ADOBE STOCK)

According to the Arbor Day Foundation, “In 2007, the Virginia Tech College of Agriculture and Life Sciences reported an estimated 50% of the eastern hemlock range had been affected.” A more recent study published by Multidisciplinary Digital Publishing Institute forecasts that “by 2030 hemlock will be almost entirely lost from forests south of southern New England…and completely lost from forests by 2050.”

HWA are small—about a quarter of an inch— but are visible due to the white, cottony substance that covers their bodies, and their congregation in clusters along the base of twigs, under needles. (See photo).

Though small, their impact is mighty. As a year-round pest, their ability to reproduce asexually results in multiple generations between July and November. They extract sap—which contains beneficial nutrients necessary for tree survival—at a rapid rate. Infested hemlocks lose color, drop needles, and look increasingly bare. More dramatically impacted hemlocks may lose limbs.

While treatment of hemlocks in forests is problematic, according to the USDA Forest Service, “…chemical controls, such as the use of systemic insecticides and horticultural oil, have proven effective in controlling adelgids in yards, gardens, and parks.”

This is done by performing either an injection or soil drench with the active ingredient imidacloprid into the vascular system. With an increased HWA population in the fall, timing treatment—whether systemic or foliar—to the spring and fall is best to maximize results; for soil drench or injection, the treatment period could extend from spring into early summer as long as the tree is not under serious stress. It’s also important to not fertilize affected hemlocks, especially while treating. Fertilization exacerbates infestation since it provides more nutrients suitable for adelgid feeding.

Spotted Lanternfly

As one of the more recent invasives, spotted lanternflies (SLF) were first reported in Pennsylvania in 2014 after arriving in a shipping container from Asia. In a few short years, they have migrated to NY, NJ, DE, CT, MA, VA, and OH.

pests Spotted Lanternfly
Spotted lanternfly. (Credit: Lee- ADOBE STOCK)

The SLF completes its life cycle in one year. Eggs hatch into nymphs in the spring, around April; nymphs mature into adults in mid-summer, usually by July; and in late summer/fall, adults mate and lay eggs. Each lifecycle stage has unique identifiers: egg masses are brown and seed-like, arranged vertically; nymphs are black with white spots, and develop red patches; adults have an inch-long wingspan. (See photo.)

SLF feed on 70 species of plants, including some ornamental trees and shrubs. Other SLF favorites include tree of heaven (Ailanthus altissima), maple, and birch. By using its piercing mouthpart, SLF puncture a trunk or leaf to extract and feed on sap. As they feed, they excrete sap onto the tree, attracting other insects. The sap can also cause an unsightly fungus called sooty mold.

The best way to treat for SLF is prevention—making the trunk inhospitable for laying eggs by using sticky traps and applying a banded wrap. Unfortunately, while traps and guards can reduce populations, they don’t keep SLF from coming to the tree. (There are no known SLF natural enemies in the U.S., though some predators like spiders and birds will eat SLF.)

If SLF is already present on trees, proven insecticides—whether systemically applied (tree injection, soil drench) or by contact to the bark (sprays)—can help with control. If using spray, apply between two and five treatments during the growing season.

Another technique is to spray horticultural oil directly on overwintering egg masses (which suffocates the eggs) in late winter through early spring. Alternatively, insecticide sprays using neem oil, pyrethroids, dinotefuran, and insecticidal soaps have all been proven to be effective at controlling nymphs and adults from late spring through early fall. However, timing must be considered since most products do not provide season-long protection.

If treating systematically with injections or soil drenching, imidacloprid or dinotefuran are most effective in controlling nymph and adult stages when timed during late spring through early summer. Note that systemic treatment is only effective when treating plants that receive insect feeding. It’s also advisable to cut down any trees of heaven on the property. This undesirable invasive weedy tree is a primary host species.

EAB, HWA, and SLF are just a few of the invasives that can wreak havoc on trees. But the more we educate ourselves—and our clients—the more we can help stop their spread. For more information, visit the USDA Pest Tracker.

Brian FeldmanMarc MayerFeldman (far left) and Mayer are both directors of technical operations at TruGreen, a leading lawn care company. Feldman, who oversees the North, and Mayer, overseeing the South, are seasoned green industry veterans. Both bring their collective horticultural, arboriculture, and agronomic expertise to help drive strategic program development and implementation, and provide chemical resource management, operations training and development, associate health and safety, regulatory compliance, and product and environmental stewardship.

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  1. It’s interesting to learn about damaging pests such as the spotted lanternfly that feeds on the sap which causes fungus growth that ultimately deteriorates tree health. Now, because of this natural phenomenon, I would understand why some homeowners need tree removal services. It’s also helpful to note that there are ways to mitigate has damage that these services can also help with.

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